I am thinking of installing some 103" Keith Black 10:1 pistons in a future build-up of my present 96". Now I already have the pistons so there is no going back and already have the SE-204 cams which might not like a lot of compression over the stock 96". The question is, just because it says on the box that the pistons are 10:1 that isn't what the motor will end up being will it and would that, plus a .045 head gasket be too much for those cams and it would be a ping monster? I've got dyno tuner fellow picked out and he seems to think we can do it but I don't want to chance having the wrong combo.

 

There are a ton of threads that discuss this, one pretty recently. Unfortunately, I can't find the thread it was in. The jist of it is this: You are correct, just because the box says 10:1 doesn't mean that's what you actually end up with. There is static compression, which is the theoretical mathematical calculation of the volume of the cylinder with the piston at Bottom Dean Center (BDC) compared to the volume of the cylinder at Top Dead Center (TDC). This gives you your static compression ratio. There are numerous variables that affect this, including deck height (meaning how far above or below the "deck" - the top of the cylinder - the piston's head is; and it's usually a few thousandths below), head gasket thickness, combustion chamber volume. Also piston dome volume, which is usually given by the piston manufacturer. Once all of these varialbes are known actual static compression ratio can be calculated. There are a number of online compression calculators available, one of which is on the KB website, another on the Big Boyz web site.

Getting all of these dimensions is typically known as "cc-ing" the motor, since head volume, piston dome volume, etc, are typically measured in cc's. While the components all have "nominal" specs, actual measurements vary due to normal production tolerances.

Corrected Compression Ratio, aka "running" or "dynamic" compression ratio is more important. This includes the timing of the intake valve closing event, which happens so many degrees After BDC (ABDC). How many degrees ABDC this happens determines where in the compression stroke the piston actually begins to compress the air it inhaled during the down stroke. The later the intake valve closes, the lower the corrected compression ratio.

Big Boyz website also has a useful camshaft comparator that lists valve timing events for many camshafts, so that it's possible to enter intake closing event in the compression calculator and get corrected compression ratio as well as cranking compression in psi. There are varying opinions of what is optimum or tolerable; if you look around you'll find some say 195 psi is optimum, a good balance between high performance and resistance to detonation, others set it higher.

I'm going to pass on what is optimum for that particular cam, as I just don't know. Determining what a certain cam likes seems to me to be a matter of personal experience gained by trial and error. I'm sure someone will chime in and fill in the blanks I've left; some have had experience with those cams and will probably advise you.

 

It will depend on actual head volume and how far down in the hole the pistons are. Setting it up for close to .030 squish will help with detonation. This will be close. http://www.bigboyzcycles.com/TwinCamComp.htm

 

Looking at the Big Boyz site and not really knowing exactly what some of my numbers will be, using a .045 HG I might be alright, using a .030 would put my CCP over 200 which is too much without the use of comp releases.

 

Are the pistons flat tops or domed? If domed, you can shave the dome off and make them flat tops to bring down the cranking compression to about 190. I have run 204s at 9.8 static and 190 cranking and experience detonation issues in the south Texas summer heat. Did you allow a deck height of about .003" in your calculations? It doesn't make a big differnce but does have an impact. You can also have the heads worked and drop the valves in the seats and/or slightly taper the chamber walls to increase chamber volume. I would not run a .045" head gasket with 204s, .030" would be best but if you need a little more you could run a .040" and probably be OK.

You can make the combination work, just have to think out of the box a bit; get creative.

 

The pistons are flat that's about all I know djl. Thanks for your input.

 

If by by going to stage 1 or 2 & increasing the volume & air mixture in cylinder wouldn’t the compression ratio increase?