Compression Ratios.....

ok i know this may be a noob question but when i read that the compression ratio for a motor is 10.6:1....what does each of the numbers stand for? all i know about this is that lower compression is better for boost....is 10.6:1 good for turbo?

The first # is the chamber volume at BDC, and the 2nd # is the volume at TDC. So the high value the 1st # is the more compressed the air fuel mixture becomes and the higher the cylinder pressure and more complete burn= more hp :icon16:

That is a simplified over view, to go into detail would be way too long for me to type.

And the ideal CR for boost is 8.5:1 for real big boost and around 9.0:1 -9.5:1 for smaller boost.

whtteg beat me to it. I'd go with 9.0:1 myself. The reasoning for lower compression is that a turbo is forcing more air into your cylinders and more air means more fuel, so theres more actual air/fuel mixture in your cylinder. Its taking up more space.

So0o0o0o0o high compression pistons are basically made for people running all motor cars ?
Cuz I knew a guy who was sayin that he put in high compression pistons and was running something like 12 - 15 psi...

So0o0o0o0o high compression pistons are basically made for people running all motor cars ?
Cuz I knew a guy who was sayin that he put in high compression pistons and was running something like 12 - 15 psi...

Yes and no. Typically all-motor cars will want high CR pistons 12+:1. It is very possible to run that level of boost with a very good tune and race gas. People lower their compression for turbo to allow a tuning margian or error. For example, say you have a high CR and boost combo and you tuned it when the temp was about 70 deg out, well summer rolls around and it becomes 90 deg. If you are still running the same tune, there are going to be big problems. For a daily driven car with a typical street tune, yes you would be better off with 9.0:1, a smaller turbo, and pump gas.

Thanks alot for clearing that up for me(damn I luv this site)...I think the guy was a BS case though

The first # is the chamber volume at BDC, and the 2nd # is the volume at TDC. So the high value the 1st # is the more compressed the air fuel mixture becomes and the higher the cylinder pressure and more complete burn= more hp :icon16:

That is a simplified over view, to go into detail would be way too long for me to type.

And the ideal CR for boost is 8.5:1 for real big boost and around 9.0:1 -9.5:1 for smaller boost.
Here is a simple explanation a third grader could understand.
imagine you have 10.6 glass's of water(BDC), now compress them into one glass(TDC), that is what the numbers mean.

first off i dont understand why turbo is good for ls/vtec cuz of high compression. and also what is the best compression for nitrous? out of these 3.
10.4:1(civic sir b16a)
10.2:1(del sol b16a3)
10.0:1(gsr b18c1)

All those are fine for nitrous and turbo.

LSvtec doesn't nescessarily have a high CR, you can build it how you like, just like any motor.

All those are fine for nitrous and turbo.

LSvtec doesn't nescessarily have a high CR, you can build it how you like, just like any motor.
ya true. but what engine would be better for n20? do you want a bigger number in the middle? like the b16a? or a small one?

what is a good comp. ratio to run turbo and nitrous together?

ya true. but what engine would be better for n20? do you want a bigger number in the middle? like the b16a? or a small one?
The number in the middle is part of the first number. Remeber the glasses of water, 10.5:1 would be ten and a half glasses.

You can run any compression ratio you like. Like it was said, it's just safer to run lower compression. Top fuel cars run in the 20:1 ratio area and they are pushing more boost than any of us ever will. Basically, if you are running 10:1 cr and somone else is pushing 9:1 cr, it takes more boost for them to accomplish the same amount of hp you are making with less boost. None of what I just said makes any sense does it?

You can run any compression ratio you like. Like it was said, it's just safer to run lower compression. Top fuel cars run in the 20:1 ratio area and they are pushing more boost than any of us ever will. Basically, if you are running 10:1 cr and somone else is pushing 9:1 cr, it takes more boost for them to accomplish the same amount of hp you are making with less boost. None of what I just said makes any sense does it?
Sure, you can run whatever you like, but your motor may not like it :)

Sure, you can run whatever you like, but your motor may not like it :)

It's all about tuning.

It's all about tuning.
No amount of tuning is going to allow a 17.5:1 on a street car :P

No amount of tuning is going to allow a 17.5:1 on a street car :P

I never said it would. Where are you gonna get 17.5:1 pistons for a b16 or b18 or any other honda motor? Other than specially made that is. My point is you don't always have to lower compression on a motor to boost it. You can leave a b18c at 11:1 and just get forged internals and it will still be able to hold a good amount of boost. I guess I should have made myself more clear for the more challenged people on the forums.

It was sarcasm,


see the :P?

I never said it would. Where are you gonna get 17.5:1 pistons for a b16 or b18 or any other honda motor? Other than specially made that is. My point is you don't always have to lower compression on a motor to boost it. You can leave a b18c at 11:1 and just get forged internals and it will still be able to hold a good amount of boost. I guess I should have made myself more clear for the more challenged people on the forums.

yes but reliability becomes an issue then. There is no room for error or changing environmental conditions.