I am about to change out cams and am trying to calculate correct static pressure needed for these new cams. Unfortunately, when I did my original 95" build and had heads done, the shop decked the heads .020 for more compression but did not cc the heads to verify volume. Now when I try use the compression calculators, I am estimating cylinder head volume and have no confidence in numbers. I was going to have heads cc'd so I could have real data, but am wondering if I can just test cold cranking pressure and calculate head volume using the CCP number. Is CCP consistent enough to calculate a reliable cylinder head volume?

 

.006-.007" per cc.
Just about 3cc's removed.
Scott

 

Thanks Scott - my problem is the heads were not cc'd before decking so I don't know what the head volume was coming from HD. I really need to cc them, but was hoping I could test for cold cranking pressure without having to pull heads, and then use a compression calculator to determine head volume. If I test CCP with same pistons, cams, altitude, etc on same
bike, would I get consistent numbers day to day?

 

Here's what I'm going to do to calculate cc before stripping my motor. I don't remember what my deck height is, though I did check it last time I had the motor apart. I do know what my head gasket thickness is, so from there I will take a piece of soft solder and slide it in the spark plug hole, doing my best to line it up with the piston pin. Roll the motor over to squish the solder and measure its thickness. From this number, subtract head gasket thickness. Now I know the deck height. Having this info and the CCP I can play with the Big Boyz compression calculator (or another, there are several available online, including on Keith Black pistons website) and "back in" to the chamber volume.

Don't ask me how well this works, as I haven't done it yet. But it should work, and I expect it will be accurate to within a couple of cc's. Surely close enough!

 

Not sure if that would work since my heads have been worked. I have all the information needed for Big Boyz calculator, minus head volume. Thats why I thought if I got ccp, I could plug in various head volume numbers until I matched actual ccp.

 

If you have al the other info then you can back into head volume that way. Assuming your engine is in good shape, the ccp should be within a few lbs and you'll be accurate within a few cc's. All you're doing is finding the unknown value by plugging in the known values. It makes no difference that the heads have been worked, the ccp is still the ccp.

 

Wow volume in this case is such a mute point. and you never test compression cold. Here's what you can do. Waste of time in my opinion but it works. loosen your pushrods off to prevent the valves from working, remove your plugs, remove the gauge from a compression test hose, fill the cylinder with and exact amount of oil, put the hose in a jar, turn it over by hand to TDC. subtract the what came out from what went in and there you go. Re set pushrods turn the engine over several times with the plugs out then fire back up. for real compression ratio, disconnect intake, leave the pushrods alone, set the piston at BDC on compression stroke, fill to the top keeping count of amount oil used, turn to TDC, calculate ratio from what came out to left in and that's real compression ratio with cam. it'll smoke a little at first but it'll burn off quick. But if you heat the engine and test the compression and the cylinders are within 10% that's all you need to know. don't waste your time worrying

 

CCP is not consistent enough to reliably calculate head volume and you don't know your deck height either. Generally, chamber volume will run between 85-86cc but I have seen as much as 89-90. The only way you will know for sure is to pull the heads, clean the chambers to remove any carbon and measure. You should also check deck height while you are at it.

Or use 85.5cc and the figures Scott provided and go from there. It's not that much trouble to pull the heads which is what I would do but I am **** retentive and have done it so many times, it's easier than changing oil.