# Anybody know how compression and hp work?

Discussion in 'Engine Building' started by gdc2000, Dec 19, 2008.

1. ### gdc2000Noob

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Don't know if this is the right forum or not so excuse me if it is in the wrong place.

"The change in horsepower due to the change in compression ratio is relative but not directly proportional."

The way I understand this is that a increase from 8:1 to 9:1 would increase horsepower while something like 13:1 to 14:1 would yield less gains.

8:1 to 9:1 would yield 3% hp gains while 13:1 to 14:1 would yield about 1.5%?

Also for every point in compression you should yield about 8hp?

All the above info was jacked off the internet. Is this stuff true or are their too many variables? Anybody got some reference material that they can refer me to?

Thanks

2. ### MthaFuknGreenGreen on the scene.

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too little is useless..

dont take my word for it though.
boost plays into it somehow too.

3. ### FLounderpower hungryVIP

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i would have to say there are a lot of variables that could affect the numbers. generally speaking more compression with everything else staying the same would make more power and there probably is a formula for it, but idk

by the way, two of the things you said contradict each other.....

4. ### gdc2000Noob

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Yea that is off the website I was looking at thats why I decided to post here. I was thinking maybe.....hell I dont know thats why I posted. My brain hurts....

5. ### gdc2000Noob

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"The lower the CR to start with, the more the gain per point of increase. That makes sense when you consider that going from 8:1 to 9:1 is a 12.5% increase but going form 11:1 to 12:1 is only a 9% change, and so on."

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Originally Posted by gdc2000
8:1 to 9:1 would yield 3% hp gains while 13:1 to 14:1 would yield about 1.5%?

This is just another quote that I found on the internet again. So what it boils down to is that I have no clue how this works. After everything I have read so far bumping the c/r up does increase the amount of hp but their is a ton variables that affect it.

6. ### MthaFuknGreenGreen on the scene.

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he means, if you keep adding more and more compression, eventually the gains are gonna stop..

in fact, if you add too much, youll probably lose performance and cause damage.

7. ### 93civicracerOh, I do a little here and there.

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You'll probably blow the head gasket with too much compression, or break a valve.

With a higher compression ratio, there's more space between TDC and BTDC strokes, which means it draws more F/A mixture in on the down stroke, and that means it has to compress it all, and the more there is to compress, the more powerful the combustion will be when it's ignited so it's going to to push that piston back down more forcefully and quicker.

That's all I can say to the best of my knowledge.

8. ### 97hatch?

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Well problem with way to much compression is you cant get good enuff fuel to controll detonation. Once your compression is so high it will ignite its self. Once you reach the self ignition point you have your self a diesel. Diesel engines run on compression only and no spark. Also head design will play a huge role in how much power you can make with the compresion you have. You could build a engine with all the exact same parts as a Pro FWD All motor car. But with out the head and intake porting they have you will only make about 40% of what they do. By the way one of the Pro All motor FWD cars makes over 425hp at the wheels but im not able to say who and its compression ratio is over 17:1

9. ### phyregod!!YTINASNI

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Well, thats if you stroke the motor, but it has alot of truth to it.

Too much compression will raise air/fuel mixture temperature to the point that the mixture explodes before it is supposed to. (Detonation).

Quoted from a wiki: Compression ratio - Wikipedia, the free encyclopedia

10. ### 93civicracerOh, I do a little here and there.

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Thanks I was kind of thinking about that right after I posted it.

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