turbo cam
08-15-2010, 08:31 AM
#42
Im having a little bit of a difficult time understanding the concept of a "turbo cam"
and here is my thinking. if you add 14.7 psi to an engine, lets say you theoretically double the output.
so, if the engine is making 400hp at 5500 rpms NA, then it would make 800hp at 5500rpms at 14.7 psi boost.
so if you install a cam that makes only 380hp at 5500rpms NA, then the turbo engine will make 760hp at 5500rpms at 14.7psi of boost.
so, in my mind, the turbo follows the NA curve and a good NA cam will also be a good turbo cam.
someone fill in the gaps for me.
and here is my thinking. if you add 14.7 psi to an engine, lets say you theoretically double the output.
so, if the engine is making 400hp at 5500 rpms NA, then it would make 800hp at 5500rpms at 14.7 psi boost.
so if you install a cam that makes only 380hp at 5500rpms NA, then the turbo engine will make 760hp at 5500rpms at 14.7psi of boost.
so, in my mind, the turbo follows the NA curve and a good NA cam will also be a good turbo cam.
someone fill in the gaps for me.
I bigger turbo will flow more air at same boost level. So 75lb/min turbo might make 500 hp at say 15psi and 50lb/min turbo might need 25psi to make 500 hp and be maxed out there.
More efficient the engine in areas of cam,heads, intake manifold,exhaust,compression, the lower the boost needed to make same power. So on a very efficient setup might get 500 hp with that 75lb/min turbo at 10psi.
And for cam more important it seems to choose as said for what you like , lope or no lope,cam surge or no cam surge,high lift is harder on valve springs.
Wilder cams are harder to tune, powerband required..
Turbos can compensate for cams, heads pretty well.
08-15-2010, 08:45 AM
#43
Double the pressure = double the power if the pressure didn't cost any power but it does. With a blower, you pay in terms of parasitic drag. With a turbo you have an exhaust restriction. Cam design for these 2 methods of building boost must obviously be different. On a blower car, I would use something similar to a nitrous cam. Overlap is ok because there should be no exhaust restriction meaning that the scavenging principles still work to help draw out the spent combustion gases. With a turbo application you can have 2 to 3x's the pressure on the back of the exhaust valve as you do on the back of the intake valve. This changes everything.
08-15-2010, 09:25 AM
#44
Lot of false info in this thread.
For starters. . . turbo's don't blow boost/fuel out the exhaust during overlap.
Secondly, boost is NOT a measure of restriction.
I wish people would quit spreading this stuff around. Next thing I know, someone will say that all FI cams should be on a 114 LSA... Ugh.
For starters. . . turbo's don't blow boost/fuel out the exhaust during overlap.
Secondly, boost is NOT a measure of restriction.
I wish people would quit spreading this stuff around. Next thing I know, someone will say that all FI cams should be on a 114 LSA... Ugh.
08-15-2010, 10:28 AM
#45
Lot of false info in this thread.
For starters. . . turbo's don't blow boost/fuel out the exhaust during overlap.
Secondly, boost is NOT a measure of restriction.
I wish people would quit spreading this stuff around. Next thing I know, someone will say that all FI cams should be on a 114 LSA... Ugh.
For starters. . . turbo's don't blow boost/fuel out the exhaust during overlap.
Secondly, boost is NOT a measure of restriction.
I wish people would quit spreading this stuff around. Next thing I know, someone will say that all FI cams should be on a 114 LSA... Ugh.
08-15-2010, 03:26 PM
#46
I was told by a very educated Cam / Airflow expert that my N/A cam I have now will work great for a turbo setup. Here is what he said when I asked him about ordering a new cam, he could have easily sold me a $400 + custom cam but recommended my current cam:
"Actually... I would run that cam with the turbo. As much as the people on the net want to talk about the turbo pretty much just makes the cam a non factor in the motor other than making it a good air pump to feed the turbine, and the best way to do that is make the cam as strong as possible NA in the RPM range you are going to run the motor, so you are pretty much all ready there!"
