Bending Rods & Blowing Gaskets with Boost or Backpressure

[tensixniner]

To keep from derailing this thread ... http://powerstrokearmy.com/forums/showthread.php?p=727108#post727108

I've copied some of the posts here, hoping someone can explain this to me.

I have read about this many times on many forums and just can't understand how this happens.


Basically two questions ...

1. How does boost or backpressure blow a headgasket?

2. How does boost or back pressure bend a rod?

it was back pressure that did it. we had made some changes to the duty cycle the night before , then nathaniel and i went out hot rodding it to put fuel in it not knowing that tadd had put that tune in. on the dyno pull when it let go it was on a skyward pull to 1200 and started sounding like a slot machine. iirc 85-90 psi .

Did i miss something ... How does backPressure bend a rod?

you missed that in physics class. just think about which way forces are applied. boost applies it one way and bp applies it another. boost was 85-90 , bp was 140 . same way bp can blow a headgasket.

I can see how 140 psi of backPressure will hold valves open.

A rod that can take several thousand psi of combustion pressure being bent with only 140 of back pressure doesn't make sence to me.

Won't boost and backPressure both be trying to push the piston down?

That's what happens. The bp is pushing down and the crank is pushing up and bingo bent rods.

No offence but Its a little more complicated then that.

Anyhow derial over. Is this motor done yet LOL

 

[Fast-6.0]

Backpressure affects cylinder pressure. You are correct that 140psi of backpressure did not bend the connecting rod. The affect of 140psi backpressure on cylinder pressure, the fact that it can't exit the cylinder, puts undo stress on the rod. Excess boost does the same thing, they both affect cylinder pressure, which is what bends the rods. Cylinder pressure in safe applications is over 3000psi.

 

[powerstroked08]

Yes there is a whole lot more to it than what I stated. Obviously timing, pulse width, and boost pressure play a big role in the life expectancy of rods. Sorry for my simplified post my bad.

 

[Ipkyss]

The simple way to look at it is the higher the boost and back pressure. The higher the cylinder pressure will be. More cylinder pressure equals more pressure up on the head gaskets and down on the pistons/rods.