So, I was wondering to myself "how much more boost would a thicker head gasket actually allow me to run", so I worded the problem as the following:

Assume a "safe" peak cylinder pressure p. Stock compression is 10:1 (GS-R motor). By using a cometic head gasket of thickness .057 in, which yields a 9.38:1 compression ratio, how many more pounds of boost can you run to maintain the same peak cylinder pressure p?

This is how I solved it:

ratio of new compression over old compression: 9.38/10.0 = .938

I will assume that the new peak pressure, befor the addition of more boost, is .938 * p = .938p


So basically, to maintain the same "p", you should divide your current boost by the ratio of the new compression over the old compression:

14.7 + 8 = 22.7psi

Extra boost that can be run: (22.7/.938) - 22.7 = 1.5psi

Is this right?! A *whopping* 1.5 pounds of boost? I don't have any kind of reference that can support or disprove these calculations, but I'm sure I'm missing something fundamental. I'm just tired and confused.... Anyone else care to take a stab at it?


Thanks,
Nick


[Modified by Nikolai, 6:36 AM 7/19/2002]

 

i now have a headache

 

nah, that wouldnt make much sense bc if you go .5 lower compression you can run more then just 1.5psi more safely, not a lot more but somewhat. i think you need like a "x2" in there

 

Yeah, you'd think you can run more boost..........But by changing the new compression figure from 9.38:1 to 8.6:1, you can run like 4.5 pounds more of boost.

I'm going to write a java applet for this, unless someone can show me that I'm wrong.......Please, show me I'm wrong, 'cause I don't want to write a bunch of wrong code, you know?!

Thanks,
Nick

 

And when do I lose the big stupid "Newbie" smiley face, where I can only make 5 posts a day?! (

 

It is encouraging that somebody here is still pursuing the lost art of mathematics. Unfortunately, your algorithim is flawed. Even if the relationship you have presented between static compression ratio, boost level, and peak cylinder pressure is correct (and it is not), it assumes that all of the other variables have remained constant. This is not possible. There are many other factors which will change once boost is increased..

Just one example (as I am tired): Intake charge temperatures.. It is impossible to hold all other variables constant and not observe an increase in IAT with a boost increase.

If you are truly interested in documented relationships between these criteria, I suggest you spend more time reading books.. Here is a good one:

"Engines, An Introduction" by John L. Lumley

Don't let the title fool you, it has a wealth of great information. E-mail me when you are finished and I will recommend further reading.

BTW, This is not intended to be a reading assignment, it is just my advice to someone who really seems to give a **** about these things... On that note, I just wanted to add that peak cylinder pressure is responsible for Tq/HP production, and isn't nearly as critical if the curve is smooth. What is important to watch for are spikes in cylinder pressure (detonation) which occur from improper fueling, improper ignition timing, and improper fuel selection among other things.

Good luck with the research.


[Modified by B20C5 Turbo, 12:31 AM 7/20/2002]