exhaust drive pressure numbers...

[Forrest Nearing]

everyone says high drive pressures are bad for your headgasket because if you've got 60-70psi exhaust pressure, that's like your head gasket seeing 60-70psi of boost...



how?



is it because of the HEAT of the exhaust charge combined w/ the pressure?



because I would imagine that the head gasket sees a LOT more pressure when you take 35psi of boost and compress it SEVENTEEN TIMES... 595psi is a little higher than 60-70psi, ain't it? and I don't know what kind of cylinder pressures are seen when the fuel is injected and starts burning, but the knocking/rattling sound we hear when the shockwave hits the top of the piston tells me it's probably pretty high!



I realize that 1:1 or lower drive pressures help prevent charge dillution (though I've got to imagine that the cams on these motors have negative overlap) and yields higher pwoer numbers than when drive exceeds boost...



but I don't understand the statement that your headgasket sees the same stress from 70psi exhaust pressure as it does from 70psi of boost.



help me out here,



Forrest

 

[Vaughn MacKenzie]

Not sure the answer Forrest, but like you say high temps combined with high pressure = a lot of heat. If you have high drive pressure of say 70psi and say 1200F EGT then you are going to have more thermal soak into the head which could maybe weaken the head gasket on that side. 1200F is 1200F on your pyro, but heat will conduct into your head casting considerably faster if that 1200F is at 70lbs of pressure versus 35lbs.



While cylinder heat during compression and firing is very high, it's fairly momentary, it's not sustained, whereas exhaust heat is.



With a lot of backpressure and heat and it rushing partially back into the cylinder then that may be increasing cylinder temps. Another contributing factor is the engine is working harder to push that air that is up against more restriction.



Vaughn

 

[Forrest Nearing]

roger all that...



so it's more of a "hey, high drive pressures and high EGT's can pop a head gasket just as easily as high boost numbers" thing...



that I can buy



Forrest

 

[lmills]

Another thing that is supposedly able to happen is the excessive backpressure causes the exhaust valves to float if the springs are not stiff enough. The drive pressure is nothing more than bottled up air pressure.