I first posted this in the Engine section. Maybe somebody here will help?

I thought I understood how to calculate compression ratio, but my results don't make sense to me and differ significantly from the number posted by the piston manufacturer.

.030 over 454
049 heads
L2465F-30 TRW pistons
.226 dome
.038 fel pro head gasket

Notes from machine shop say the heads were milled and cc's at 119. It's been awhile since I built this engine, and my notes also say 25.7 cc dome, 1.64 comp. distance. As best as I can remember, zero deck height.

The literature with the pistons says that with 119cc heads, the compression ratio will be 10.86. My calculations, not allowing for gasket thickness, come up with 10.1. When I factor in the gasket, I get 9.2.

Why is TRW's number so much higher?

Does the gasket really drop the compression nearly a whole point, or am I calculating something wrong?

I would say you've missed a decimal place somewhere. A 454 with a 25cc dome and 119cc heads is roughly 10:1

I get 10.19 cr

I first posted this in the Engine section. Maybe somebody here will help?

I thought I understood how to calculate compression ratio, but my results don't make sense to me and differ significantly from the number posted by the piston manufacturer.

.030 over 454
049 heads
L2465F-30 TRW pistons
.226 dome
.038 fel pro head gasket

Notes from machine shop say the heads were milled and cc's at 119. It's been awhile since I built this engine, and my notes also say 25.7 cc dome, 1.64 comp. distance. As best as I can remember, zero deck height.

The literature with the pistons says that with 119cc heads, the compression ratio will be 10.86. My calculations, not allowing for gasket thickness, come up with 10.1. When I factor in the gasket, I get 9.2.

Why is TRW's number so much higher?

Does the gasket really drop the compression nearly a whole point, or am I calculating something wrong?
=
The compression at zero deck is 10.1 with your numbers.
I think the 2465 is a 27cc dome but this is from memory & that would make it 10.2 with a zero deck.
Your block would have to have been cut to 9.775 or approx .025 from the nominal 9.800 factory deck height for zero deck.
Most Felpros are .039 or .041 which is not a big deal from .038 you used.
If the deck were uncut you would be approx 9.7

Can't tell you why the numbers you have from the manufacturer are 10.86 unless it was a misprint or you read them for a 112 cc head at zero deck??

Thanks for the responses. Greatly appreciated.

I'll dig back through my file and find the receipt from the machine shop. That will answer the question about whether the block was decked. It's been awhile, but my memory is telling me the block was decked.

Thanks, again.

TRW piston number you're using is computed with a .028 head gasket thickness, info taken directly from TRW parts book. You will be .2 (two tenths/lower) using the .038 thickness. They show 10.19 with a 116.9 cc head with .028 gasket. This will put you at 9.99 theoretically. These numbers include no milling figured in on your end. Hope it helps. Thanks, Gary.