Hi All,

I was thinking of doing BB kit/heads and wondering how compression affects horsepower. I'm trying to determine what the ideal compression is that will run on pump gas and not destroy my starter.

Also is the horsepower gained by raising compression and adding the required compression releases worth the expense? Meaning, if I can get 100HP with 9.5:1 that starts easily without releases how much power would be expected if I raised the compression to 10.2:1 and go through the trouble of the releases? Is it safer to keep it on the low side to avoid tuning issues and detonation issues?

I'm certain there's more to it so any explanations to help me understand is truly appreciated.

Thanks,
J.

 

3% more power per point raise in compression.....roughly

 

The proper compression, for the cam/cubic inch, being used, can make, or break the build.
10.2 cr with, say a Wood 9B, will be a stone turd, and on the other hand, 10.2 cr with and Andrews 26, will spark knock(detonation) so loud, it rip the ring lands off the piston.
Scott

 

The first thing you need to do is to stop thinking in terms of static compression; it's just a number and the engine never sees it while operating. That is why you can't generalize and say 3% increase in power for every point of increase in compression. As Scott has pointed out 10.2 static CR will produce completely differernt results with different cams. The reason is that different cams have different intake valve timing, i.e., the intake valves close at different positions in the operating cycle. In Scott's example, the Woods 9B intake closes at 50* ABDC and the Andrews 26 closes at 35* ABDC. The cam with the earlier intake close event captures more swept volume of A/F to be compressed as the piston approaches TDC. The engines sees this compression which is technically referred to as dynamic or corrected compression and is measured numerically, i.e., 9:1, 10:1, etc. Cranking compression is also measured this way but as PSI; just a different calculation. There are other factors that enter into the calculation, such as chamber volume, head gasket thickness and deck height. Go to the BigBoyz website and look at their compression calculator; that should clarify the compression discussion.

There may be some debate about what is considered reasonable corrected and cranking compression for a street build but there does seem to be a consensus that 9.3 corrected and 190psi cranking are reasonable and manageable targets. Personally, I would install compression releases anytime cranking compression exceeds 185psi. They may not be needed but they will definitely extend battery and starter life.

If your '08 is stock, i.e., stock heads and head gasket and deck height is typical for the later models, your static compression should be a tad above 9:1; chamber volume does vary so it could be a little more or less. The CR 575 intake closesa at 35* ABDC which would put your corrected CR about 8.6 and cranking CR about 174psi. That's pretty mild and not getting full benefit of the cam's capability to make power; you have room to grow.

 

Thanks djl. I spoke to Wes at Cyclerama and he suggested a compression of 10.2 would be manageable. Since its his cam I guess he would know

 

Yeah, I guess he would but near 200psi CCP (sea level) and corn fuel don't play well together and that setup would not be for me; don't care who's cam it is. You may not be able to live with what Wes is willing to live with. Having said that, you may live at an altitude that will lower the CCP but take a look and think about compression releases. A good tune will be important, so find a good tuner.

Heads will have to come off so have some head work done while they are off.

 

The heads are being done and compression releases being installed. I have an EMS tuner so hopefully they'll be able to figure it out.

I guess that's the point of my question. If lowering the compression to say 10:1 makes the bikes less susceptible to detonation and is easier to tune but will only cost a few HP that makes more sense to me. On road trips you can't always guarantee high quality gas.

 

Minus 1/4 point of squeeze, won't kill it.
You'll be in good shape.
Scott

 

wes`s 575 cams,103 set @ 10.2-1 & decent heads (150+ cfm @ 10"),105/110 shouldnt be a problem

 

how bout with a 107?