Why LSA doesn't matter
08-09-2014, 08:14 PM
#43
Moderator
This simple on-line calculator will help you determine overlap:
http://www.wallaceracing.com/overlap-calc.php
Even though the page says to "Enter advertised duration", enter the durations at .050 and you will have the overlap at .050.
Entering the duration and LSA values from Martin's post #1 will give you the same overlap values that he calculated the "long way".
Someone correct me if these novice beliefs of mine are wrong.
As overlap increases:
1. The cam is oriented towards higher rpm.
2. While chop will increase, the vacuum at idle/cruise will decrease.
IMHO, somewhere around 10 degrees overlap there will not be enough vacuum to safely operate normal power brakes. (I use Hydraboost which doesn't need vacuum.)
3. As overlap increases, gas mileage will decrease quickly.
4. As overlap increases, emissions will soar.
As far as I know, no "stock" cam from any US auto manufacturer from any year (even 1960s) has positive overlap. Using the calculator on any stock cam specs always shows a negative overlap.
Thanks again for the excellent thread.
http://www.wallaceracing.com/overlap-calc.php
Even though the page says to "Enter advertised duration", enter the durations at .050 and you will have the overlap at .050.
Entering the duration and LSA values from Martin's post #1 will give you the same overlap values that he calculated the "long way".
Someone correct me if these novice beliefs of mine are wrong.
As overlap increases:
1. The cam is oriented towards higher rpm.
2. While chop will increase, the vacuum at idle/cruise will decrease.
IMHO, somewhere around 10 degrees overlap there will not be enough vacuum to safely operate normal power brakes. (I use Hydraboost which doesn't need vacuum.)
3. As overlap increases, gas mileage will decrease quickly.
4. As overlap increases, emissions will soar.
As far as I know, no "stock" cam from any US auto manufacturer from any year (even 1960s) has positive overlap. Using the calculator on any stock cam specs always shows a negative overlap.
Thanks again for the excellent thread.
08-11-2014, 10:26 AM
#44
I think you're close to spot on with all of your points, but I want to talk about what exactly overlap is for the masses so that they can envision what is actually going on and see what is happening during overlap. As I said in my first post I didn't want to get into further discussion on cam timing without touching on the very basic items first. I won't get into full blown resonance tuning here, but this will help visualize what occurs during valve overlap.
First let's start with Webster's definition of overlap.
over·lap verb \ˌō-vər-ˈlap\
: to lie over the edge of (something) : to cover part of the edge of (something)
: to happen at the same time as something else
: to have parts that are the same as parts of something else
"To cover part of the edge of something." "To occur at the same time as something else."
Those two stand out the most to me when trying to define valve overlap in an engine like we are concerned with here. Valve overlap in a running engine is when the intake valve is opened before the exhaust valve closes. We use overlap to help cleanse the cylinder and combustion chamber of spent exhaust gasses that the piston cannot push out. We also use it to help get the flow of air/fuel from the intake tract moving into the cylinder before the piston begins to start downwards on the intake stroke. This is accomplished by resonance tuning.
Before the benefits of longer duration camshafts were discovered, the first internal combustion piston driven engines were extremely throttled back and could not rev. These engines were not meant for anything but industrial use and thus never meant for even moderate RPM. Even when the first passenger cars emerged valve overlap and long duration camshafts were not employed as RPM's were kept very low.
When the benefits of valve overlap were first discovered it was described as a supercharging effect and even to some, a fifth engine cycle. Opening the intake valve before the exhaust valve closed was never really thought of before that time. They wouldn't open the intake valve until the piston had passed top dead center and they would close the intake valve at bottom dead center. They also would open the exhaust valve at bottom dead center and close the exhaust valve at top dead center. Resulting in an engine that was extremely power limited, had no engine acceleration and could not rev whatsoever.
Mrvedit stated OEM camshafts have no valve overlap and even negative valve overlap. That is true @.050, but not true at lower lobe lifts where the lobe has even more duration than it does @.050.
Every single passenger car on the road today has valve overlap. This means that the intake valve is opening before the exhaust valve is closed. For this to happen the intake valve has to open while the piston is still on the exhaust stroke. This means it has not reached top dead center yet. Modern engines would not be able to achieve the RPM that even a Honda Accord can make peak power at without some amount of valve overlap. Not to mention the amount of power even that Honda Accord engine makes. To get a true idea of how much overlap a camshaft has, and how much vacuum it may be able to pull at idle you need to look at the seated duration specs. .050" is not where the lobe begins to open or close. There are obviously lower lobe lift points below .050". Those lower lifts have more duration than higher lifts, and how much more is determined by the ramp rate and shape of the lobe.
A slower opening and most of the time milder ramp rate lobe will have more valve overlap at lower lobe lifts because of added duration at those lower lobe lifts. If you add duration between sections of ramp(one lift point to the next) you can slow down the velocity and acceleration of that ramp. A faster opening more aggressive ramp rate lobe will have less valve overlap at lower lobe lifts for the same reason as I mentioned above. This produces higher vacuum, more throttle response and more off idle torque in my experience.
When camshaft durations become longer and valve overlap is introduced because of it, the engine loses vacuum signal and MPG suffers. Why is this? When the piston is moving at 600-1000rpm it is moving very slowly. Just because the engine is moving that slowly doesn't mean that the valves aren't open for any less time than they would be at 7000rpm. Because of this the engine is experiencing a literal vacuum leak and is struggling to run. Not only that, but exhaust gas contamination is polluting the intake charge and also causing the engine to run poorly thus struggling to run. This doesn't allow full combustion to occur and produces higher emissions as a byproduct. This is why your cammed engine will always smell rich at idle and why even leaning it out will not get the smell to go away.
First let's start with Webster's definition of overlap.
over·lap verb \ˌō-vər-ˈlap\
: to lie over the edge of (something) : to cover part of the edge of (something)
: to happen at the same time as something else
: to have parts that are the same as parts of something else
"To cover part of the edge of something." "To occur at the same time as something else."