I'm running a 224 cam with a tight LSA and it has a big split, 10 degrees of overlap @ .050. If you check out my dyno results my 346 makes more power than most people through the entire curve, 365 ftlbs @ 3000 rpm and 400 ftlbs @ 3800 rpm while pulling clean to 6800 rpm. I will know shortly how this works.
"Actually... I would run that cam with the turbo. As much as the people on the net want to talk about the turbo pretty much just makes the cam a non factor in the motor other than making it a good air pump to feed the turbine, and the best way to do that is make the cam as strong as possible NA in the RPM range you are going to run the motor, so you are pretty much all ready there!"
I'm running a 224 cam with a tight LSA and it has a big split, 10 degrees of overlap @ .050. If you check out my dyno results my 346 makes more power than most people through the entire curve, 365 ftlbs @ 3000 rpm and 400 ftlbs @ 3800 rpm while pulling clean to 6800 rpm. I will know shortly how this works.
08-15-2010, 06:04 PM
#47
1. Turbo's don't blow boost out the exhaust during overlap because the overwhelming majority of turbo combo's have more exhaust pressure than intake pressure. So, the opposite would happen. Exhaust would go into the intake port. I haven't even been able to convince myself this is so bad, either, when you consider what it does to your e/i pressure difference, and how it could reduce pumping losses, plus a couple of other factors.
2. Boost isn't a measure of restriction because you should have near or above boost pressure in the cylinder when the intake valve closes on the compression stroke. If your NA motor had 105% VE, then you have about 15.4 psia in the cylinder at IVC. If you boost it to, say, 10 psi (24.7 psia), then you'd have about 26 psi in the cylinder, or 11.3 psig. If boost were restriction, then you could remove all restriction and get zero boost. But if you have zero boost, then you have zero power increase also.
3. The 114 LSA thing is soooo overplayed. Keep in mind that the stock Buick 3.8 turbo cam is on a 106 and the aftermarket ones rarely go over 110 and they make gobs of power and work well even over 30 psi.
Mike
2. Boost isn't a measure of restriction because you should have near or above boost pressure in the cylinder when the intake valve closes on the compression stroke. If your NA motor had 105% VE, then you have about 15.4 psia in the cylinder at IVC. If you boost it to, say, 10 psi (24.7 psia), then you'd have about 26 psi in the cylinder, or 11.3 psig. If boost were restriction, then you could remove all restriction and get zero boost. But if you have zero boost, then you have zero power increase also.
3. The 114 LSA thing is soooo overplayed. Keep in mind that the stock Buick 3.8 turbo cam is on a 106 and the aftermarket ones rarely go over 110 and they make gobs of power and work well even over 30 psi.
Mike
08-15-2010, 06:21 PM
#49
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1. Turbo's don't blow boost out the exhaust during overlap because the overwhelming majority of turbo combo's have more exhaust pressure than intake pressure. So, the opposite would happen. Exhaust would go into the intake port. I haven't even been able to convince myself this is so bad, either, when you consider what it does to your e/i pressure difference, and how it could reduce pumping losses, plus a couple of other factors.
2. Boost isn't a measure of restriction because you should have near or above boost pressure in the cylinder when the intake valve closes on the compression stroke. If your NA motor had 105% VE, then you have about 15.4 psia in the cylinder at IVC. If you boost it to, say, 10 psi (24.7 psia), then you'd have about 26 psi in the cylinder, or 11.3 psig. If boost were restriction, then you could remove all restriction and get zero boost. But if you have zero boost, then you have zero power increase also.
3. The 114 LSA thing is soooo overplayed. Keep in mind that the stock Buick 3.8 turbo cam is on a 106 and the aftermarket ones rarely go over 110 and they make gobs of power and work well even over 30 psi.
