Mountaineer
Comp D's Doe Killer
- Joined
- Apr 13, 2007
- Messages
- 699
Ok here all this time I thought high drive pressure caused hg failure.
Tell us what's up .
Forrest informed us that.....
there was an old wives' tale floating around the boards some time ago that high drive pressure was just as hard on your head gasket as high boost pressure. IE "if you've got 75psi of drive pressure with 50psi of boost, it's just like having 75psi of boost as far as your head gasket is concerned!" this is false
take 50psi of boost... then multiply it by your static compression ratio. call it 16.5:1 for the sake of argument. that's 825psi of pressure at TDC with no fuel/combustion. now add a bunch of fuel and ignite it before TDC and what happens to cylinder pressure as the piston reaches TDC? It goes SKY HIGH! that's where the awesome power these trucks make comes from it pushes the piston down and moves a 7000lb truck forward and a surprising pace!
then that piston comes back up and pushes the exhaust gas against the turbine wheel...
say you have 75psi of drive pressure at 50psi of boost. That's 75psi of pressure between the piston and the turbine on the exhaust stroke... that's it... 75psi... your head gasket has to hold 75psi of pressure... it sees more pressure than that while you're starting your engine.
high drive pressure is a measurement of restriction on the exhaust stroke, and if you're way above 1:1 boost/drive, you're leaving horsepower on the table, but you're not overstressing your head gasket
Tell us what's up .
Forrest informed us that.....
there was an old wives' tale floating around the boards some time ago that high drive pressure was just as hard on your head gasket as high boost pressure. IE "if you've got 75psi of drive pressure with 50psi of boost, it's just like having 75psi of boost as far as your head gasket is concerned!" this is false
take 50psi of boost... then multiply it by your static compression ratio. call it 16.5:1 for the sake of argument. that's 825psi of pressure at TDC with no fuel/combustion. now add a bunch of fuel and ignite it before TDC and what happens to cylinder pressure as the piston reaches TDC? It goes SKY HIGH! that's where the awesome power these trucks make comes from it pushes the piston down and moves a 7000lb truck forward and a surprising pace!
then that piston comes back up and pushes the exhaust gas against the turbine wheel...
say you have 75psi of drive pressure at 50psi of boost. That's 75psi of pressure between the piston and the turbine on the exhaust stroke... that's it... 75psi... your head gasket has to hold 75psi of pressure... it sees more pressure than that while you're starting your engine.
high drive pressure is a measurement of restriction on the exhaust stroke, and if you're way above 1:1 boost/drive, you're leaving horsepower on the table, but you're not overstressing your head gasket
