compression ratio

[2000aleroff]

can someone explain what compression ratio means exactly because im having a hard time understanding it.

[WhiteV6]

http://stuff.dewsoftoverseas.com/engine.htm

You have to understand how an engine works first, which is based on compression itself. You take the volume of a cylinder with the piston at top dead bottom. Then you move the piston up the bore to top dead center. The difference in volume from this point and what you had at bottom dead center is called the compression ratio.

Specifically, its called the static compression ratio.

[2000aleroff]

i know all of that but what is the difference between 11:5:1 and 9:1? i know its like 900cc's of air sucked in at the bottem of the rotation and 90cc's at the top of the combustion but is 14:1 better then 12.5:1? and what C/R is in bikes and formula cars that causes them to be able to go over 10,000 rpms?

[xXManwhoreXx]

High Compression is used for Powerful N/A cars and lower (like 9:1) is for forced induction. High compression engines usually use higher octane gas to get rid of nock, cause high compression causes it.... normally.

I don't think it is that compression ratio that lets them rev that high though, I think it is the entire build. I know most cars that try to rev that high get detonation.... which sucks...

[[ion] C2]

It's all of the rotating assembly and valvetrain that allow for high RPMs... it's about balance, precision, spring rates.

[mfuller]

i know all of that but what is the difference between 11:5:1 and 9:1? i know its like 900cc's of air sucked in at the bottem of the rotation and 90cc's at the top of the combustion but is 14:1 better then 12.5:1? and what C/R is in bikes and formula cars that causes them to be able to go over 10,000 rpms?

Okay, here's a severely watered down summary:

(Static) Compression ratio is simply the ratio of cylinder volume when the piston is at the bottom of it's stroke compared to the volume of a cylinder when the piston is at the top of it's stroke.
In general, there are really only two ways to make more power without forced induction: displacement and revs. That's why motorcycles can make a lot of power from small engines - lotsa revs (like 16,000 rpm, although lightweight reciprocating assemblies don't hurt).
Also, in general, higher compression ratios make more power, but it's a balancing act. The more you compress the air/fuel mixture, the bigger boom you get when the spark plug ignites the mixture. But as you raise the compression, it generates heat, which can lead to premature combustion. That is, the air/fuel mixture can ignite before the spark plug generates the spark. This leads into detonation and engine damage.
Now, higher octane fuels allow the air/fuel mixture to be compressed more before they ignite, which is why engines with high compression ratios usually require higher octane fuel. Tuning plays a part in this, along with spark timing, cylinder head efficiency, the cam you run, and a bunch of other thing that frankly, I don't completely understand.
There are other things that can be done to allow for higher compression ratios. Take direct fuel injection, where the fuel is sprayed directly into the combustion chamber of the cylinder head rather than the intake port. This helps to keep the air/fuel mixture cooler, so you can compress it more which generates more power.
A good example is GM's High Feature 3.6L DOHC V6. When it was first introduced in the Cadillac CTS, it made 263HP. Through some careful tweaking, you would see around 275HP from the same engine in some of GM's crossover SUVs (GMC Acadia and so forth). The direct injection version of the same engine makes over 300HP; you'll find it in the new Camaro and other cars.
Modern diesels are all direct injection motors, and usually run much higher compression ratios than gasoline engines (20:1 or so).

Hope that explains some of it (in a very watered-down way).

[2000aleroff]

thanks ya that makes a lot of sence, ill look into direct injection and see what i can learn from it

[mfuller]

thanks ya that makes a lot of sence, ill look into direct injection and see what i can learn from it
Just so you know, direct injection will be extremely difficult to retrofit (I won't say impossible, but it's damn close). It would require a different cylinder head, an entirely new fuel system, computer tuning, etc. It's great for you to read up on, but the only viable way to get it is buying a vehicle that already incorporates it.

[Spilner521]

Yeah I would lean more towards impossible ^

The combustion chambers and pistons have different shapes to them than a normal fuel injected engine, on top of everything else.