Mike
2. Boost isn't a measure of restriction because you should have near or above boost pressure in the cylinder when the intake valve closes on the compression stroke. If your NA motor had 105% VE, then you have about 15.4 psia in the cylinder at IVC. If you boost it to, say, 10 psi (24.7 psia), then you'd have about 26 psi in the cylinder, or 11.3 psig. If boost were restriction, then you could remove all restriction and get zero boost. But if you have zero boost, then you have zero power increase also.
3. The 114 LSA thing is soooo overplayed. Keep in mind that the stock Buick 3.8 turbo cam is on a 106 and the aftermarket ones rarely go over 110 and they make gobs of power and work well even over 30 psi.
Mike
your first statement doesn't take into account the pressure waves that are in the exhaust and intake runners.It is very possible to get intake presure in the exhaust just as the exhaust can go back into the intake.This can only be studied properly using high end engine simulation such as Optimum Power's Virtual 4 stroke or other such as that.
the second is only true if the intake port and valve are large enough to fill the cylinder to intake manifold pressure before ivc.On larger engine with smaller heads and cams it is very possible to not have complete cylinder filling.In fact most turbo engine have a very low VE while not boosted.Boost is actually the measurment of unused air-that's why as the engine gets larger with the same turbo and same airflow the boost goes down.
lsa doesn't mean anything valve events do.just because the actory buick cam was on a 106 doesn't mean if you add 20 deg of duration to the intake and exhaust lobe you still want to run a 106 lsa.We run some of our race stuff on a 118 and 119 lsa.
08-15-2010, 06:50 PM
#50
Shawn, I can't disagree with most of your post, but. . .
On an NA engine, where the intake and exhaust pressure are close to 0 psig, it's pretty difficult to lose an appreciable amount of air/fuel out the exhaust. So, with 10 to 30 psi of exhaust pressure superimposed, I'd say it would be pretty difficult to bypass the cylinder during overlap.
I still have to disagree with this. Boost is a good thing. The only way you are going to make more power than a well-tuned NA engine (110% VE) with a boosted engine, is to have. . . well. . . boost. The whole goal is to have the greatest mass of air in the cylinder as possible before compression begins. In order for this mass to be at greater than atmospheric pressure, you have to have boost. Now, installing better heads or the like should result in a lower boost number, all things being equal. But, you are improving the VE, meaning more mass in the cylinder and less left in the plenum where boost is measured.
It would be more accurate (though totally impractical) to measure pressure in the cylinder before IVC, rather than the plenum. Then, it would all be clear that you want as much "boost" as possible, and changing heads would increase the number. Then, getting, say, 10 psi "boost" would clearly NOT be indicative of restriction.
An engine should not be considered a "restriction orifice", where pressure forces more mass through a hole. Do away with the orifice, and you get maximum flow and no pressure. Rather, it's a Positive Displacement device, so filling the cylinders is the goal, not forcing mass flow through.
your first statement doesn't take into account the pressure waves that are in the exhaust and intake runners.It is very possible to get intake presure in the exhaust just as the exhaust can go back into the intake.This can only be studied properly using high end engine simulation such as Optimum Power's Virtual 4 stroke or other such as that.
I still have to disagree with this. Boost is a good thing. The only way you are going to make more power than a well-tuned NA engine (110% VE) with a boosted engine, is to have. . . well. . . boost. The whole goal is to have the greatest mass of air in the cylinder as possible before compression begins. In order for this mass to be at greater than atmospheric pressure, you have to have boost. Now, installing better heads or the like should result in a lower boost number, all things being equal. But, you are improving the VE, meaning more mass in the cylinder and less left in the plenum where boost is measured.
It would be more accurate (though totally impractical) to measure pressure in the cylinder before IVC, rather than the plenum. Then, it would all be clear that you want as much "boost" as possible, and changing heads would increase the number. Then, getting, say, 10 psi "boost" would clearly NOT be indicative of restriction.
An engine should not be considered a "restriction orifice", where pressure forces more mass through a hole. Do away with the orifice, and you get maximum flow and no pressure. Rather, it's a Positive Displacement device, so filling the cylinders is the goal, not forcing mass flow through.
