How does compression ratio relate to hp?

[84convertablegt]

ok, i was wondering if anyone had a link to a compression ratio to horsepower calcualtor or could explain how c.r. relates to hp? i understand that 289's have 58cc heads with a 9.3:1 c.r. which seems low. id hate to give anything up by buying 64cc late model heads, seems like it would really hurt things. even if i bought 58 cc heads, i wouldnt be gaining any compression. is the only way to gain it to mill the heads or change pistons? i wouldnt be concerned except that im under the impression that compression ratio can really boost hp levels

 

[golf4283]

I think compression ratio mostly helps determind what kind of gasoline you run. Say 89 92 or 93 octane.

 

[84convertablegt]

well, i know it does more than that. higher compression motors just make way more horsepower. its the same concept as supercharging. cramming more air and fuel in the chamber tighter before you fire it off. needing higher octane gas is a byproduct of this. that way the gas will resist lighting off longer without a spark and causing detonation.

 

[golf4283]

You just answered your question . Higher compression equals getting more air into the engine. More air = more horsepower.

 

[grego37]

If you plan on running pump gas and your running iron heads, I wouldnt go over 9:1, if your running aluminum heads, I wouldnt go over 10:1. Find extra HP else where in your cam, Intake, carb, headers.
You can build a powerful motor that runs on pump gas.

These days Pump gas is only going to get worse, Out here in Calif. the highest octane is 91.

 

[84convertablegt]

ok, i know i kinda answered my own question. but i knew all that. i was wondering what the actual direct relation is in math terms. like a link to a calculator or something. like if i did go to 64cc heads, what it would do to me?

 

[Max Power]

Higher compression does NOT get more air into the engine. Higher compression squeezes the A/F mixture tighter so when it ignites, it creates more force.

As a general rule, every 5cc in a small block Ford equals about 1 point in compression, give or take.

Also, every point of compression is worth about 4% total horsepower. This varies widely based on engine design and things like camshaft selection in using that compression. Example, you really can't run an RV cam on an 11:1 motor like you would on an 8:1 motor, as the detonation would give you false horsepower readings. You would have to switch cams to make the test consistant.

Every test I have seen is about 4% per point in CR.

 

[66 BLAKE 96]

Another factor in compression ratio would be power adders. Generally speaking boost (Supercharger/Turbo) likes low compression. Nitrous likes higher compression. I know it's general, and not exactly what you're asking about, but that's what I got.

 

[grego37]

84convertablegt,

There's no way to figure this out on paper or with some calculator unless you know:
the cc volume of the cylinder when your piston is at TDC, (head gasket thickness included). This can be done if your heads are removed, and the shortblock is on an engine stand.


Or,
if you know the exact compression ratio you have now, and you know the exact cc of your current heads, someone can figure it out.

example :
you currently have 9:1 compression and your current heads have 85cc combustion chambers, and the replacement heads have 64cc
the formula would be 9:1 minus 21cc = new compression ratio.
good luck

 

[84convertablegt]

Higher compression does NOT get more air into the engine. Higher compression squeezes the A/F mixture tighter so when it ignites, it creates more force.

As a general rule, every 5cc in a small block Ford equals about 1 point in compression, give or take.

Also, every point of compression is worth about 4% total horsepower. This varies widely based on engine design and things like camshaft selection in using that compression. Example, you really can't run an RV cam on an 11:1 motor like you would on an 8:1 motor, as the detonation would give you false horsepower readings. You would have to switch cams to make the test consistant.

Every test I have seen is about 4% per point in CR.

im basically getting my figures based on stock small block specs. so basically going from a 58cc head to a 64cc head would probably leave me around a 8.0:1 compression ratio. good info guys, i was curious as to the best thing to do when it comes to buying heads. i dont plan on running a supercharger, although i love boost. just trying to keep the classic all n/a so was trying to get a general idea. grego37, thanks for the info, im not trying to get exact numbers here, just an idea of where ill be at with a possible combination.

 

[BornInAFord]

But it's not the static CR you should be worried about, anyway. The dynamic CR is what affects the power. This comes from piston vs valve timing, overlap, etc, and is not just filling up the cylinder, closing the valve, and compressing the air. This doesn't directly answer your OP, but is something to throw into the equation.
Daniel

 

[kruizinkareem]

I know it's wayyy late, but here's a good video talking about compression and power


View: https://www.youtube.com/watch?v=0XAGo1B_w20

 

[DBegley]

Too high of compression using this crap fuel we have access to is a potential issue. Detonation is not a good thing. It's not just the compression, it's the whole set up for you motor. I am dealing with fixing or replacing the 302 in my 68 Mustang. It's a street car. I want a torque curve that starts at low rpm and hits the max about 4500-5000 RPM. Cam, heads, headers, intake, carb or EFI all affect the torque curve. I have been looking at Stroker motors with 9.5:1 compression putting out 425 HP 400 TQ on 91 octane. Compression is just one part of the equation. I just talked with a fellow that re-build motors for a living. He wants to fix my 68 block. He feels that we can take that old setup, add new forged pistons, bearings, turn the crank, etc and get me 350 HP. That includes him taking my stock cast iron heads and tweaking them. I'm seriously thinking of picking up some Flo Tek aluminum heads and swap my stock ones for these. $299 each for new heads. The other part of this equation is how everything works together. As it turns out my aluminum Cobra intake and Motorsport Cam are low torque street setup. And those Flo Tek heads are great for getting a lower torque curve. What do you want your car & engine to do and how deep is your pocket?