Those two stand out the most to me when trying to define valve overlap in an engine like we are concerned with here. Valve overlap in a running engine is when the intake valve is opened before the exhaust valve closes. We use overlap to help cleanse the cylinder and combustion chamber of spent exhaust gasses that the piston cannot push out. We also use it to help get the flow of air/fuel from the intake tract moving into the cylinder before the piston begins to start downwards on the intake stroke. This is accomplished by resonance tuning.
Before the benefits of longer duration camshafts were discovered, the first internal combustion piston driven engines were extremely throttled back and could not rev. These engines were not meant for anything but industrial use and thus never meant for even moderate RPM. Even when the first passenger cars emerged valve overlap and long duration camshafts were not employed as RPM's were kept very low.
When the benefits of valve overlap were first discovered it was described as a supercharging effect and even to some, a fifth engine cycle. Opening the intake valve before the exhaust valve closed was never really thought of before that time. They wouldn't open the intake valve until the piston had passed top dead center and they would close the intake valve at bottom dead center. They also would open the exhaust valve at bottom dead center and close the exhaust valve at top dead center. Resulting in an engine that was extremely power limited, had no engine acceleration and could not rev whatsoever.
Mrvedit stated OEM camshafts have no valve overlap and even negative valve overlap. That is true @.050, but not true at lower lobe lifts where the lobe has even more duration than it does @.050.
Every single passenger car on the road today has valve overlap. This means that the intake valve is opening before the exhaust valve is closed. For this to happen the intake valve has to open while the piston is still on the exhaust stroke. This means it has not reached top dead center yet. Modern engines would not be able to achieve the RPM that even a Honda Accord can make peak power at without some amount of valve overlap. Not to mention the amount of power even that Honda Accord engine makes. To get a true idea of how much overlap a camshaft has, and how much vacuum it may be able to pull at idle you need to look at the seated duration specs. .050" is not where the lobe begins to open or close. There are obviously lower lobe lift points below .050". Those lower lifts have more duration than higher lifts, and how much more is determined by the ramp rate and shape of the lobe.
A slower opening and most of the time milder ramp rate lobe will have more valve overlap at lower lobe lifts because of added duration at those lower lobe lifts. If you add duration between sections of ramp(one lift point to the next) you can slow down the velocity and acceleration of that ramp. A faster opening more aggressive ramp rate lobe will have less valve overlap at lower lobe lifts for the same reason as I mentioned above. This produces higher vacuum, more throttle response and more off idle torque in my experience.
When camshaft durations become longer and valve overlap is introduced because of it, the engine loses vacuum signal and MPG suffers. Why is this? When the piston is moving at 600-1000rpm it is moving very slowly. Just because the engine is moving that slowly doesn't mean that the valves aren't open for any less time than they would be at 7000rpm. Because of this the engine is experiencing a literal vacuum leak and is struggling to run. Not only that, but exhaust gas contamination is polluting the intake charge and also causing the engine to run poorly thus struggling to run. This doesn't allow full combustion to occur and produces higher emissions as a byproduct. This is why your cammed engine will always smell rich at idle and why even leaning it out will not get the smell to go away.
08-11-2014, 12:30 PM
#45
So what's the big deal about overlap then you say? If it causes these kinds of problems at idle and cruising speeds, why is it so beneficial? Why would it be utilized if it causes poor gas mileage and idle quality? The answer is simple. If an internal combustion piston driven engine is to ever come remotely close to achieving 100% VE or even exceeding 100% VE overlap has to be employed.
When the exhaust valve first opens high pressure gasses are expelled from the cylinder. This is due to the pressure differential between lower atmospheric pressure in the exhaust port and higher pressure spent exhaust gas in the cylinder. Think of a compressed air tank when you open the release valve. The high pressure compressed air in the tank will flow out of the open valve due to the pressure differential that exists between the contents of the tank and what's present in the atmosphere. This is what is occurring during the exhaust valve opening event.
With that said, there is more to the pressure differential between the contents of the cylinder and atmospheric pressure. There is a high pressure sonic wave that travels at 1400-1700fps from the exhaust port down the primary tube at the speed of sound. As temperature increases so does the speed of sound, so instead of travelling at 760mph, this wave is travelling at 1150 mph+. The high pressure sonic wave is separate from the particle gasses that compose the spent exhaust gas in the cylinder which travel at a much slower speed of 250-350fps. This sonic wave is propelled out of the cylinder at the very beginning of the EVO event. This high pressure wave creates a low pressure suction wave behind it. Think of an 18 wheeler driving down the freeway and you're standing on the side of the road as it passes by you. As it goes past you can feel a low pressure vacuum try and suck you in behind it. This is what rocks your car when a fast moving vehicle goes past you. An 18 wheeler can create this effect at a much lower speed than a lighter smaller vehicle. This is due to the 18 wheelers mass and the amount of inertia it carries versus a smaller and lighter sports car travelling at a higher rate of speed. When this outgoing rush of high pressure exits the exhaust valve it creates a low pressure vacuum behind it and pulls the spent exhaust gas in the cylinder out with it. As RPM increases this effect becomes more pronounced as there is more exhaust gas exiting the engine thus it has more mass and carries more inertia. While all of this is happening the piston is being forced down the bore from the pressure present in the cylinder. This expanding exhaust gas is a byproduct from the combustion event and even though the exhaust valve is now open and is relieving pressure, the piston still carries momentum on its way to bottom dead center.
As the piston begins to move back up the bore on the exhaust stroke and is pushing the rest of the spent exhaust gas out, something else is happening in the primary as well. That same sonic wave that was released upon the EVO event has now reflected itself back up the primary pipe as a negative pressure wave due to the resulting change in area at the collector. When the primary tube diameter changes and reaches the collector (or atmosphere in the case of zoomies) it allows that high pressure positive wave to expand. When it expands it creates a reflected negative pressure wave. The greater the change in area the more intense the reflection is. This reflected wave is also of the same intensity as the positive wave. Depending on the RPM the engine is operating, VE% at that RPM, EVO timing, length of the primary pipe, diameter of the primary pipe and collector length/design will all determine just how intense that wave is in both directions. Certain collector(merged or stamped) and primary designs (straight or stepped) can create higher or lower intensity waves for narrower or broader RPM ranges.
