Heh, you'd probably have to either be a postwhore, REALLY bored, or actually want to help out. I understand if no-one wants to explain it. How does it affect everything? In what circumstances would one want to lower it? Thanks. Just trying to expand my knowlege.

Compression Ratio (epsilon) = (Vh + Vc) / Vc (those should be subscripts)

Vh = volume displaced by piston moving between TDC and BDC
Vc = compression space, the space left at the top of the cylinder above the piston at TDC

I think one may want to lower the compression ratio if the engine has high boost, to avoid detonation of the air/fuel mixture when it is compressed.

You can probably find out here: http://www.howstuffworks.com/category.htm?cat=Auto

great site

Thanks for the help guys. I'll check that site out.

That basic equation accounts for most of it. But but the "area above the piston" is made up of a few things other than just the combustion chamber of the head. It includes dish/dome of the actual piston, the thickness of the headgasket and the area between the cylinder wall and piston, above the last piston ring.

what is our stock compression ratio?

So that's why Forcefed on the 5thgen forum was saying something about a different headgasket to decrease the compression to 8:1. So that helps avoid pinging? Is that the only advantage, or does it change the engine "properties" (for lack of a better word).

I think stock compression is like 11:1 ?? :gotme:

Yes, of course that's all contributes and is included in the "space left at the top of the cylinder above the piston" as I said. Try quoting people accurately in the future.

it's simple
 

really!!!

10:1 ratio means that the volume of Air/Fuel mixture when the piston is at it's lowest position (BDC) will be compressed TEN TIMES smaller it's volume when the piston goes up to it's highest position (TDC).

and of course when air is compressed it heats up, so when ever you increase the compression it heats up more, thus couasing Pre-Ignition (Detonation, ping, knock ect...)of the air/fuel mixture. and vise-versa.

so if you add forced induction (already compressed and hot air) it will amplify the "pre-ignition" symptoms of an engine, so tuners like to lower the compression to help that side effect.

see simple huh?

TDC = Top D(?) Cylinder ? what does the D stand for? Thanks for the info I understand it completely :) Are you going to the dragway next wed? I'm going again and my friend in the SI is going also.





Ok I definately understand all of what you guys were saying. Woo hoo. :greenboun:

and I'm getting impatient. This thing won't send.

Top Dead Center and Bottom Dead Center, referring to the position of the crank and piston during the cycle.

I probably won't be going this Wednesday. The next time will be when my friend finishes some work on his Eclipse, in a week or two. Have fun!

Re: it's simple
 

Ok, so does forced induction include CAIs? No one wants to have this pre-ignition problem, so what does one do to lessen the compression ratio to something like 8:1? For example, if one got an Injen CAI installed (assuming this falls under the forced induction category) would this screw with the ignition timing of your engine? If so, how is this fixed?

Stock compression on a 2k2 is 10.3

Brian

Re: Re: it's simple
 

CAI's or HAI's do not fall in the forced induction category. Forced induction is the use of supercharges or turbos where the air charge is pressurized prior to entering the intake manifold. That increase of pressure also increases the temp. of the air and possibly causing pre-ignition. That is when you would want to look at lowering your compression ratio.

Re: Re: Re: it's simple
 

Thanks for the clarification.

A "Ram Air" intake like the Camaro/Firebird offer raises the intake pressure slightly at speed, providing a few HP boost. I know a few 240SX guys tried it, but haven't seen any Maximas with it.

:laugh: I think WILLSE had a 2000 back when he posted the question.

2000-2001 is 10:1