Join Date: Jan 2009
Thanked 64 Times in 60 Posts
Well what about this though, just for instance. I'm not an engineer so I could be missing something here, but see if you can follow my reasoning. If you are forcing Air into the combustion chamber at say 45psi, and the valve closes at the bottom of the piston stroke. You are have a given volume at 45psi. Now the piston comes up and compresses that volume at a ration of 16.3:1 in an ETC or 17:1 in an ETH So you're essentially coming up with a cylinder pressure of 733.5 psi which is more than double that of a stock truck wastegated at 18-22psi. Then add to that the injection and combustion event, and your cylinder pressures sky rocket because of the expanding gases that are driving the piston. This is why advancing the timing too much is detrimental to head gaskets. When advanced, the majority of the injection event occurs closer to TDC when cylinder pressures are highest, and the combustion occurs when the compression is higher therefore increasing the cylinder pressures even more. The end result is peak pressure in the thousands of PSI range or higher. Which makes 90 psi seem inconsequential. Also, I don't see what the exhaust would be pushing off of to cause the head to lift, its all captured within the head and the manifold. So I don't see how it could really has any effect on the head gasket. Just As the pressurized air in the intake manifold doesn't have an effect on it either until it enters the cylinder.
The major detrimental effect I can see of a high drive pressure ratio, is a large pressure gradient across the shaft of the turbo, putting it up against stresses that it wasn't designed to with stand and also over speeding. I don't have an answer as to why the head gaskets tend to fail on the exhaust side? My best guess would be because of excessive heat on the side because the exhaust manifold is much hotter than the intake manifold, and excessive heat is just another element of stress to throw into the mix on that side.
I don't think you sound like a D*** and no hard feelings, this is just what makes sense to me, but if there is something I am missing that causes exhaust manifold pressure to cause failures, definitely share, I'm interested.
2000 2500 reg. cab 4x4 auto 577hp
09 6.7L QC LB stick, 4x4.
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