CR's responsible for how much Hp?

[tyoneal]

Hello:

Can someone educate me.

If two engines make 600 hp @ 11 to 1 Compression, about how much would the power drop if the Compression was dropped to:

9:5 to 1
9:0 to 1
8.5 to 1
8:0 to 1

Thanks I really appreciate it.

BTW: The two engines, one is a 377 ci. and the other is a 427 ci.

Both are running Pump gas @ 91 octane.

Thanks,

Ty O'Neal

[Steve1968LS2]

I've heard that for every drop (or gain) of 1 point in compression it's equates to between 4 and 6-percent.

[RussMS]

Found a good article from a fairly reputable magazine :D j/k I love PHR

http://www.popularhotrodding.com/tech/0311_phr_compression_ratio_tech/index.html

[ProdigyCustoms]

Yeah, it really is not so much about the HP drop as the "SNAP" in the motor. More compression, more responsive.

[Efi69Cam]

There are two factors that come into play when talking about CR. One is static CR which is computed by the standard computation, the other is the dynamic ratio which is indicative of cylinder pressure. The camshaft plays a bigger role than the static CR in determining the dynamic ratio.

A 12:1 engine with a lot of overlap might run fine on pump gas, albeit at a limited RPM range. That same engine cammed with less overlap would build too much cylinder pressure and likely detonate on pump gas. Lower the CR and the engine will no longer detonate and will have a wider power band.

What this means is that there really is no way to compare HP changes due to static CR changes without discussing the camshaft.

[tyoneal]

There are two factors that come into play when talking about CR. One is static CR which is computed by the standard computation, the other is the dynamic ratio which is indicative of cylinder pressure. The camshaft plays a bigger role than the static CR in determining the dynamic ratio.

A 12:1 engine with a lot of overlap might run fine on pump gas, albeit at a limited RPM range. That same engine cammed with less overlap would build too much cylinder pressure and likely detonate on pump gas. Lower the CR and the engine will no longer detonate and will have a wider power band.

What this means is that there really is no way to compare HP changes due to static CR changes without discussing the camshaft.
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Good point:

Both Cam Shafts have 114 LSA.

[kadettcqp]

In my opinion it would be hard to adress an exact formula for this matter, there are simply to many things that come into play. Because another fact that comes into play is how efficient the engine is buildt. And for what CR the engine is buildt. If for instance the engine is buildt in any matter for 11:1 CR on 91 octane and can run with "optimum" ignition advance without detonation then you will have less power on lower CR. But if the engine needs to advance the ignition to much in order to run 91 with 11:1 then you could actually gain HP with less CR and less timing. It all comes down to the excact build of the engine. Therefore i would not be able to make any reasonable guess on how the power would change. CR is not always an indicator on HP it is an factor that you can play with along with the fuel used, cam etc.

[tyoneal]

In my opinion it would be hard to adress an exact formula for this matter, there are simply to many things that come into play. Because another fact that comes into play is how efficient the engine is buildt. And for what CR the engine is buildt. If for instance the engine is buildt in any matter for 11:1 CR on 91 octane and can run with "optimum" ignition advance without detonation then you will have less power on lower CR. But if the engine needs to advance the ignition to much in order to run 91 with 11:1 then you could actually gain HP with less CR and less timing. It all comes down to the excact build of the engine. Therefore i would not be able to make any reasonable guess on how the power would change. CR is not always an indicator on HP it is an factor that you can play with along with the fuel used, cam etc.
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Thanks you for the answer. Looks like I have some more reading to do to get my arms around this subject.

Thanks again,

Ty