As this negative wave is traveling back up the primary tube, the piston is approaching top dead center. When the negative pressure wave travels up the primary tube and exhaust port, it is changing the pressure in the primary tube and exhaust port to a lower pressure than the remaining contents of the cylinder. This pulls the spent exhaust gas out of the cylinder and into the primary. The reason this occurs is because the spent exhaust gas in the cylinder is of a higher pressure than what is presently in the exhaust port and primary tube due to the reflected wave. An internal combustion piston driven engine revolves around pressure differential in filling and emptying the cylinder. Without that pressure differential, no cylinder fill or evacuation would occur.
Since there is a clearance volume in the combustion chamber, the piston can't push all of the spent exhaust gas out. So there needs to be a way to remove it if you want to fill the cylinder to its maximum capacity on the intake stroke. When grinding a cam for an engine, the camshaft is specified to set the RPM that peak HP and TQ occur along with how we want to shape the power curve for the application the cam is being used in. If the cam is ground correctly for the application, it will take advantage of the arrival of this negative pressure wave and use it to its advantage to fill and exhaust the cylinder.
If valve timing is correctly timed and the headers properly designed, the negative pressure in the exhaust port will stick around long enough between peak torque and peak HP to pull that last bit of spent exhaust gas out of the clearance volume that the piston cannot push out. The exhaust valve needs to be open far enough to allow this to happen so timing its closing event is paramount for it to be of maximum benefit. When that exhaust valve closes will determine at what RPM this occurrence will be of maximum benefit. The earlier it closes the lower the RPM it benefits the engine. The later it closes the higher the RPM it benefits the engine. The design of the header plays just as much if not more of a role in all of this than the actual cam timing itself. The two must be designed to work together for optimum results and maximum benefit.
An engine can also utilize this negative pressure in the exhaust port to help coax higher pressure air/fuel from the intake tract into the cylinder before the piston even starts to travel downwards on the intake stroke. This is done by opening the intake valve before the exhaust valve closes. The lower pressure in the exhaust port and cylinder allows the higher pressure intake charge to begin to gain velocity in the intake tract before the piston even starts to pull on the intake port. Yes you will lose some air/fuel charge by doing this, but at the same time we're removing every last bit of spent exhaust gas from the cylinder and jump starting the intake charge. This would be like giving a runner a head start against his competition. Even though some intake charge is lost during overlap, the cylinder can now be filled to a higher % than it could of been had we shut the exhaust valve sooner, not evacuated all spent exhaust gasses, not jump started the intake charge and had spent exhaust gas displacing fresh intake charge.
Controlling that charge loss and utilizing this resonance tuning to your advantage is paramount to obtaining every bit of power from your engine in the operating range it’s utilized. It could be said that more overlap is geared more towards higher RPM. When the piston is moving slower in a lower RPM engine, the overlap period does not need to last as long as it would in a higher RPM engine. Since the piston is moving slower, the longer those valves are open the more charge loss and reversion occurs. Less time is needed for both valves to be open to achieve the same end goal. Conversely in a higher RPM engine more time is needed for both valves to be open as the piston is moving faster and less time is available for cylinder evacuation and filling. Because of this we must give the exhaust port more time to get that last bit of spent gas out of the clearance volume and more time to jump start the intake charge.
Just because a camshaft has some overlap doesn't always mean it's oriented for higher RPM operation. Or if a camshaft has more than 10 degrees of overlap@.050 it is oriented for higher RPM operation. 10 degrees would be a good rule to follow for a 346-364 c.i. engine to determine whether the camshaft was more so geared for a lower RPM combination or a higher RPM combination. A 4” crank stroker engine such as a 408-427 c.i. engine would “soak up” 10 degrees of overlap a lot better than a smaller 346-364 c.i. engine. The stroker engine needs that added overlap due to its increase in piston speed from a longer stroke crankshaft. Even if the operating range doesn’t increase when comparing optimal cam timing for a 346-364 c.i. engine versus a 408-427 c.i. engine, piston speed increases. As a result, the overlap time period must also increase.
I went a little bit deeper than I wanted to on this post, but I felt like some of this had to be explained in detail for it to be understood. I don’t proclaim to be a physics major or have a degree in fluid dynamics or continuum mechanics. What I do feel I have is a strong understanding of how a running engine fills and exhausts the cylinder. By understanding these events and understanding the how and why it works allows you to peer into the mechanics of why your engine does what it does. As I said before, getting people to use what is between their ears and understanding the physics of a running engine is the ultimate epiphany in engine building.
Enjoy.
When the exhaust valve first opens high pressure gasses are expelled from the cylinder. This is due to the pressure differential between lower atmospheric pressure in the exhaust port and higher pressure spent exhaust gas in the cylinder. Think of a compressed air tank when you open the release valve. The high pressure compressed air in the tank will flow out of the open valve due to the pressure differential that exists between the contents of the tank and what's present in the atmosphere. This is what is occurring during the exhaust valve opening event.
With that said, there is more to the pressure differential between the contents of the cylinder and atmospheric pressure. There is a high pressure sonic wave that travels at 1400-1700fps from the exhaust port down the primary tube at the speed of sound. As temperature increases so does the speed of sound, so instead of travelling at 760mph, this wave is travelling at 1150 mph+. The high pressure sonic wave is separate from the particle gasses that compose the spent exhaust gas in the cylinder which travel at a much slower speed of 250-350fps. This sonic wave is propelled out of the cylinder at the very beginning of the EVO event. This high pressure wave creates a low pressure suction wave behind it. Think of an 18 wheeler driving down the freeway and you're standing on the side of the road as it passes by you. As it goes past you can feel a low pressure vacuum try and suck you in behind it. This is what rocks your car when a fast moving vehicle goes past you. An 18 wheeler can create this effect at a much lower speed than a lighter smaller vehicle. This is due to the 18 wheelers mass and the amount of inertia it carries versus a smaller and lighter sports car travelling at a higher rate of speed. When this outgoing rush of high pressure exits the exhaust valve it creates a low pressure vacuum behind it and pulls the spent exhaust gas in the cylinder out with it. As RPM increases this effect becomes more pronounced as there is more exhaust gas exiting the engine thus it has more mass and carries more inertia. While all of this is happening the piston is being forced down the bore from the pressure present in the cylinder. This expanding exhaust gas is a byproduct from the combustion event and even though the exhaust valve is now open and is relieving pressure, the piston still carries momentum on its way to bottom dead center.
