my take on drive pressure...

[Forrest Nearing]

I'm posting this in the products/accessories forum because it's not a 1st, 2nd, or 3rd gen thing, it applies to all generations, and it primarily applies to those who have increased performance of their trucks



after driving around with a drive pressure gauge hooked up for a good while and running it w/ different levels of fueling and with different turbos, I am here to say that drive pressure DOES NOT have the same effect on a head gasket as boost. with a stock HX35-12 turbo set at 35psi, drive pressure or TIP (turbine inlet pressure) will peg a 60psi gauge with ease as you get up over 3000rpm.



let's think about this...



at 35psi of manifold pressure, when you compress that air SIXTEEN times smaller or 16. 5, etc. the pressure in the cylinder jumps up there pretty good... 35psi at 16. 5:1 compression is 577. 5psi... 40psi at 16. 5psi is 660psi.



then add a little fuel and fire about 16-18 degrees before TDC and the cylinder pressure jumps WAY up there!



so what is drive pressure? it's the pressure that exists between the piston and the exhaust turbine... so 60psi of drive pressure is 60psi of pressure in the cylinder... probably a little more due to restriction in the cylinder head, but the moral of the story is that it's FAR less pressure than the head gasket sees on the compression stroke with a bone stock engine at WOT



high drive pressure means you're leaving some horsepower on the table, but it doesn't necessarily mean you're going to pop a head gasket.



that's my take on it anyhow

 

[PC12Driver]

I've wondered about that too. Once the intake valve opens, and certainly once the piston starts on the downstroke, it would seem that the cylinder pressure would have equalized to whatever is in the intake side, regardless of the drive pressure.



Maybe the soot/particulate-filled exhaust that remains in the cylinder (a little will get pushed back upstream during valve overlap at TDC) is less compressable than a clean intake charge, resulting in higher pressures at TDC during combustion?

 

[Forrest Nearing]

ooh, good call! I recently witnessed a teardown of an engine with unknown boost:TIP ratio that had a lot of soot in the intake bowls and ports...



1:1 or less is obviously the goal for best performance, but...

 

[WUnderwood]

I think that is one of the best write ups of drive pressure I have seen

 

[ENafziger]

I agree, but...

Great write up Forrest! You've got me thinking...



I agree with the reasoning behind your post. However, the calculations for cylinder pressure are wrong (I was just corrected on this myself, and had to go back to the old Mechanical Engineering books... kindof sad when you've only been out of school 4 years! :-laf ).



It looks like you figured cylinder pressure the same way I initially thought. A boost pressure of 35 psi multiplied by the compression ratio of 16. 5 gives a pressure of around 577 psi. However, it doesn't quite work this way.



The way to calculate cylinder pressure is as follows:



Cylinder Pressure = Boost Pressure * ( Compression Ratio ^ Gamma)





Gamma here is approximated as 1. 4 for air. The value for exhaust gas is 1. 33. You can think of this kindof as the heat capacity of the medium that is being compressed.



So, for the case here, it would be:



Cylinder Pressure = (35 psi) * (16. 5 ^ 1. 4) = 1,772 psi





If you calculate it the way you did (and the way I did at first), only the change in volume is being considered. However, when you compress something, it gets very hot... and hot gas expands. Thus, the "extra" cylinder pressure comes from the heat expansion of the gas.



This website The Diesel Engine explains things a little more technically, but allows you to put in values, and it does the math for you. (It doesn't allow you to put boost pressure though).





I have also heard that high drive pressure is what kills headgaskets, and I always took that to be true. However, your post got me thinking, and I agree with you... I don't see how that drive pressure would have a direct effect on harming a headgasket.



I'm probably missing something, but the only thing I can think of, is that when drive pressure begins to far exceed boost pressure, you are operating the turbo out of it's efficiency range. Thus, you are creating much more heat to achieve the boost you are getting... leading to higher intake air temperatures, and as a result, higher cylinder pressure.



Thanks for sparking this discussion... I'm interested in seeing where it leads. --Eric