Computing Compression Ratio - check my math?

[fzmax]

This is the first time I'm using a dome piston and want to be sure my procedure and math are correct. The engine in question is a .030 over 318 (3.940 bore) and the pistons are KB 399's:

65 - Head CC
2.2 - Deck (piston is .011 in the hole at TDC)
7.1 - Compressed Gasket Volume
1 - Above Top Ring
4.6 - Dome Volume
_____________________
70.7 - Clearance Volume

661.3 - Swept Volume

10.35 - Compression Ratio

I think the above is pretty straight forward, but I want to be sure I measured the dome correctly as my number is different than what KB says it should be.

What I did was install a piston and rod ensuring the dome was completely below the deck. I then cc'ed the block just as you would a cylinder head and got 39 cc's. Then I measured the deck height which was now .218 down. If I calculated that correctly a flat top piston would have netted 43.6 cc's which gives means I've lost 4.6 cc's from the clearance volume due to the dome. The KB catalog says 6 cc's so that's why I am little concerned.

 

[Rat Patrol]

KB asks for gasket thickness, gasket bore size, bore size, stroke, piston relief, - if you can provide that stuff somebody can cross check your entries?

I understand you are using the "volume method" - but it may be what you are putting into the KB calculator is wrong?

 

[fzmax]

I didn't use KB's calculator, those are either the actual measured cc's or calculated from actual spacial measurements like the deck height. For the gasket volume I used a gasket bore of 4.170 and a compressed thickness of .032 which comes out to 7.1 cc's. Bore and stroke are 3.940 and 3.310 respectively.

 

[MoparAnglia]

I didn't use KB's calculator, those are either the actual measured cc's or calculated from actual spacial measurements like the deck height. For the gasket volume I used a gasket bore of 4.170 and a compressed thickness of .032 which comes out to 7.1 cc's. Bore and stroke are 3.940 and 3.310 respectively.

The piston dome and compression ratio calculations look sound to me. I honestly wouldn't lose sleep over 1.4cc difference in the piston dome. If you're running that close to the ragged edge on dynamic compression ratio then a rethink might be required on cam timing anyway.

 

[fzmax]

I honestly wouldn't lose sleep over 1.4cc difference in the piston dome.

Therein lies my problem. I get way to anal about this stuff at times. I posted that at 12:30 last night, I should have been sleeping for over an hour. I just wanted to make sure I wasn't missing something major. Thanks

 

[MoparAnglia]

Therein lies my problem. I get way to anal about this stuff at times. I posted that at 12:30 last night, I should have been sleeping for over an hour. I just wanted to make sure I wasn't missing something major. Thanks

Ha ha, I hear ya. It's surprising how often stuff like that springs to mind late in the evening.

It's never a bad thing paying very close attention to what's going into an engine build though. Looks like you're building a pretty strong 'teen.