08-15-2010, 09:47 PM
#51
1. Turbo's don't blow boost out the exhaust during overlap because the overwhelming majority of turbo combo's have more exhaust pressure than intake pressure. So, the opposite would happen. Exhaust would go into the intake port. I haven't even been able to convince myself this is so bad, either, when you consider what it does to your e/i pressure difference, and how it could reduce pumping losses, plus a couple of other factors.
2. Boost isn't a measure of restriction because you should have near or above boost pressure in the cylinder when the intake valve closes on the compression stroke. If your NA motor had 105% VE, then you have about 15.4 psia in the cylinder at IVC. If you boost it to, say, 10 psi (24.7 psia), then you'd have about 26 psi in the cylinder, or 11.3 psig. If boost were restriction, then you could remove all restriction and get zero boost. But if you have zero boost, then you have zero power increase also.
3. The 114 LSA thing is soooo overplayed. Keep in mind that the stock Buick 3.8 turbo cam is on a 106 and the aftermarket ones rarely go over 110 and they make gobs of power and work well even over 30 psi.
Mike
2. Boost isn't a measure of restriction because you should have near or above boost pressure in the cylinder when the intake valve closes on the compression stroke. If your NA motor had 105% VE, then you have about 15.4 psia in the cylinder at IVC. If you boost it to, say, 10 psi (24.7 psia), then you'd have about 26 psi in the cylinder, or 11.3 psig. If boost were restriction, then you could remove all restriction and get zero boost. But if you have zero boost, then you have zero power increase also.
3. The 114 LSA thing is soooo overplayed. Keep in mind that the stock Buick 3.8 turbo cam is on a 106 and the aftermarket ones rarely go over 110 and they make gobs of power and work well even over 30 psi.
Mike
so here is my question, would the best NA cam for a particular combo(rpm, gear ratio, etc) also inevitably be the best "turbo cam" for that combo?
or could there be changes made to an optimal NA cam that would create more power in a boosted situation than said NA cam?
08-16-2010, 06:20 AM
#52
...so here is my question, would the best NA cam for a particular combo(rpm, gear ratio, etc) also inevitably be the best "turbo cam" for that combo?
or could there be changes made to an optimal NA cam that would create more power in a boosted situation than said NA cam?
or could there be changes made to an optimal NA cam that would create more power in a boosted situation than said NA cam?
If you design a very well-matched turbo combo and manage to get the same exhaust pressure as boost. Say, it's 15 psi boost and 15 psi exhaust pressure. In this case, you would cam it optimally for NA and it would make max power on boost too. The engine itself would essentially just know that it's running under higher atmospheric pressure. [You MIGHT consider tweaking something to get quicker spool, though, but at the expense of power.]
Now, modify the pressure from there to match your combo, and think about what changes.
For a typical turbo combo, you might have 15 psi more exhaust pressure than intake. This is like an NA engine with a really crappy exhaust system. I've been told that you would generally want more exhaust duration in this case, to better evacuate the cylinder. My personal "turbo cam" was 224/236. I tried a 226/226 and lost power. My pressures were 17 psi boost and 34 psi exhaust pressure.
For a typical SC combo, you might have the same 15 psi on the intake side, but drop to near 0 on the exhaust. In this case, the exhaust side is in much better shape than the scenarios above. To me, this doesn't necessarily mean you need more exhaust duration, like most people say. It does, though, mean you should limit overlap to prevent bypassing the cylinder. As I mentioned above, bypassing the chamber can be done on NA, so you bet it could happen with 15 psi motive pressure pushing air through during overlap.
I don't claim to know all the answers on cam selection, and my experience is on the small end of the spectrum. However, I haven't seen very much dyno info on camshafts in FI. Most everything you read is theories. . . and many of those have errors.
08-16-2010, 07:00 AM
#53
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Shawn, I can't disagree with most of your post, but. . .
On an NA engine, where the intake and exhaust pressure are close to 0 psig, it's pretty difficult to lose an appreciable amount of air/fuel out the exhaust. So, with 10 to 30 psi of exhaust pressure superimposed, I'd say it would be pretty difficult to bypass the cylinder during overlap.
