LSX is ready to be dropped in tomorrow (PICS!)
05-19-2009, 09:57 PM
#1141
FormerVendor
Helicoil- I don't think you're really reading what I have posted.
You can't just look at it as how much pressure is on the back of the valve, but how much pressure is on both sides of the valve.
When the intake valve opens air flows into the cylinder and the pressure starts to equalize between the intake manifold and cylinder. Like if you connected a full air tank to an empty one, open the valve in the middle and the pressure equalizes.
As the intake valve is closing the piston is chasing it up the cylinder though the cylinder is still filling due to the inertia of the intake charge.
So, at intake valve closing pressure in the cylinder, or on the other side of the valve is not zero, it is close to or even higher than intake manifold pressure at that point, so you cannot subtract the pressure on the backside of the valve without considering the pressure on the top of the valve. There has to be a differential for it to matter and in this case it is not signifigant.
Now, as the intake valve is closing and the piston is chasing it up the bore, pressure in the intake port and cylinder is similar, the very instant the intake valve touches it's seat, while the piston is allready moving upwards, the compression cycle starts and pressure in the cylinder immediately and greatly exceeds intake manifold pressure so the valve is certainly not going to bounce at that point.
Now on from there the valve is closed and stable on it's seat. Through the power stroke, and the exhaust stroke while the intake is on the base circle. Boost pressure at that point is not going to blow the valve off it's seat.
If you can explain to me how at any point in time intake manifold pressure is a contributing factor in valvetrain instability I am all ears.
I understand what you have posted has been a rule of thumb for quite some time but when you break it down into what is actually happening in the engine this rule of thumb does not make sense as it is not taking all the factors into consideration.
You can't just look at it as how much pressure is on the back of the valve, but how much pressure is on both sides of the valve.
When the intake valve opens air flows into the cylinder and the pressure starts to equalize between the intake manifold and cylinder. Like if you connected a full air tank to an empty one, open the valve in the middle and the pressure equalizes.
As the intake valve is closing the piston is chasing it up the cylinder though the cylinder is still filling due to the inertia of the intake charge.
So, at intake valve closing pressure in the cylinder, or on the other side of the valve is not zero, it is close to or even higher than intake manifold pressure at that point, so you cannot subtract the pressure on the backside of the valve without considering the pressure on the top of the valve. There has to be a differential for it to matter and in this case it is not signifigant.
Now, as the intake valve is closing and the piston is chasing it up the bore, pressure in the intake port and cylinder is similar, the very instant the intake valve touches it's seat, while the piston is allready moving upwards, the compression cycle starts and pressure in the cylinder immediately and greatly exceeds intake manifold pressure so the valve is certainly not going to bounce at that point.
Now on from there the valve is closed and stable on it's seat. Through the power stroke, and the exhaust stroke while the intake is on the base circle. Boost pressure at that point is not going to blow the valve off it's seat.
If you can explain to me how at any point in time intake manifold pressure is a contributing factor in valvetrain instability I am all ears.
I understand what you have posted has been a rule of thumb for quite some time but when you break it down into what is actually happening in the engine this rule of thumb does not make sense as it is not taking all the factors into consideration.
Last edited by qqwqeqwrqwqtq; 05-19-2009 at 10:04 PM.
05-21-2009, 04:17 PM
#1142
Helicoil- I don't think you're really reading what I have posted.
You can't just look at it as how much pressure is on the back of the valve, but how much pressure is on both sides of the valve.
When the intake valve opens air flows into the cylinder and the pressure starts to equalize between the intake manifold and cylinder. Like if you connected a full air tank to an empty one, open the valve in the middle and the pressure equalizes.
As the intake valve is closing the piston is chasing it up the cylinder though the cylinder is still filling due to the inertia of the intake charge.
So, at intake valve closing pressure in the cylinder, or on the other side of the valve is not zero, it is close to or even higher than intake manifold pressure at that point, so you cannot subtract the pressure on the backside of the valve without considering the pressure on the top of the valve. There has to be a differential for it to matter and in this case it is not signifigant.
Now, as the intake valve is closing and the piston is chasing it up the bore, pressure in the intake port and cylinder is similar, the very instant the intake valve touches it's seat, while the piston is allready moving upwards, the compression cycle starts and pressure in the cylinder immediately and greatly exceeds intake manifold pressure so the valve is certainly not going to bounce at that point.
