yOnKiNaToR

i want to know what compression does for an engine and what if it gives power by raising it. i would assume that you would gain power by raising compression but how much? how much is a "safe" limit? how do you give an engine more compression?

Howard

There are several ways to increase compression, you can machine the heads to reduce the dome depth, but this leads to cam chain length problems and also repositions the intake and exhaust manifold mounting surfaces. Next you can replace the pistons with higher compression pistons, which will have a higher dome height, this may cause interference with the valves though depending on the cams you are running. Last, you can go with turbos or supercharging, which would compress the air/fuel being delivered to the combustion chamber. As far as how much power you can achieve by increasing the compression ratio I would assume there would be a simple formula for calculating this but I have yet to come across one. I know sveral guys in the club who have bumped the CR up to 12:1 running 108 octane race fuel with no problems, but they built full race motors with all the required upgrades ($$$) in order to have a bullet proof high output race motor.
good luck !

yOnKiNaToR

well, i don't think i should even attempt to raise my compression any higher than it is...10.2:1 is probably high enough for street applications. it just amazes me that drag cars and high performance cars can have ratios like 15:1...

Danno

The reason higher-compression gives you more power is what happens on the opposite end of the compression-stroke, the power stroke of the engine. If you have an 8:1 engine, you are making that gas expand to 8x it's original size, extracting a certain amount of engergy out of it. By going to 10:1 compression, you are making that gas expand a larger ratio of it's original size, thus making it work more and extracting more energy out of it.

And yes, there's only so much compression you can force a mixture to endure before it detonates or pre-ignites. But this limit is not cast in stone, it has a lot to do with combustion chamber design, the width of the piston makes a difference. The included valve-angle. The most efficient shape for a combustion chamber is actually a sphere with the spark-plug right in the center. That way, when it ignites, the flame-front has the minimum amount of travel to ignite the entire mixture. Only once the entire mixture has ignited, can it truly exert maximum force on the pistons

jpitman

As you increase compression ratio the peak power goes up (within some limits), but the rpm of peak torque and power also rises, which can produce driveability problems. There is BIG difference in low end torque between say a 9.35:1 and a 10.5:1 motor, which is VERY obvious in street driving a 5 speed manual box, but less so in an auto. Try it if you can find the relevant cars - US and Euro 928s, 79-83.
The limiting factor is usually the fuel quality - tendency to ping (detonate, which destroys pistons, reduces power) rises, unless the octane is raised (as in previous posts). If you already have a CR in the 10 range, I wouldnt play around with it for a few hp.
jp

Steve Weiner-Rennsport Systems

"yonk":

The real-world limitations of compression ratio in street cars is, as already mnentioned, dictated by the locally available grades of fuel.

Other factors are bore size, in the case of Porsche air-cooled engines. Big-bore engines have a far greater tendency to leave pockets of fuel/air close to the squish band that tend to create the conditions for detonation compared to small-bore engines.

As a good rule-of thumb, here are some guidelines that will help determine what is possible or not with pump gas at 91-94:

Street engines up to 9.7:1 can use pump gas and single ignition.

Street engines over 9.8:1 must use twin ignition in big-bore engines and smaller bore ones really should, as well.

Remember, the threshold of detoantion is NOT a fixed value. This changes with air temp, cylinder head temp, variations in pump gas, mixture strength, ignition timing, humidity, to name a few.

You should not have so much compression or ignition timing that it will detonate under hot conditions or high load.

Never, ever depend upon your ears to determine whether any engine is operating in detonation or not.
In most cases, it will be too late.

I've more info on this subject on our website at:

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