Engine/Transmission (1998.5 - 2002) Can Somebody Explain Drive Pressures & Measurement?

[HeavyHauler]

What I meant by "exponentially" - is your ratio going to increase as your actual boost pressure increases. Meaning, can you have a ratio of 1:1 at 10psi, then 1. 3:1 at 20 psi, and something like 2:1 at 30 psi? It sounds like your answer confirms this.



That's why I asked the question about drive pressures in daily drivers not being an issue.



So if we're running high boost pressures with the stock headgasket, should we really be more concerned with drive pressures than actual boost pressure? That's to say, if I'm running 45psi of boost and my drive pressure ratio at 45psi is 1. 12:1, then am I gonna blow a head gasket? (assuming that it will blow at exactly 50psi)



I hope I made that clear, and please speak up if I'm not making sense - I might be way off base with my thinking.

 

[RustyJC]

Other than head gasket failure, are they any other problems that can be caused by high drive pressure ratios?

Loss of potential BHP due to high pumping losses on the exhaust stroke. The piston is having to push the exhaust gases out into the exhaust manifold against the drive pressure in the manifold. The higher the drive pressure, the more BHP it takes to shove the gases out into the exhaust manifold.



Rusty

 

[nps]

So if we're running high boost pressures with the stock headgasket, should we really be more concerned with drive pressures than actual boost pressure? That's to say, if I'm running 45psi of boost and my drive pressure ratio at 45psi is 1. 12:1, then am I gonna blow a head gasket? (assuming that it will blow at exactly 50psi)

You should be worried about both, however boost is more of a concern for the head gasket because it adds to the cylinder pressure on the compression stroke - more boost and more cylinder pressure before the piston starts its upward travel. Drive pressure is not subject to cylinder compression.

 

[Cooker]

So if we're running high boost pressures with the stock headgasket, should we really be more concerned with drive pressures than actual boost pressure?

Let's not foget that EGTs can also alter the ability of the head gasket to hold up.