Compression Calculator

72bluNblu said
I have KB 243's in my 340, they measured .018" over the deck (which is the spec with the stock deck height). The compression calculators I was using, including the Wallace calculator, had some issue dealing with the deck height and the piston volumes because of the way that piston is shaped. It looks flat but it actually has a .030" protrusion, so the shoulder of the piston sits .012" below the deck but the top is .018" over. I'm not entirely sure where the 6cc volume comes from that they advertise, but the tech I spoke to at KB straight up told me not to use it. Instead, he told me to use a deck height of 0 and a 7.3cc volume, with that volume accounting for the protrusion and the valve reliefs.

View attachment 1715876349

From the tech...

I ended up running a different head gasket than the one I gave the Tech and my head chambers ended up 1cc larger, so it calculated a little differently when I updated the gasket
View attachment 1715876343

Also, with the KB 243's if you do end up with the tops of the pistons .018" over the deck like they're supposed to be you'll need to check your piston to head clearance if you run closed chamber heads. You may need to adjust that clearance with a thicker head gasket. I run open chamber 308's on mine.

In the picture of post number 7, the deck surface appears to show markings from previous decking. Do you know for sure if yours was previously decked?

As for the numbers, I think I agree with KB on this one, or at least they are pretty close. If we treat that as a flat top piston, which typically has around 5 or 6 cc valve pockets, you would add the volume of that little step around the piston to the valve notch volume, and that is how they get 7.3 cc. But, the numbers say that "flat top" would sit about .018" above the deck, assuming 9.6" deck height, and the numbers measured from 72bluNblu confirm this on his engine. With those numbers in the calculator on the diamondracing.net website, it comes out to 9.99 to 1, and that is including .305" top ring land height.

plumkrazee70 said
Thank you sir. A little more aggressive than what I want. (a nice torque street car). Ill take the compression numbers back to Ken and go from there. Thank you all. I wanted to be sure I was on the right track.

Is this engine already assembled? If not, maybe you can test the half inch down fill volume and that will give you exact numbers. You would "travel" the piston down half and inch from true TDC, not just measure the piston to the top of the deck directly. Subtract measured half inch down volume from a calculated perfect half inch tall cylinder with your 4.08" in bore and that will give you the true cc of that piston. Then we know how close to 6cc or 7.3 cc KB was on their estimate.