As the piston begins to move back up the bore on the exhaust stroke and is pushing the rest of the spent exhaust gas out, something else is happening in the primary as well. That same sonic wave that was released upon the EVO event has now reflected itself back up the primary pipe as a negative pressure wave due to the resulting change in area at the collector. When the primary tube diameter changes and reaches the collector (or atmosphere in the case of zoomies) it allows that high pressure positive wave to expand. When it expands it creates a reflected negative pressure wave. The greater the change in area the more intense the reflection is. This reflected wave is also of the same intensity as the positive wave. Depending on the RPM the engine is operating, VE% at that RPM, EVO timing, length of the primary pipe, diameter of the primary pipe and collector length/design will all determine just how intense that wave is in both directions. Certain collector(merged or stamped) and primary designs (straight or stepped) can create higher or lower intensity waves for narrower or broader RPM ranges.
As this negative wave is traveling back up the primary tube, the piston is approaching top dead center. When the negative pressure wave travels up the primary tube and exhaust port, it is changing the pressure in the primary tube and exhaust port to a lower pressure than the remaining contents of the cylinder. This pulls the spent exhaust gas out of the cylinder and into the primary. The reason this occurs is because the spent exhaust gas in the cylinder is of a higher pressure than what is presently in the exhaust port and primary tube due to the reflected wave. An internal combustion piston driven engine revolves around pressure differential in filling and emptying the cylinder. Without that pressure differential, no cylinder fill or evacuation would occur.
Since there is a clearance volume in the combustion chamber, the piston can't push all of the spent exhaust gas out. So there needs to be a way to remove it if you want to fill the cylinder to its maximum capacity on the intake stroke. When grinding a cam for an engine, the camshaft is specified to set the RPM that peak HP and TQ occur along with how we want to shape the power curve for the application the cam is being used in. If the cam is ground correctly for the application, it will take advantage of the arrival of this negative pressure wave and use it to its advantage to fill and exhaust the cylinder.
If valve timing is correctly timed and the headers properly designed, the negative pressure in the exhaust port will stick around long enough between peak torque and peak HP to pull that last bit of spent exhaust gas out of the clearance volume that the piston cannot push out. The exhaust valve needs to be open far enough to allow this to happen so timing its closing event is paramount for it to be of maximum benefit. When that exhaust valve closes will determine at what RPM this occurrence will be of maximum benefit. The earlier it closes the lower the RPM it benefits the engine. The later it closes the higher the RPM it benefits the engine. The design of the header plays just as much if not more of a role in all of this than the actual cam timing itself. The two must be designed to work together for optimum results and maximum benefit.
An engine can also utilize this negative pressure in the exhaust port to help coax higher pressure air/fuel from the intake tract into the cylinder before the piston even starts to travel downwards on the intake stroke. This is done by opening the intake valve before the exhaust valve closes. The lower pressure in the exhaust port and cylinder allows the higher pressure intake charge to begin to gain velocity in the intake tract before the piston even starts to pull on the intake port. Yes you will lose some air/fuel charge by doing this, but at the same time we're removing every last bit of spent exhaust gas from the cylinder and jump starting the intake charge. This would be like giving a runner a head start against his competition. Even though some intake charge is lost during overlap, the cylinder can now be filled to a higher % than it could of been had we shut the exhaust valve sooner, not evacuated all spent exhaust gasses, not jump started the intake charge and had spent exhaust gas displacing fresh intake charge.
Controlling that charge loss and utilizing this resonance tuning to your advantage is paramount to obtaining every bit of power from your engine in the operating range it’s utilized. It could be said that more overlap is geared more towards higher RPM. When the piston is moving slower in a lower RPM engine, the overlap period does not need to last as long as it would in a higher RPM engine. Since the piston is moving slower, the longer those valves are open the more charge loss and reversion occurs. Less time is needed for both valves to be open to achieve the same end goal. Conversely in a higher RPM engine more time is needed for both valves to be open as the piston is moving faster and less time is available for cylinder evacuation and filling. Because of this we must give the exhaust port more time to get that last bit of spent gas out of the clearance volume and more time to jump start the intake charge.
Just because a camshaft has some overlap doesn't always mean it's oriented for higher RPM operation. Or if a camshaft has more than 10 degrees of overlap@.050 it is oriented for higher RPM operation. 10 degrees would be a good rule to follow for a 346-364 c.i. engine to determine whether the camshaft was more so geared for a lower RPM combination or a higher RPM combination. A 4” crank stroker engine such as a 408-427 c.i. engine would “soak up” 10 degrees of overlap a lot better than a smaller 346-364 c.i. engine. The stroker engine needs that added overlap due to its increase in piston speed from a longer stroke crankshaft. Even if the operating range doesn’t increase when comparing optimal cam timing for a 346-364 c.i. engine versus a 408-427 c.i. engine, piston speed increases. As a result, the overlap time period must also increase.
I went a little bit deeper than I wanted to on this post, but I felt like some of this had to be explained in detail for it to be understood. I don’t proclaim to be a physics major or have a degree in fluid dynamics or continuum mechanics. What I do feel I have is a strong understanding of how a running engine fills and exhausts the cylinder. By understanding these events and understanding the how and why it works allows you to peer into the mechanics of why your engine does what it does. As I said before, getting people to use what is between their ears and understanding the physics of a running engine is the ultimate epiphany in engine building.
Enjoy.
08-11-2014, 12:41 PM
#46
Yeah yeah. All that is great, but what is the best LSA for maximum power? 
This stuff is actually really simple to figure out. I created a device that can tell you ideal overlap for any engine. Now all you need is one of these:
Seriously though Martin, you better stop all this. This place is in danger of becoming ls1TECH again.
This stuff is actually really simple to figure out. I created a device that can tell you ideal overlap for any engine. Now all you need is one of these:
Seriously though Martin, you better stop all this. This place is in danger of becoming ls1TECH again.
