Compression Ratio

[Spoolitup]

I would like to know what the relationship between air/fuel mixture and compression ratio is. also why does detonation occur and when it does y r there head gasket issues. on the same note i was talking to a friend who put a supercharger on his already tricked out motor and said that he had to de-tune it because the head was lifting off of the block. y did this happen
any help would be greatly appreciated

[MagicRat]

This should be in the technical section and a mod may move it there later.
Here goes:
Detonation is often caused by excessive cylinder pressure at the point of ignition. The air fuel mixture explodes violently rather than being burned.
An increase in compression ratio can increase cylinder pressure

A supercharger or a turbo can increase cylinder pressure as well because it stuffs more air into an engine than a non supercharged engine.

Uusually turbo or supercharged engines have lower compression ratios than a normal engine, to make sure the cylinder pressure does not get too high.
Your buddy would have to reduce the compression ratio of the engine after the supercharger was bolted on. This is done by increasing the volume of the combustion chamber, that is, one makes more room in the cylinder when the piston is all the way up. One can do this by dished pistons, or cylinder heads with a larger chamber, or...sometimes, by a thicker head gasket.

Finally, a leaner air fuel mixture (more air and less fuel) is somewhat more likely to detonate, all other things being equal.

Class dismissed.

[-Josh-]

errr...once again i get beaten to the punch, but you forgot to mention that the stoichiometric compression ratio that's desired is 14.7:1
Not that it matters, you already answered his question..

[MagicRat]

Originally Posted by Z28Josh

errr...once again i get beaten to the punch, but you forgot to mention that the stoichiometric compression ratio that's desired is 14.7:1
Not that it matters, you already answered his question..

Not to sound like a smart ass or anything but that ratio you mention is the ideal fuel to air ratio, (as measured by mass, not volume)

A compression ratio is the relative proportion of the combustion chamber volume with the piston at bottom dead centre and at top dead centre.

[silver_chevelle]

sorry but im gonna go ahead and agree with magic on this one...couild not have said it better myself.

btw. long post, too long 4 me 2 read.
the cyl pressure he was talkint about wants to go somewhere...hopefully out of the valve but if not it will push the head gasket (blowing) out of the way and now you have a possibly ruined block or head. "great!"

btw...you can safely run high compression, higher octane. Hi octain burns slower (and colder) pretty simple stuff. once its explained

[Murray B.]

Originally Posted by Spoolitup

...relationship between air/fuel mixture and compression ratio is. also why does detonation occur ... put a supercharger on his already tricked out motor and said that he had to de-tune it because the head was lifting off of the block. any help would be greatly appreciated

As far as I know there is no direct relationship between fuel/air mixture and compression although I understand that higer compression engines can run slightly leaner. Compressing a gas raises it temperature according to the relationship PV=nRT. In a diesel, with 18:1 compression, the air gets so hot that the fuel lights as soon as it is injected. A high compression engine is more efficient and produces more power (for less fuel). A Chevy 454 with 8.5:1 compression produced about 220 H.P. but at 13.5:1 compression it produced about 450 H.P. Blowing the high compression motor probably would not double horsepower again but just blow the heads off. Those 2000 H.P. dragster motors are very special beasts and such outputs are impossible for regular cars.