Help determine static compression ratio

[jim_howard_pdx]

I have 62 CC heads
I have 20 CC dishes on my pistons
I have a head gasket that compresses to .039 inches
I have piston to deck clearance of .025 inches

I have a 4.030 bore
I have a 3.5 inch stroke

Anyone know the calculation to determine the static compression ratio?

[PKRWUD]

Using my weak math skills, I come up with 11.3:1. I did most of this in my head, but it should be close. I will try and come up with a more precise answer later when I have a little more time (math gives me headaches).

Take care,
~Chris

[Eric4Nitrous]

I'm not that good at math myself either..but i came up with 11.5:1?? Sounds about right i guess.

[302 LX Eric]

I thought that dish pistons were for lower compression and that dome pistons were for higher compression? If so, 11.3-11.5 sounds too high. I was thinking more in the area of 9.3 or 9.4:1 compression.

E

[PKRWUD]

Well, so much for doing it in my head. I just sat down with the numbers and a calculator, and came up with 8.4:1 compression. Here are my notes:

Compression equals Cylinder Volume plus Chamber Volume divided by Chamber Volume. So:

731.3351 + 99.033001 = 830.3681

830.3681 / 99.033001 = 8.3847615:1 (or, 8.4:1)

Cylinder Volume: 731.3351cc

Chamber Volume: 99.033001cc

62 + 20 + 6.6535166 + 10.379485 = 99.033001cc

Combustion Chamber Volume: 62cc
Valve Reliefs/Dish: 20cc
Piston to Deck .025": 6.6535166cc
Head Gasket .039": 10.379485cc

[Eric4Nitrous]

Man my math skills are BAD. I think i saw where i made my mistake though.. DOH.

[jim_howard_pdx]

Thanks for the help.

Some of the deck hight goes away as the rod grows under temp, and my heads were shaved, so I figure I lost some cc's. Even so, the final compression ratio should not exceed 8.5-9.0.

In the future, I should be able to strap on a super charger without any worries--other than having an INSANE ride.

[jim_howard_pdx]

Hey Peckerwood,

I went back and remeasured all the important stuff. Head gasket compressed will be .039. Deck is actually .018 inches. Dish measured at 15 CC. The original combustion chamber was 64 CC but the head was shaved 0.1 inches. The shop told me it would raise me about 1/2 a compression point..... I figure the volume is somewhere between 59 and 62 CC

I used the EZ Engine Analyzer to calculate compression ratio. This is what it said.

8.96 to 1 at 62 CC or 9.51 at 59 CC.

Thanks for the help. You were right on with your second calculation based on my original data.

At 9.5 to one compression I will probably need a water injector if I strap on a supercharger.

Oh well, I love steam cleaning my exhaust and combustion chambers anyway.

You have to just love today's pump gas........