08-11-2014, 01:47 PM
#48
Moderator
Wow, great information. Perhaps the best thread ever on camshaft theory. Certainly the best "article" I have ever read on the subject.
I never considered that when comparing two cams with the same amount of overlap, the one with more-aggressive ramps would have better idle/vacuum than the cam with less-aggressive ramps. "Gut feel" was that aggressive ramps give an aggressive idle - but the opposite appears to be true. That helps explains why an LS engine with roller lifters idles better than a flat-tappet SBC with similar cam specs - the roller lifters allow more aggressive ramps.
To compare different cams, I always calculate the overlap @.050. Unlike advertised or even .050 duration, a few degrees of overlap make a huge difference. When I switched from a cam with 12 degrees overlap to one with 9 degrees overlap, my wife commented that she no longer hears me drive into the garage. I also lost about 15HP, but it does drive better on the street.
I never considered that when comparing two cams with the same amount of overlap, the one with more-aggressive ramps would have better idle/vacuum than the cam with less-aggressive ramps. "Gut feel" was that aggressive ramps give an aggressive idle - but the opposite appears to be true. That helps explains why an LS engine with roller lifters idles better than a flat-tappet SBC with similar cam specs - the roller lifters allow more aggressive ramps.
To compare different cams, I always calculate the overlap @.050. Unlike advertised or even .050 duration, a few degrees of overlap make a huge difference. When I switched from a cam with 12 degrees overlap to one with 9 degrees overlap, my wife commented that she no longer hears me drive into the garage. I also lost about 15HP, but it does drive better on the street.
08-11-2014, 02:30 PM
#49
UD Harold was big on using decreased seat to seat duration to improve torque, area under the curve and vacuum at idle.
If you search a lot of his older stuff, and even newer stuff you'll find that he holds those fast open slow close asymmetrical ramps in high regard. He used decreased duration on the opening side of his ramps to decrease overlap on the opening event. He felt this decreased reversion and increased idle vacuum and emissions. Which I have found to be true in most cases as well.
Kip from Cam Motion had some interesting information regarding added seated duration in regards to added overlap. He mentioned that a lot of times the added duration merely resulted in more pre-loading of the valve train and didn't actually increase seat to seat overlap much if any. I would assume as long as you didn't add a large amount of duration at lower lobe lifts that this would have to hold true as well.
I will say that in regards to lobe design, more than 90% of the lobes used today are asymmetrical ramps.
If you search a lot of his older stuff, and even newer stuff you'll find that he holds those fast open slow close asymmetrical ramps in high regard. He used decreased duration on the opening side of his ramps to decrease overlap on the opening event. He felt this decreased reversion and increased idle vacuum and emissions. Which I have found to be true in most cases as well.
Kip from Cam Motion had some interesting information regarding added seated duration in regards to added overlap. He mentioned that a lot of times the added duration merely resulted in more pre-loading of the valve train and didn't actually increase seat to seat overlap much if any. I would assume as long as you didn't add a large amount of duration at lower lobe lifts that this would have to hold true as well.
I will say that in regards to lobe design, more than 90% of the lobes used today are asymmetrical ramps.
08-11-2014, 02:51 PM
#50
I never considered that when comparing two cams with the same amount of overlap, the one with more-aggressive ramps would have better idle/vacuum than the cam with less-aggressive ramps. "Gut feel" was that aggressive ramps give an aggressive idle - but the opposite appears to be true. That helps explains why an LS engine with roller lifters idles better than a flat-tappet SBC with similar cam specs - the roller lifters allow more aggressive ramps.
So, when conventional small block chevy guys marvel how how high an LS will RPM with such a small cam, they weren't considering the duration at the valve as opposed to the duration at the camshaft. Hell, neither was I until Kip pointed this out to me. Going from a 1.5 rocker arm to a 1.7 rocker arm showed to provide 5 degrees more duration at .050" and 8 degrees more duration @ .200" at the valve.
08-11-2014, 02:53 PM
#51
The roller is part of the difference between the SBC and LSx engines. However, there are a few more things at play. Recently in a conversation with Kip Fabre, he sent me some screen shots of two engines with identical camshaft specs, one with 1.5 rockers as a SBC would have and one with 1.7 rockers like an LSx would have. The engine with 1.7 rockers not only had more lift as you would expect for a given cam lobe, but also had notably more duration AT THE VALVE both at .050" and moreso at .200". So, even with the same seat timing, the duration at higher lifts and area under the curve were notable more with the LSx at the valve.
So, when conventional small block chevy guys marvel how how high an LS will RPM with such a small cam, they weren't considering the duration at the valve as opposed to the duration at the camshaft. Hell, neither was I until Kip pointed this out to me. Going from a 1.5 rocker arm to a 1.7 rocker arm showed to provide 5 degrees more duration at .050" and 8 degrees more duration @ .200" at the valve.
So, when conventional small block chevy guys marvel how how high an LS will RPM with such a small cam, they weren't considering the duration at the valve as opposed to the duration at the camshaft. Hell, neither was I until Kip pointed this out to me. Going from a 1.5 rocker arm to a 1.7 rocker arm showed to provide 5 degrees more duration at .050" and 8 degrees more duration @ .200" at the valve.
08-11-2014, 05:08 PM
#53
Hold on to your butts people! (Always loved that line from Jurassic Park)
08-11-2014, 10:39 PM
#54
UD Harold was big on using decreased seat to seat duration to improve torque, area under the curve and vacuum at idle.
If you search a lot of his older stuff, and even newer stuff you'll find that he holds those fast open slow close asymmetrical ramps in high regard. He used decreased duration on the opening side of his ramps to decrease overlap on the opening event. He felt this decreased reversion and increased idle vacuum and emissions. Which I have found to be true in most cases as well.
If you search a lot of his older stuff, and even newer stuff you'll find that he holds those fast open slow close asymmetrical ramps in high regard. He used decreased duration on the opening side of his ramps to decrease overlap on the opening event. He felt this decreased reversion and increased idle vacuum and emissions. Which I have found to be true in most cases as well.
