Compression ratios and HP

[Chuck 98 RT/10]

Does HP increase or decrease exponentially or equally with each point of compression ratio increase? That is, if I go from 9:1 to 10:1 will the amount of HP increase be the same as if I went from 11:1 to 12:1. Or will it be more? Or would it be less? Any idea on how much of an increase it would be?

 

[SGViperGTS]

Chuck,

I've seen a chart of this before, but I'm not sure of the exact details. However, the power gain decreases for each additional point of compression (law of diminishing returns). Personally, I wouldn't go too much higher than stock compression unless it's a race car using some type of race gas. First, you won't really make that much extra horsepower (maybe 10-20 if you raise the compression 1-2 points). Also, you'll probably risk detonation much higher than 10.5:1 on pump gas.

Ben.

 

[Tom Welch]

Chuck,

It is not as simple to gain power by increasing compression RATIO.

While an engine is nothing more than a finely tuned air pump and the more you can compress the air the more power it will make, there are other factors involved. The engine must have an explosion in the cylinder in order to continue running. When Less air is compressed(increasing compression by decreasing the volume of the cylinder)a smaller explosion can occcur which negates the increase in compression. Therefore;
1. the amount of atomized fuel/air must increase.
2. the spent air must be more efficient in its exit from the compression chamber.

In a nutshell, raising compression is definately a benefit if you also do the following:
1. Increase the time that the valves stay open and closed for the entrance of fuel, resulting explosion, power stroke, then hasty exhaust of the spent gases.

This is accomplished with a better camshaft, larger valves, stronger valvesprings(to get the valves open and closed quicker)
and heads that can flow more air.

Don't increase compression by just shaving the heads without considering the above as a mismatch in your basic combination will probably occur resulting in possible detonation, and probably little gain in performance.

bottom line, save your money and get a nitrous kit!!!

tom

"woman who fly lying on her back have crack up"

 

[Mike Brunton]

Tom,

I gotta disagree with you on this one. When you increase compression ratio, you are not really changing how much air gets into the cylinder. The intake valves open when the piston is on the intake stroke, and the gas/air mixture is "sucked" in. If you deck the heads and lower the cylinder volume, you might be changing the amount of air that can get into the cylinder by a tiny bit, but you're also getting more "bang" out of that mixture by compressing it more and allowing it to push harder on the piston at ignition.

Like Ben said, it is a diminishing return, so when you get to a certain point, it's not worth going too much further. I would agree that you would gain only a little on a stock car by shaving the heads. Having said that, if I had my heads off, I'd probably have them decked a little - but I'd be real careful of the compression ratio - wouldn't want to go too much over stock. An nitrous kit is certainly gonna give you more bang for your buck.


Dave,

I just noticed you are back on the forum. I don't know when you signed back on, but I am glad to see you here contributing to the board. I think everyone respects your technical abilities, even if they have other "issues" with you. I hope you'll continue to chime in when you have some knowledge to share. I'm just saying this to voice my appreciation of your input - and I hope you don't stop sharing your knowledge and leave the board if people start to give you grief over past incidents.

 

[Chuck 98 RT/10]

I wasn't really considering upping the ratio as much as I wanted to find out if there were "diminishing returns."

However, your answers bring up another question - Rather than shaving the heads couldn't taller pistons be used??? That is, if anyone makes them.

Thanks for all of your input. This board is great! For a change of pace check out Tom's discussion board at btwviper.com. No we're not in business together!

 

[Travis Warden]

Rule of thumb is every point is 15 hp. Same for head flow increase; rule of thumb is 2.2hp/CFM