I still have to disagree with this. Boost is a good thing. The only way you are going to make more power than a well-tuned NA engine (110% VE) with a boosted engine, is to have. . . well. . . boost. The whole goal is to have the greatest mass of air in the cylinder as possible before compression begins. In order for this mass to be at greater than atmospheric pressure, you have to have boost. Now, installing better heads or the like should result in a lower boost number, all things being equal. But, you are improving the VE, meaning more mass in the cylinder and less left in the plenum where boost is measured.
It would be more accurate (though totally impractical) to measure pressure in the cylinder before IVC, rather than the plenum. Then, it would all be clear that you want as much "boost" as possible, and changing heads would increase the number. Then, getting, say, 10 psi "boost" would clearly NOT be indicative of restriction.
An engine should not be considered a "restriction orifice", where pressure forces more mass through a hole. Do away with the orifice, and you get maximum flow and no pressure. Rather, it's a Positive Displacement device, so filling the cylinders is the goal, not forcing mass flow through.
On an NA engine, where the intake and exhaust pressure are close to 0 psig, it's pretty difficult to lose an appreciable amount of air/fuel out the exhaust. So, with 10 to 30 psi of exhaust pressure superimposed, I'd say it would be pretty difficult to bypass the cylinder during overlap.
I still have to disagree with this. Boost is a good thing. The only way you are going to make more power than a well-tuned NA engine (110% VE) with a boosted engine, is to have. . . well. . . boost. The whole goal is to have the greatest mass of air in the cylinder as possible before compression begins. In order for this mass to be at greater than atmospheric pressure, you have to have boost. Now, installing better heads or the like should result in a lower boost number, all things being equal. But, you are improving the VE, meaning more mass in the cylinder and less left in the plenum where boost is measured.
It would be more accurate (though totally impractical) to measure pressure in the cylinder before IVC, rather than the plenum. Then, it would all be clear that you want as much "boost" as possible, and changing heads would increase the number. Then, getting, say, 10 psi "boost" would clearly NOT be indicative of restriction.
An engine should not be considered a "restriction orifice", where pressure forces more mass through a hole. Do away with the orifice, and you get maximum flow and no pressure. Rather, it's a Positive Displacement device, so filling the cylinders is the goal, not forcing mass flow through.
your looking at the exhaust pressure as though it's a constant.We know though testing and simulation that this is not true.When the exhaust valve opens you get a pressure drop and a pressure wave that reflects back from the collector-these waves cause low pressure areas in the exhaust.
08-16-2010, 07:56 AM
#54
your looking at the exhaust pressure as though it's a constant.We know though testing and simulation that this is not true.When the exhaust valve opens you get a pressure drop and a pressure wave that reflects back from the collector-these waves cause low pressure areas in the exhaust.
Obviously, advanced engine simulation software or better yet, extensive dyno testing, are better than hypothesizing about the average pressures. But, the vast majority of us don't have these tools at our disposal.
Last edited by engineermike; 08-16-2010 at 08:26 AM.
08-16-2010, 08:03 AM
#55
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Shawn, I can't disagree with most of your post, but. . .
On an NA engine, where the intake and exhaust pressure are close to 0 psig, it's pretty difficult to lose an appreciable amount of air/fuel out the exhaust. So, with 10 to 30 psi of exhaust pressure superimposed, I'd say it would be pretty difficult to bypass the cylinder during overlap.
I still have to disagree with this. Boost is a good thing. The only way you are going to make more power than a well-tuned NA engine (110% VE) with a boosted engine, is to have. . . well. . . boost. The whole goal is to have the greatest mass of air in the cylinder as possible before compression begins. In order for this mass to be at greater than atmospheric pressure, you have to have boost. Now, installing better heads or the like should result in a lower boost number, all things being equal. But, you are improving the VE, meaning more mass in the cylinder and less left in the plenum where boost is measured.