The roller is part of the difference between the SBC and LSx engines. However, there are a few more things at play. Recently in a conversation with Kip Fabre, he sent me some screen shots of two engines with identical camshaft specs, one with 1.5 rockers as a SBC would have and one with 1.7 rockers like an LSx would have. The engine with 1.7 rockers not only had more lift as you would expect for a given cam lobe, but also had notably more duration AT THE VALVE both at .050" and moreso at .200". So, even with the same seat timing, the duration at higher lifts and area under the curve were notable more with the LSx at the valve.
So, when conventional small block chevy guys marvel how how high an LS will RPM with such a small cam, they weren't considering the duration at the valve as opposed to the duration at the camshaft. Hell, neither was I until Kip pointed this out to me. Going from a 1.5 rocker arm to a 1.7 rocker arm showed to provide 5 degrees more duration at .050" and 8 degrees more duration @ .200" at the valve.
So, when conventional small block chevy guys marvel how how high an LS will RPM with such a small cam, they weren't considering the duration at the valve as opposed to the duration at the camshaft. Hell, neither was I until Kip pointed this out to me. Going from a 1.5 rocker arm to a 1.7 rocker arm showed to provide 5 degrees more duration at .050" and 8 degrees more duration @ .200" at the valve.
08-12-2014, 12:30 PM
#55
Here is a legend for the following two diagrams. A "key" if you will.
Light blue - Intake port pressure
Dark blue - Cylinder pressure
Red - exhaust port pressure
Let's take a look at what is occurring between top dead center on the power stroke to top dead center on the intake stroke.
The above diagram depicts an engine that is close to being "in tune". Note that when the intake valve opens, pressure in the cylinder and in the exhaust are lower than pressure in the intake tract. This allows higher pressure air/fuel charge to begin to flow into the cylinder before the piston passes top dead center. If cylinder pressure and/or exhaust pressure was equal or higher than intake pressure there would be reversion and port stall.
Note that after the initial spike in exhaust pressure upon the EVO event(sonic wave), the wave begins to go strongly negative and that negative pressure lasts until top dead center. Around mid exhaust stroke is when this wave will start to go negative in a well tuned system. Note that cylinder pressure also goes strongly negative as well. This results in decreased pumping losses as the more pressure present in the cylinder as the piston begins the exhaust stroke while it is pushing up, the more pressure it has to "pump" against. This slows the piston's momentum and piston speed thus creating a loss in power.
Note that this wave stays negative all the way until the piston reaches top dead center. This is what is keeping cylinder pressure lower than atmospheric which is what allows cylinder fill to be "jump started" before the piston begins the intake stroke.
The wave going strongly negative around mid exhaust stroke is from good exhaust tuning and proper primary length, primary diameter and proper collector length, diameter and design. The header design and engine RPM determines when this wave arrives and its intensity. The header design is what sets the harmonics, the camshaft just capitalizes on it. Much like I mentioned that the intake tracts length and cross section sets the operating range more than the cam does and the cam merely accentuates those points.
Now let's take a look at what is occurring between top dead center on the intake stroke and top dead center on the compression stroke.
As you can see once the piston passes top dead center and begins its intake stroke cylinder pressure is negative. As the piston travels downwards on the intake stroke, it leaves a low pressure void behind it. This creates a vacuum and pulls on the intake port. Thus the "suction wave" you're seeing in the diagram. As the piston travels down the bore on the intake stroke it is also gaining speed. The suction wave peaks right after maximum piston speed on the intake stroke. 70-75* ATDC is where maximum piston speed is reached and thus where the piston is demanding the most CFM from the induction system.
Even though the hardest draw on the intake port occurs around 70-75* ATDC that doesn't mean that the air/fuel charge responds instantly to it. Since air has mass and cannot accelerate instantaneously and there being a set amount of distance between the piston and the plenum(atmosphere) there is a delay in the signal reaching the intake port that the piston creates. This delay is seen at both ends of the intake tract. The below diagram depicts this.
Here is a legend(key) for this diagram:
Intake port inlet - Red
Intake port exit - Blue
Intake valve opening - purple
Exhaust valve closing - green
As you can see, there is a significant delay between the suction wave and the compressive wave seen at both ends of the intake tract. Due to the length of the intake tract and due to the fact that air has mass and cannot instantaneously accelerate. Just because the piston is drawing the hardest on the head @70-75* ATDC doesn't mean that is when the compressive wave will arrive at the cylinder. Not until after maximum piston speed occurs will primary ramming(inertia ram) occur. Just remember that the suction wave and the compressive wave are a result of piston motion and that piston motion drives everything. In a perfect world we'd have the intake center line occurring at maximum piston speed and we'd jerk the intake valve open instantaneously to that intake center line because that's when primary ramming would be occurring. In a world with physics though this doesn't happen and there is significant delay. Thus why the ICL normally occurs in most EFI LS engines between 106-112 degrees ATDC.
So with that said, around 90-95* ATDC is when primary ramming and inertia ram starts to build. This is a by product of the piston reaching maximum speed @70-75* ATDC and the draw it placed on the intake tract. Once the piston reaches 180* ATDC or BDC on the intake stroke inertia ram is really starting to pack every last bit of air mass into the cylinder that it can. As you can see right before BDC on the intake stroke cylinder and intake pressure are actually above atmospheric. The intake pressure rising above atmospheric from the building of inertia in the intake tract is literally creating a supercharging effect. This continues well into the compression stroke and is what allows the cylinder to continue to fill even with cylinder pressure rising above atmospheric.
As I've touched on before, the length of the intake tract is one of the primary determining factors in how long this wave will build for and then consequently die off. The shorter the intake tract the sooner this wave will build to its maximum and the sooner it will die off. The longer the intake tract the longer it takes to build and the longer it takes to die off.
In the first diagram being shown above this "die off" occurs a little before 90* after bottom dead center. In this diagram you can see that the IVC event is actually occurring a little later than it should and charge reversion is occurring because of it. If we want to trap and keep as much mass in the cylinder as possible the intake valve must close right at the point that cylinder pressure from the rising piston compressing the air/fuel charge overcomes the inertia from the air mass in the intake tract's momentum while the engine is in tune. Again, piston motion drives everything!
