How much HP is comprerssion worth?

How much HP is gained going from a 10:1 engine to a 13:1 engine - with all other components being the same? Meaning same cam, manifold, carb, ignition, heads - just changing pistons.
Perhaps one of those desktop dyno programs could provide the answer.

It would be dumb to build an engine with no other changes except compression.

As static compression comes up, cams can get more radical without sacraficing areas of performance, such as starting, and off idle.

Even if cams arent optimised, one engine to the other because of the compression will require differnt fuel, and of course differnt ignition timming events.

Its near impossible to do a straight apples to apples comparison, if you run 32 degrees advance and 110 octain fuel in one combination its a dog, and if your run it the other its crisp.

If you run 38 degrees advance and 92 octain in one combination, you've beaten the second one to death.........

4604X4 wrote:How much HP is gained going from a 10:1 engine to a 13:1 engine - with all other components being the same? Meaning same cam, manifold, carb, ignition, heads - just changing pistons.
Perhaps one of those desktop dyno programs could provide the answer.

You should see about 10% more.

rmcomprandy wrote:

4604X4 wrote:How much HP is gained going from a 10:1 engine to a 13:1 engine - with all other components being the same? Meaning same cam, manifold, carb, ignition, heads - just changing pistons.
Perhaps one of those desktop dyno programs could provide the answer.

You should see about 10% more.

Now THAT is the polite way to answer a ???. Randy,you are such a sweet man.
See you next Saturday

kim wrote:It would be dumb to build an engine with no other changes except compression.

As static compression comes up, cams can get more radical without sacraficing areas of performance, such as starting, and off idle.

Even if cams arent optimised, one engine to the other because of the compression will require differnt fuel, and of course differnt ignition timming events.

Its near impossible to do a straight apples to apples comparison, if you run 32 degrees advance and 110 octain fuel in one combination its a dog, and if your run it the other its crisp.

If you run 38 degrees advance and 92 octain in one combination, you've beaten the second one to death.........

It was a hypothetical question. I was just wanting to understand why someone would build a 13:1 engine for street/strip car seeing as how much extra trouble it is getting proper fuel/octane for street driving. For 10% more HP, it would not be worth it to me (for a 100% strip car it would not be an issue of course).

Percentage increases tend to increase up to about 12:1 or so the the percentage of gain starts to deminish.

You may see a 10% gain going from 9:1 to 12:1..... But may only see 5% going from 12:1 to say 15:1..... Purely hypothetical of course.

10% of a 650 horsepower engine is a 65 horsepower gain and that is nothing to be simply sneezed at.

Although, on the street, how often is any engine at Wide Open Throttle...?

I agree Randy, people have spent a whole lot of money far fewer HP gained.

Wasn't trying to give an butthole answere, just being honest, if going through the expense of getting domed pistons, shaved heads, thinner head gaskets, its not likly someone would not change the rest of the package to maximise all that investment.

Ancient rule of thumb was 4% per full compression point was a rough idea of what to expect when doing one combination versus another one.

As Dfree pointed out, there is a pretty distinct wall where dollars for gain makes it impractical to "target" more compression. about 14-1 the ability to do anything more radical becomes less affective. Cant really gain much more from more cam, etc.. You do get more HP, with something say 16-1 or even 18-1 than a 14-1 engine, but the cost of fuel, the cost of parts, the net gain, makes it impracticle for builders to target those thresholds. An additional, it was about 18-1 where Glidden found out 10 head bolts/studs arent real affective in keeping the head sealed to the cylinder, and thus the development of the 18 bolt heads.

4604X4 wrote:How much HP is gained going from a 10:1 engine to a 13:1 engine - with all other components being the same? Meaning same cam, manifold, carb, ignition, heads - just changing pistons.
Perhaps one of those desktop dyno programs could provide the answer.

Most have replied with a diversified bundle of knowledge, and of course an efficient internal combustion engine will provide efficiency of the components provided. I do believe higher compression ratios, given of course the platform you choose is a candidate for the stress a higher compression will exert on given areas of the project, it's beneficial, to a point!

Not to compare NHRA P/S ideas with popular bracket series engines, I must quote Mr. Morgan, owner of Larry Morgan Racing. "We've gone from not a lot of compression to a lot of compression, and we feel compression is a good thing to a certain extent. When you turn an engine at 11,000, it takes away from up high."

In short, I say operate your engine at a compression level you can manage. Yes, compression will be conductive to increasing an engine's power.

Just my old B.S.,
Dave.