Now on from there the valve is closed and stable on it's seat. Through the power stroke, and the exhaust stroke while the intake is on the base circle. Boost pressure at that point is not going to blow the valve off it's seat.
If you can explain to me how at any point in time intake manifold pressure is a contributing factor in valvetrain instability I am all ears.
I understand what you have posted has been a rule of thumb for quite some time but when you break it down into what is actually happening in the engine this rule of thumb does not make sense as it is not taking all the factors into consideration.
You can't just look at it as how much pressure is on the back of the valve, but how much pressure is on both sides of the valve.
When the intake valve opens air flows into the cylinder and the pressure starts to equalize between the intake manifold and cylinder. Like if you connected a full air tank to an empty one, open the valve in the middle and the pressure equalizes.
As the intake valve is closing the piston is chasing it up the cylinder though the cylinder is still filling due to the inertia of the intake charge.
So, at intake valve closing pressure in the cylinder, or on the other side of the valve is not zero, it is close to or even higher than intake manifold pressure at that point, so you cannot subtract the pressure on the backside of the valve without considering the pressure on the top of the valve. There has to be a differential for it to matter and in this case it is not signifigant.
Now, as the intake valve is closing and the piston is chasing it up the bore, pressure in the intake port and cylinder is similar, the very instant the intake valve touches it's seat, while the piston is allready moving upwards, the compression cycle starts and pressure in the cylinder immediately and greatly exceeds intake manifold pressure so the valve is certainly not going to bounce at that point.
Now on from there the valve is closed and stable on it's seat. Through the power stroke, and the exhaust stroke while the intake is on the base circle. Boost pressure at that point is not going to blow the valve off it's seat.
If you can explain to me how at any point in time intake manifold pressure is a contributing factor in valvetrain instability I am all ears.
I understand what you have posted has been a rule of thumb for quite some time but when you break it down into what is actually happening in the engine this rule of thumb does not make sense as it is not taking all the factors into consideration.
I am certainly not saying the valve is going to be opened by boost, I am simply saying it's closing is affected by the pressures behind it (inside the plenum). This is largely due to the dynamics created in the closing ramp of the cam lobe and the speed (frequency) of the valve events. Valvetrain harmonics are bad enough to try and control without the negatives of reduced valve spring pressures in a performance engine.
I comprehend what you are saying completely, but, pressures will only be equal if there is no restriction, look at your example of the air tanks. This is not the case on an engine with an intake valve bowl of say 2.0" (one side), a moving valve head 2.080" (restriction - as it nears the seat), and a 4.0" diameter piston x 5.5" long cylinder (other side).
Pressure will only be generated (or equalized) once the complete cylinder volume has been filled.
How are you determining that there is the same pressure in the cylinder as there is in the plenum before the intake valve seats?
I am not sure I can explain it any clearer, but I can post plenty of pics of 'rattled' valve keepers, galled retainers, and worn keeper grooves in intake valves taken from high boost engines. Although the method I listed is a 'rule of thumb' it has served many well, for many years. Why mess with something that works?
05-21-2009, 06:03 PM
#1143
FormerVendor
If IVC was AT BDC then maybe there would be a large differential across the valve.
Now for example if your IVC is at 45deg abdc that means the cylinder is still filling as the piston is moving upwards in the bore due to inertia. (as with any engine, forced induction or not).
It's because of that inertia, pressure wave, etc that a naturally aspirated engine can see higher than atmospheric pressure in the cylinder. Those same forces exist in a supercharged engine as well, likely even to a greater extent due to much higher port velocity.
Now, at IVC as the piston is speeding up the bore the very instant that valve touches it's seat the piston is compressing the intake charge greatly eclipsing manifold pressure. Pressure in the cylinder must be higher than intake pressure at that point so I see no way possible the valve is going to bounce.
Another example would be very high boost diesel engines. If you calculate their boost pressure and seat pressure with your rule of thumb many are running at a calculated NEGATIVE seat pressure. I don't think they would run very well if that was actually the case.
I've seen plenty of cars come in with valvetrains pounded to death in naturally aspirated cars, so damaged parts isn't really compelling evidence in this case.
I'm not trying to mess with something that works, merely pointing out that the rule of thumb is flawed and not as simple as it's made out to be.
05-21-2009, 11:25 PM
#1144
Because of the IVC point. If there was a huge pressure difference at IVC than that valve event is WAY too early, like 50deg or so.
If IVC was AT BDC then maybe there would be a large differential across the valve.