Light blue - Intake port pressure
Dark blue - Cylinder pressure
Red - exhaust port pressure
Let's take a look at what is occurring between top dead center on the power stroke to top dead center on the intake stroke.
The above diagram depicts an engine that is close to being "in tune". Note that when the intake valve opens, pressure in the cylinder and in the exhaust are lower than pressure in the intake tract. This allows higher pressure air/fuel charge to begin to flow into the cylinder before the piston passes top dead center. If cylinder pressure and/or exhaust pressure was equal or higher than intake pressure there would be reversion and port stall.
Note that after the initial spike in exhaust pressure upon the EVO event(sonic wave), the wave begins to go strongly negative and that negative pressure lasts until top dead center. Around mid exhaust stroke is when this wave will start to go negative in a well tuned system. Note that cylinder pressure also goes strongly negative as well. This results in decreased pumping losses as the more pressure present in the cylinder as the piston begins the exhaust stroke while it is pushing up, the more pressure it has to "pump" against. This slows the piston's momentum and piston speed thus creating a loss in power.
Note that this wave stays negative all the way until the piston reaches top dead center. This is what is keeping cylinder pressure lower than atmospheric which is what allows cylinder fill to be "jump started" before the piston begins the intake stroke.
The wave going strongly negative around mid exhaust stroke is from good exhaust tuning and proper primary length, primary diameter and proper collector length, diameter and design. The header design and engine RPM determines when this wave arrives and its intensity. The header design is what sets the harmonics, the camshaft just capitalizes on it. Much like I mentioned that the intake tracts length and cross section sets the operating range more than the cam does and the cam merely accentuates those points.
Now let's take a look at what is occurring between top dead center on the intake stroke and top dead center on the compression stroke.
As you can see once the piston passes top dead center and begins its intake stroke cylinder pressure is negative. As the piston travels downwards on the intake stroke, it leaves a low pressure void behind it. This creates a vacuum and pulls on the intake port. Thus the "suction wave" you're seeing in the diagram. As the piston travels down the bore on the intake stroke it is also gaining speed. The suction wave peaks right after maximum piston speed on the intake stroke. 70-75* ATDC is where maximum piston speed is reached and thus where the piston is demanding the most CFM from the induction system.
Even though the hardest draw on the intake port occurs around 70-75* ATDC that doesn't mean that the air/fuel charge responds instantly to it. Since air has mass and cannot accelerate instantaneously and there being a set amount of distance between the piston and the plenum(atmosphere) there is a delay in the signal reaching the intake port that the piston creates. This delay is seen at both ends of the intake tract. The below diagram depicts this.
Here is a legend(key) for this diagram:
Intake port inlet - Red
Intake port exit - Blue
Intake valve opening - purple
Exhaust valve closing - green
As you can see, there is a significant delay between the suction wave and the compressive wave seen at both ends of the intake tract. Due to the length of the intake tract and due to the fact that air has mass and cannot instantaneously accelerate. Just because the piston is drawing the hardest on the head @70-75* ATDC doesn't mean that is when the compressive wave will arrive at the cylinder. Not until after maximum piston speed occurs will primary ramming(inertia ram) occur. Just remember that the suction wave and the compressive wave are a result of piston motion and that piston motion drives everything. In a perfect world we'd have the intake center line occurring at maximum piston speed and we'd jerk the intake valve open instantaneously to that intake center line because that's when primary ramming would be occurring. In a world with physics though this doesn't happen and there is significant delay. Thus why the ICL normally occurs in most EFI LS engines between 106-112 degrees ATDC.
So with that said, around 90-95* ATDC is when primary ramming and inertia ram starts to build. This is a by product of the piston reaching maximum speed @70-75* ATDC and the draw it placed on the intake tract. Once the piston reaches 180* ATDC or BDC on the intake stroke inertia ram is really starting to pack every last bit of air mass into the cylinder that it can. As you can see right before BDC on the intake stroke cylinder and intake pressure are actually above atmospheric. The intake pressure rising above atmospheric from the building of inertia in the intake tract is literally creating a supercharging effect. This continues well into the compression stroke and is what allows the cylinder to continue to fill even with cylinder pressure rising above atmospheric.
As I've touched on before, the length of the intake tract is one of the primary determining factors in how long this wave will build for and then consequently die off. The shorter the intake tract the sooner this wave will build to its maximum and the sooner it will die off. The longer the intake tract the longer it takes to build and the longer it takes to die off.
In the first diagram being shown above this "die off" occurs a little before 90* after bottom dead center. In this diagram you can see that the IVC event is actually occurring a little later than it should and charge reversion is occurring because of it. If we want to trap and keep as much mass in the cylinder as possible the intake valve must close right at the point that cylinder pressure from the rising piston compressing the air/fuel charge overcomes the inertia from the air mass in the intake tract's momentum while the engine is in tune. Again, piston motion drives everything!
Last edited by Sales@Tick; 08-12-2014 at 01:07 PM.
08-12-2014, 12:56 PM
#56
The last diagram I'm going to show for now depicts the same engine in the above diagram running at an RPM above where it is "in tune".
Legend for this diagram:
Light blue - Intake pressure
Dark blue - Cylinder pressure
Red- Exhaust pressure
Let's again take a look at what is occurring between TDC on the power stroke and TDC on the intake stroke.
In this diagram note that the sonic wave upon EVO is not as sharp or as pointy as it was when the engine was "in tune". Note that the exhaust pressure also does not drop as quickly as it did when the engine was in tune. As a by product cylinder pressure doesn't drop as quickly and note that there is much higher pressure present in the cylinder mid-stroke versus when the engine was in tune. This creates much higher pumping losses as the piston now has to fight against that pressure as it starts to come back to TDC on the exhaust stroke from BDC.
What is happening here is the exhaust primary is either too long or too small of a diameter for the engine to be in tune at this engine speed. The higher the RPM the engine will operate the shorter and larger the primary must become. As you can see the reflected wave resulting from the initial sonic wave is arriving much too late during overlap. As a result cylinder pressure is higher than intake pressure upon the IVO event. This creates reversion and port stall. Note that not until TDC does cylinder pressure drop lower than intake pressure which finally allows air/fuel mixture to start to gain momentum in the intake tract. When this occurs the VE% that the engine can generate is greatly diminished. The engine is not able to generate as much cylinder fill as it could have had the air/fuel charge been allowed to build velocity and jump start cylinder fill before TDC.