It would be more accurate (though totally impractical) to measure pressure in the cylinder before IVC, rather than the plenum. Then, it would all be clear that you want as much "boost" as possible, and changing heads would increase the number. Then, getting, say, 10 psi "boost" would clearly NOT be indicative of restriction.
An engine should not be considered a "restriction orifice", where pressure forces more mass through a hole. Do away with the orifice, and you get maximum flow and no pressure. Rather, it's a Positive Displacement device, so filling the cylinders is the goal, not forcing mass flow through.
On an NA engine, where the intake and exhaust pressure are close to 0 psig, it's pretty difficult to lose an appreciable amount of air/fuel out the exhaust. So, with 10 to 30 psi of exhaust pressure superimposed, I'd say it would be pretty difficult to bypass the cylinder during overlap.
I still have to disagree with this. Boost is a good thing. The only way you are going to make more power than a well-tuned NA engine (110% VE) with a boosted engine, is to have. . . well. . . boost. The whole goal is to have the greatest mass of air in the cylinder as possible before compression begins. In order for this mass to be at greater than atmospheric pressure, you have to have boost. Now, installing better heads or the like should result in a lower boost number, all things being equal. But, you are improving the VE, meaning more mass in the cylinder and less left in the plenum where boost is measured.
It would be more accurate (though totally impractical) to measure pressure in the cylinder before IVC, rather than the plenum. Then, it would all be clear that you want as much "boost" as possible, and changing heads would increase the number. Then, getting, say, 10 psi "boost" would clearly NOT be indicative of restriction.
An engine should not be considered a "restriction orifice", where pressure forces more mass through a hole. Do away with the orifice, and you get maximum flow and no pressure. Rather, it's a Positive Displacement device, so filling the cylinders is the goal, not forcing mass flow through.
I like to think about it this way...
On cycle n, the engine consumes a certain amount of air...the turbo is providing more air than the engine can consume, this is what creates the positive pressure in the intake...more air provided than the engine can take in.
now on the next cycle (n+1). There is a certain pressure in the intake, when the valve opens more air than can normally be consumed at atmospheric is taken in, but once again the turbo is still providing enough excess air to provide a constant pressure (determined by the Wastegate).
so yes...boost is a good thing, but still is a measure of unused air...
this is obviously simplified and do not take the dynamic components of pressure waves into account...
08-16-2010, 08:14 AM
#56
Let's think about it this way for a sec. . .
If you design a very well-matched turbo combo and manage to get the same exhaust pressure as boost. Say, it's 15 psi boost and 15 psi exhaust pressure. In this case, you would cam it optimally for NA and it would make max power on boost too. The engine itself would essentially just know that it's running under higher atmospheric pressure. [You MIGHT consider tweaking something to get quicker spool, though, but at the expense of power.]
Now, modify the pressure from there to match your combo, and think about what changes.
For a typical turbo combo, you might have 15 psi more exhaust pressure than intake. This is like an NA engine with a really crappy exhaust system. I've been told that you would generally want more exhaust duration in this case, to better evacuate the cylinder. My personal "turbo cam" was 224/236. I tried a 226/226 and lost power. My pressures were 17 psi boost and 34 psi exhaust pressure.
For a typical SC combo, you might have the same 15 psi on the intake side, but drop to near 0 on the exhaust. In this case, the exhaust side is in much better shape than the scenarios above. To me, this doesn't necessarily mean you need more exhaust duration, like most people say. It does, though, mean you should limit overlap to prevent bypassing the cylinder. As I mentioned above, bypassing the chamber can be done on NA, so you bet it could happen with 15 psi motive pressure pushing air through during overlap.
I don't claim to know all the answers on cam selection, and my experience is on the small end of the spectrum. However, I haven't seen very much dyno info on camshafts in FI. Most everything you read is theories. . . and many of those have errors.