Now for example if your IVC is at 45deg abdc that means the cylinder is still filling as the piston is moving upwards in the bore due to inertia. (as with any engine, forced induction or not).
It's because of that inertia, pressure wave, etc that a naturally aspirated engine can see higher than atmospheric pressure in the cylinder. Those same forces exist in a supercharged engine as well, likely even to a greater extent due to much higher port velocity.
Now, at IVC as the piston is speeding up the bore the very instant that valve touches it's seat the piston is compressing the intake charge greatly eclipsing manifold pressure. Pressure in the cylinder must be higher than intake pressure at that point so I see no way possible the valve is going to bounce.
Another example would be very high boost diesel engines. If you calculate their boost pressure and seat pressure with your rule of thumb many are running at a calculated NEGATIVE seat pressure. I don't think they would run very well if that was actually the case.
I've seen plenty of cars come in with valvetrains pounded to death in naturally aspirated cars, so damaged parts isn't really compelling evidence in this case.
I'm not trying to mess with something that works, merely pointing out that the rule of thumb is flawed and not as simple as it's made out to be.
If IVC was AT BDC then maybe there would be a large differential across the valve.
Now for example if your IVC is at 45deg abdc that means the cylinder is still filling as the piston is moving upwards in the bore due to inertia. (as with any engine, forced induction or not).
It's because of that inertia, pressure wave, etc that a naturally aspirated engine can see higher than atmospheric pressure in the cylinder. Those same forces exist in a supercharged engine as well, likely even to a greater extent due to much higher port velocity.
Now, at IVC as the piston is speeding up the bore the very instant that valve touches it's seat the piston is compressing the intake charge greatly eclipsing manifold pressure. Pressure in the cylinder must be higher than intake pressure at that point so I see no way possible the valve is going to bounce.
Another example would be very high boost diesel engines. If you calculate their boost pressure and seat pressure with your rule of thumb many are running at a calculated NEGATIVE seat pressure. I don't think they would run very well if that was actually the case.
I've seen plenty of cars come in with valvetrains pounded to death in naturally aspirated cars, so damaged parts isn't really compelling evidence in this case.
I'm not trying to mess with something that works, merely pointing out that the rule of thumb is flawed and not as simple as it's made out to be.
I am going to have to say I disagree on your statement that 'added' valve spring pressures are not needed on supercharged engines exceeding two times atmospheric conditions. I know what works for me and keeps valvetrains happy in my boosted engines. Again, just trying to share a little information and not just saying it because it is 'rule of thumb' for many builders, which it is in fact.
I am also well aware cylinder fill continues well beyond BDC on performance camshafts, my point is there is not FULL manifold pressure in the cylinder until the entire cylinder volume is filled. How can there be? Pressure is a result of volume, and a restriction such as an intake port and a valve reduces volume (in this case, cylinder filling), as well as pressure in the cylinder before compression can be made (valve close)
There is a lot more area in the cylinder than there is across the valve which is never completely out of the way. Valve timing works against pressure equalization, just as your air tank example. Just because you open the valve on the empty air tank does not mean the pressures are equal between both tanks (the full one and the empty one), there is a pressure differential, created by the hose (restriction) connecting the two. Not until all of the empty air tank volume is filled can pressure be made. Even with inertia, or forced air (boost) it does not mean there will be equal pressures in both the cylinder and the intake, or again in your example, the air tanks. The main reason why is time, which is why we choose to keep increasing the time with more race oriented camshafts (longer duration lobes, etc.)
The cylinder of an engine can fill completely due to it being at a lower pressure than manifold pressure (just as in a highly efficient NA combo), however, the cylinder does not see complete manifold pressure, there is a pressure differential which is due to the valve (the restriction) and the valve timing. Can we really prove it otherwise?
My point about knowing the pressure in the cylinder before the intake valve closes was made because I didn't think you had measured these pressures. Obviously, it is not something we can do, or do easily. It is a theory many have, as it is easy to assume that the cylinder is at the same pressure as the manifold. There are many different physic laws which state it differently.
Things are starting to get off track with this guys thread anyway.....as well as the topic at hand. I'll end it by saying you cannot go wrong erroring on the side of higher valve springs pressures provided all other necessities are put in place, such as the right lifters, pushrods, valves, retainers, and keepers. Your engine and your valvetrain will appreciate it. With rufertic sp? running 26 psi and 150 pounds of seat pressure I would bet a dollar he won't get 50 good dragstrip laps without a problem. Not that I am wishing BAD, I just don't see it happening with that much boost and that valvetrain. Although I do hope he gets much more than that for all the heartache experienced in the build. BTW, it turned out great and sounds pretty stout!