Taking a look at what is occurring between TDC on the intake stroke and TDC on the compression stroke, the depressions in the cylinder and intake port are much greater. The higher the piston speed the greater the depression that is created in the cylinder. Note that at this higher piston speed and engine RPM, the IVC event is actually more appropriate. This is a good visual to show that the higher the piston speed and/or engine RPM the later the intake valve needs to close to be optimum.
What's most telling to me in this diagram is the cylinder pressure and it may of not been apparent to you at first until I mentioned it. Even though the depressions are higher in this diagram than they were in the first diagram and even though the IVC event is more appropriate the cylinder pressure is much lower than the first diagram. Cylinder pressure is a direct by product of trapped air mass, and the more air mass that is trapped in the cylinder the higher the cylinder pressure rises. The more cylinder pressure that is created the more torque is created as a by product. The more torque that is created the more HP increases.
Look at the cylinder pressure in the first diagram in my last post right before BDC on the intake stroke. It's already above atmospheric pressure. Now look at cylinder pressure in the last diagram at the same piston position. It's not even at atmospheric yet! Not until nearly 75* ABDC does cylinder pressure reach atmospheric. This shows a huge loss in trapped air mass from the engine being out of tune. If this doesn't show you guys just how important overlap is, I don't know what will! Utilizing overlap to it's greatest benefit while the engine is in tune allows cylinder fill to reach a level it never could of been reached had it not otherwise been utilized. This also should show just how important having the proper header and intake manifold on your engine is for the operating range that it's being used in.
I have some more diagrams that I will show later that coincide with what I've shown here. I have some diagrams that show the differences in how a stepped header differs from a straight tube header when it comes to pressure in the exhaust port and cylinder. I also have some diagrams from some engines I've reversed engineered in Engine Analyzer Pro and Dynomation.
I hope these visuals were helpful and that they allowed those that didn't or couldn't fully grasp the literary portion of this thread to understand what is occurring during the 4 strokes of an internal combustion piston driven engine.
Legend for this diagram:
Light blue - Intake pressure
Dark blue - Cylinder pressure
Red- Exhaust pressure
Let's again take a look at what is occurring between TDC on the power stroke and TDC on the intake stroke.
In this diagram note that the sonic wave upon EVO is not as sharp or as pointy as it was when the engine was "in tune". Note that the exhaust pressure also does not drop as quickly as it did when the engine was in tune. As a by product cylinder pressure doesn't drop as quickly and note that there is much higher pressure present in the cylinder mid-stroke versus when the engine was in tune. This creates much higher pumping losses as the piston now has to fight against that pressure as it starts to come back to TDC on the exhaust stroke from BDC.
What is happening here is the exhaust primary is either too long or too small of a diameter for the engine to be in tune at this engine speed. The higher the RPM the engine will operate the shorter and larger the primary must become. As you can see the reflected wave resulting from the initial sonic wave is arriving much too late during overlap. As a result cylinder pressure is higher than intake pressure upon the IVO event. This creates reversion and port stall. Note that not until TDC does cylinder pressure drop lower than intake pressure which finally allows air/fuel mixture to start to gain momentum in the intake tract. When this occurs the VE% that the engine can generate is greatly diminished. The engine is not able to generate as much cylinder fill as it could have had the air/fuel charge been allowed to build velocity and jump start cylinder fill before TDC.
Taking a look at what is occurring between TDC on the intake stroke and TDC on the compression stroke, the depressions in the cylinder and intake port are much greater. The higher the piston speed the greater the depression that is created in the cylinder. Note that at this higher piston speed and engine RPM, the IVC event is actually more appropriate. This is a good visual to show that the higher the piston speed and/or engine RPM the later the intake valve needs to close to be optimum.
What's most telling to me in this diagram is the cylinder pressure and it may of not been apparent to you at first until I mentioned it. Even though the depressions are higher in this diagram than they were in the first diagram and even though the IVC event is more appropriate the cylinder pressure is much lower than the first diagram. Cylinder pressure is a direct by product of trapped air mass, and the more air mass that is trapped in the cylinder the higher the cylinder pressure rises. The more cylinder pressure that is created the more torque is created as a by product. The more torque that is created the more HP increases.
Look at the cylinder pressure in the first diagram in my last post right before BDC on the intake stroke. It's already above atmospheric pressure. Now look at cylinder pressure in the last diagram at the same piston position. It's not even at atmospheric yet! Not until nearly 75* ABDC does cylinder pressure reach atmospheric. This shows a huge loss in trapped air mass from the engine being out of tune. If this doesn't show you guys just how important overlap is, I don't know what will! Utilizing overlap to it's greatest benefit while the engine is in tune allows cylinder fill to reach a level it never could of been reached had it not otherwise been utilized. This also should show just how important having the proper header and intake manifold on your engine is for the operating range that it's being used in.
I have some more diagrams that I will show later that coincide with what I've shown here. I have some diagrams that show the differences in how a stepped header differs from a straight tube header when it comes to pressure in the exhaust port and cylinder. I also have some diagrams from some engines I've reversed engineered in Engine Analyzer Pro and Dynomation.
I hope these visuals were helpful and that they allowed those that didn't or couldn't fully grasp the literary portion of this thread to understand what is occurring during the 4 strokes of an internal combustion piston driven engine.
08-12-2014, 03:44 PM
#59
How do the measure the cylinder pressure? It would have to be a hell of a sensor that could handle the pressure and heat of the combustion cycle, yet still accurately measure the relatively delicate vacuum pulses of the intake and exhaust cycles.
Last edited by speedtigger; 08-12-2014 at 03:51 PM.
08-12-2014, 04:01 PM
#60
I guess this is more so theoretical then, and not "actual data" from a LS engine.
I have compiled numerous graphs like these along with pressure volume loop diagrams as well, but have none that I'm aware of that have come from a LS engine. I'm a nerd for stuff like this. I always find I can learn something from studying pressure traces from different engines.