This is what I'm coming up with,not sure if it's right,sounds right though.

Bore:4.124"
Stroke:3.76"
Piston: TRW-L2240NF30
Displacement:401.8ci
Connecting rod:6.135"
Piston deck clearence:?Haven't measured this yet,going with 0 for now
Piston dome height:.182"
Dome volume:-21cc
Compression distance:1.760"
Head gasket;bore:4.320"
compressed thickness:.040"
volume:9.607cc
Cylinder head chamber volume:101+/-cc(290 casting)

I get a compression ratio of:9.96845998730929 with these numbers.

I got 10.18

How do you get a piston with .182" of dome to come out with negative 21cc of volume? Either your valve reliefs are 1" deep or there is a math error in there somehwere. That piston should have a net volume of about +15cc.
If those pistons are really -21cc, then your CR is 7.25:1.

I have the same exact setup and Speedpro lists the 2240nf.030 pistons as 9:6 comp with your setup so i think your right on.

Also,my machinest said since he had to deck the block & heads a few thosands the comp might be closer to 9:7-9:8 comp.

Scott

I also get 10.18 w/.000 deck.

I'd like to know how you guys are getting 10:1CR with a negative 21cc dome. That is a huge dished piston.

It is a dome,that's how Speed Pro has it listed.I think it means it subtracts that much volume from the combustion chamber volume is the only thing I can think of.
Spec.s as listed:
Bore (in): 4.124 in.
Bore (mm): 104.750mm
Piston Material: Forged aluminum
Piston Style: Dome
Compression Distance (in): 1.760 in.
Dome Height (in): 0.182 in.
Piston Head Volume (cc): -21.00cc
Valve Reliefs: Single
Wrist Pin Style: Press fit
Pin Diameter (in): 0.989 in.
Top Ring Thickness: 5/64 in.
Second Ring Thickness: 5/64 in.
Oil Ring Thickness: 3/16 in.

http://static.summitracing.com/global/images/prod/large/trw-l2240nf30_w.jpg

I used this calculator:http://www.csgnetwork.com/compcalc.html

Jim,in case your not aware the speedpro 2240nf.030 pistons do not have Neg domes,they are farily short raised domes and thats where the 9:6-10:1 comp estimates are comming from.

The neg (- dome #) in his original post has to be a mistake/typo,i have these same exact pistions in my 396 and they are definately domed.

scott

It is a dome,that's how Speed Pro has it listed.I think it means it subtracts that much volume from the combustion chamber volume is the only thing I can think of.


Yes, that's correct.

Thanks guys,it's been helpful.I forgot that my desktop dyno had a compression math calculator on it,it came up with 9.97:1.Funny how the same numbers come up different.I've got links to a few online calc.s,I think for grins I'm going to do the eqaution on all of them.

Dang,you put"compression ratio calculator" in a google search and you get all kinds of them.Some require more info than others(good thing).This was interesting:http://www.kb-silvolite.com/news.php?action=read&N_id=30

Here's some of the totals i got:9.969,9.97(desktop dyno)9.98,10.18(3 times*),10.21,10.273,10.277(5 times).

*The 2 that asked for cam timing came back with 10.188 static compression and 7.595 dynamic.

Cam is Comp 294S.

Epistuff, please explain your comment about the TRW2240 piston "...should have a net volume of +15cc's" (it's specd at +21cc's). Are you taking into consideration ring grooves or something for the final volume? I am running that same piston in my 396, but it's the .60-over one.

The reason I'm asking is that, depending on which cc you plug into a compression ratio calculator, you end up with a big difference in final CR.

Thanks, Charles