05-22-2009, 09:44 AM
#1145
FormerVendor
Well, I am ok with agreeing to disagree.
Basically where I keep trying to go with this is ivc points are reached by finding which one results in the highest power for a particular combination. If with 45-55 abdc ivc their was still a huge pressure differential across the intake valve the cylinder is still filling and massive power gains would be had by closing the intake valve 70-90 deg abdc. Why don't we see that? Because maximum cylinder filling occurs in the normal range of what we would commonly see an ivc point for this application. It's for that reason that pressure in the cylinder -has- to at least be close to manifold pressure at IVC (though IMO is likely equal or higher).
I'll bring up my example again of diesels running at a calculated negative seat pressure as well.
I myself like to run 'higher' spring pressures on most combinations and agree it's better to err on the high side but for reasons other than this rule of thumb.
Getting back to the original post, if I was the one to spec the valvetrain I would have chosen different components and higher spring pressure but I do not believe at this point in time that is the reason for the low #'s so far.
Basically where I keep trying to go with this is ivc points are reached by finding which one results in the highest power for a particular combination. If with 45-55 abdc ivc their was still a huge pressure differential across the intake valve the cylinder is still filling and massive power gains would be had by closing the intake valve 70-90 deg abdc. Why don't we see that? Because maximum cylinder filling occurs in the normal range of what we would commonly see an ivc point for this application. It's for that reason that pressure in the cylinder -has- to at least be close to manifold pressure at IVC (though IMO is likely equal or higher).
I'll bring up my example again of diesels running at a calculated negative seat pressure as well.
I myself like to run 'higher' spring pressures on most combinations and agree it's better to err on the high side but for reasons other than this rule of thumb.
Getting back to the original post, if I was the one to spec the valvetrain I would have chosen different components and higher spring pressure but I do not believe at this point in time that is the reason for the low #'s so far.
Last edited by qqwqeqwrqwqtq; 05-22-2009 at 09:56 AM.
05-22-2009, 08:57 PM
#1146
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Great technical discussion guys.
For the record, my valvetrain is not perfect but was only thrown together to get by temporally with these 317s. I still have to upgrade to six bolt heads and I will be sure to get the best valvetrain for my set-up at that time. I'm ok with it for now as long as it's not causing an immediate problem. I don't plan on running it the way it is for more than 10 passes, let alone 50. This is an ongoing project and just really didn't have the money or patients to wait until I had the money for 100% of the best parts. I basicly just needed to get it up and running for some baseline numbers and to be able to get a little enjoyment out of the car before I lost interest which almot still happened. Now that it's back up and running the excitement is back and I can relax for a little while and start making a list of future upgrades. A few big ones that I know I need are fuel system, converter and heads. Once the set-up is 100%, it will be a lot easier to tell just how well it turns out. For now, I'm just happy it's alive.
For the record, my valvetrain is not perfect but was only thrown together to get by temporally with these 317s. I still have to upgrade to six bolt heads and I will be sure to get the best valvetrain for my set-up at that time. I'm ok with it for now as long as it's not causing an immediate problem. I don't plan on running it the way it is for more than 10 passes, let alone 50. This is an ongoing project and just really didn't have the money or patients to wait until I had the money for 100% of the best parts. I basicly just needed to get it up and running for some baseline numbers and to be able to get a little enjoyment out of the car before I lost interest which almot still happened. Now that it's back up and running the excitement is back and I can relax for a little while and start making a list of future upgrades. A few big ones that I know I need are fuel system, converter and heads. Once the set-up is 100%, it will be a lot easier to tell just how well it turns out. For now, I'm just happy it's alive.
05-27-2009, 07:59 PM
#1147
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Got new numbers in but I made a new thread with a vid and the gragh because this thread ogt out of hand too long lol.
https://ls1tech.com/forums/forced-in...l#post11664832
https://ls1tech.com/forums/forced-in...l#post11664832
05-30-2009, 03:57 PM
#1149
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05-31-2009, 09:33 AM
#1150
Thanks a lot bud! I still can't believe that my build kept people interested enough to read through this many pages, it's like a book, lol. I'm really happy people are enjoying it though. This site is the best and I learned 99% of what I know here so hopefully some people can learn a little from my mistakes and maybe use the info here to help to build thier own. Very cool
