please explain compression ratio

[AaronRVegas]

so i am wondering how to change the compression ratio of an engine.

how does a turbo affect compression ratio?

what needs to be upgraded to raise compression ratio?

does raising the compression ratio cause the engine internals to wear prematurely? does a 8.8:1 cr engine last longer than a 12:1 cr engine?



please do not post if you do now know what you are talking about. i am looking for correct answers from knowledgeable people. not retards saying "jus tighten tha boltz more yo tha raise da cr"

[Slice One]

the compression ratio on a engine can be change in a couple ways, one of the easiest is to mill the head, it will raise it a couple of .'s low compression is best for turbo setup, and i wouldnt say that a high compression engine lasts any less then a low compression engine, because high compression engines are usually built with better internals. another more expensive way to raise the compression is high compression pistons, and would warrent the use of new internals and forged rods.(if you want it to last and want to beat on it)

CR as explained on wikipedia

"Picture a cylinder and its combustion chamber with the piston at the bottom of its stroke containing 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio."

So to answer you question about how a turbo effects a compression ratio , it doesnt, it does what a higher compression ratio would if there were no turbo, makes for a more effecient burning of the fuel.

[AaronRVegas]

well i saw on the site when i searched CR about different pistons. some being 10.5:1 and some being 12:1. i guess its just counterintuitive to me to think that just installing pistons would raise the cr. my brain wants to think that to raise the cr you have to add more air.

what i dont understand is how installing those pistons raises the cr. maybe i am misunderstanding how it works. it seems to me like more air has to be forced in to raise the ratio. or is it the amount of air does not change just that is compressed more?

[chrischhorn]

Higher compression pistons are dome shaped while lower compression pistons tend to have a dish shape. Reason being, the dome shape reduces the amount of volume in the combustion chamber. So for example (just using the number reference from above) with dome shaped pistons, instead of there being a 100cc's left in the combustion chamber, it would reduce it to 80cc due to the dome reducing room for volume. Now you have 1000:80 cc which reduced to a fraction would be 12.5:1 compression. If you go with a dish shaped (lower compression) it would increase the left over volume to say 120cc so it would be 1000:120 = 8.3:1 compression ratio. These numbers are just for reference but to basicly to explain how the pistons and their shape effect the ratio.

[91kid]

The compression ratio of an internal-combustion engine is a value that represents the ratio of the volume of its combustion chamber; from its largest capacity to its smallest capacity.

In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.


Picture a cylinder and its combustion chamber with the piston at the bottom of its stroke containing 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.\

-wikipedia

[bcjammerx]

first thread I made asking about this:
http://www.cb7tuner.com/vbb/showthread.php?t=138665

Followed by this thread with more info on the same topic:
http://www.cb7tuner.com/vbb/showthread.php?t=144501

does that help any?

[AaronRVegas]

chris -- what i am gathering from your post is that compression ratio is just compressing the existing air more. the shape of the piston effecting the volume of air inside the chamber.

91kid - thanks for posting

bcjammer - it does and it does not help. its good info but still a bit confusing to me.


so is it better to run low compression with a turbo or high compression?

i am still wondering if the shape of the piston is the only thing that affect CR? are there any other factors?

[91kid]

The available air inside the cylinder affects the CR so deck height and head volume would play a part in the CR

turbo usually runs a lower compression as the turbo compresses the air before it enters the engine.

[AaronRVegas]

ok that makes a little more sense to me now.

i am pretty noobish in my knowledge of turbos. i thought a turbo was just a fan that blew a higher volume of air. i.e. adding more air means you can add more fuel and produce more heat.

well wouldnt running a higher compression with a turbo make it that much better? maybe there are some other factors i am ignorant to about a turbo. idk. i read stuff in the forced induction section but it just seems that the information is too advanced for me right now.

[wed3k]

too much compression will lead to a potentially leaner condition. and lean condition can ultimately cause pre-ignition or detonation.

with a turbo setup, you basically want the engine to be your solid foundation. you make adjustments with the turbo which is why turbo choice is so critical. it can make or break your setup. start at a low compression so at low or no boost the engine is stable and easy to tune. when boost begins to come on, then you can begin backing off timing and adding fuel.

[AaronRVegas]

too much compression will lead to a potentially leaner condition. and lean condition can ultimately cause pre-ignition or detonation.

with a turbo setup, you basically want the engine to be your solid foundation. you make adjustments with the turbo which is why turbo choice is so critical. it can make or break your setup. start at a low compression so at low or no boost the engine is stable and easy to tune. when boost begins to come on, then you can begin backing off timing and adding fuel.

thanks for the info wed3k

so you are saying that too small or big of a turbo will cause problems? like i said i am ignorant about turbos. i am very curious to learn about them. i would someday love to make a turbo build.

by backing off timing you mean change the ignition timing right? like the placement of the cam in relation to the dizzy? like i said i am ignorant but very curious about this.

i always thought if you installed a turbo you had to up your injectors as well.

[wed3k]

well the computer automatically retards timing to a certain degree. Cam timing you dont adjust unless you want to dial in an aftermarket cam.

small turbos, the boost comes on very quickly. like the dsm setups like 14b and big 16's come on pretty quick which is good for power but they create a lot of heat. and hot air again, can lead to lean conditions.

big turbos, they take a lot of volume to spool so generally they take longer but produce a lot more power.

there's a lot of ways to fight lean conditions.