If you design a very well-matched turbo combo and manage to get the same exhaust pressure as boost. Say, it's 15 psi boost and 15 psi exhaust pressure. In this case, you would cam it optimally for NA and it would make max power on boost too. The engine itself would essentially just know that it's running under higher atmospheric pressure. [You MIGHT consider tweaking something to get quicker spool, though, but at the expense of power.]
Now, modify the pressure from there to match your combo, and think about what changes.
For a typical turbo combo, you might have 15 psi more exhaust pressure than intake. This is like an NA engine with a really crappy exhaust system. I've been told that you would generally want more exhaust duration in this case, to better evacuate the cylinder. My personal "turbo cam" was 224/236. I tried a 226/226 and lost power. My pressures were 17 psi boost and 34 psi exhaust pressure.
For a typical SC combo, you might have the same 15 psi on the intake side, but drop to near 0 on the exhaust. In this case, the exhaust side is in much better shape than the scenarios above. To me, this doesn't necessarily mean you need more exhaust duration, like most people say. It does, though, mean you should limit overlap to prevent bypassing the cylinder. As I mentioned above, bypassing the chamber can be done on NA, so you bet it could happen with 15 psi motive pressure pushing air through during overlap.
I don't claim to know all the answers on cam selection, and my experience is on the small end of the spectrum. However, I haven't seen very much dyno info on camshafts in FI. Most everything you read is theories. . . and many of those have errors.
thats a good way to think about it, though in your case the cam that made more power was the one closer to normal NA cam specs.
on the backpressure issue, my thinking is that correct turbine size selection,A/R, and downpipe size are the main factors when it comes to backpressure.
so, if those were optimized wouldnt a "turbo cam" become a moot point?
08-16-2010, 08:17 AM
#57
I like to think about it this way...
On cycle n, the engine consumes a certain amount of air...the turbo is providing more air than the engine can consume, this is what creates the positive pressure in the intake...more air provided than the engine can take in.
now on the next cycle (n+1). There is a certain pressure in the intake, when the valve opens more air than can normally be consumed at atmospheric is taken in, but once again the turbo is still providing enough excess air to provide a constant pressure (determined by the Wastegate).
so yes...boost is a good thing, but still is a measure of unused air...
this is obviously simplified and do not take the dynamic components of pressure waves into account...
On cycle n, the engine consumes a certain amount of air...the turbo is providing more air than the engine can consume, this is what creates the positive pressure in the intake...more air provided than the engine can take in.
now on the next cycle (n+1). There is a certain pressure in the intake, when the valve opens more air than can normally be consumed at atmospheric is taken in, but once again the turbo is still providing enough excess air to provide a constant pressure (determined by the Wastegate).
so yes...boost is a good thing, but still is a measure of unused air...
this is obviously simplified and do not take the dynamic components of pressure waves into account...
08-16-2010, 08:21 AM
#58
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fireball, Im confused on this. maybe you can help me out. I thought that if the turbo was supplying more air than the engine could take in at a certain rpm and VE ratio, thats when a surge situation exists, ie putting more air than the engine can consume under positive boost.
surge happens when you fall to the left side of a compressor map (surge line)
08-16-2010, 08:25 AM
#59
Something else to mind, is that the less of a DIFFERENCE between intake and exhaust pressure, the less you need an FI specific cam.
Some stock exhaust systems can impose 10+ psi of exhaust pressure, at nearly 0 psi on the intake side. So, you could say anything less than 10 psi difference falls in the realm of NA. That said, the lower boost numbers shouldn't require anything special. Folks drop $4000 on a supercharger system and think they need an SC cam to run 5 psi boost.
Some stock exhaust systems can impose 10+ psi of exhaust pressure, at nearly 0 psi on the intake side. So, you could say anything less than 10 psi difference falls in the realm of NA. That said, the lower boost numbers shouldn't require anything special. Folks drop $4000 on a supercharger system and think they need an SC cam to run 5 psi boost.
08-16-2010, 08:32 AM
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very intelligent conversation here guys, i applaud you for keeping it calm and collective. Please do continue, i believe ive learned more in the last couple days then ive learned in a long time here.
thanks. lol
thanks. lol