Hey all... I've got a set of big block heads with casting # 336781. How would I find the compression ratio for these? Based on my engine casting number book, they could either be from a passenger car or a truck from 1973-1976, however, since they are oval port, I am inclined to think they are from a passenger car.

Any ideas or suggestions on where to go to find this info would be greatly appreciated.

Thanks!

:beers:

http://members.uia.net/pkelley2/crc.htm
You'll need your piston info and bore/deck info.

336781.....73-85...oval...OPEN...454, 118cc chamber, 256/114cc ports

Michou
There is a ton of info on these heads here on TC. Use the SEARCH feature.
These heads have different chamber sizes.....119cc is common, some are smaller, some reported as large as 121cc....so you must cc your own chamber for exact info on what you have.

781s came on mid 70s cars and pickups.
If you have factory dished pistons and a stock-style .040" gasket and typical stock piston .023" down the hole for deck height and your heads are on the large side for chamber volume, you could have whopping 7.9:1 CR.

A flattop piston, smaller deck height, and a shim gasket with somewhat more favorable size you could have 8.9:1CR.

Small dome pistons can get you 9.0:1 up to 9.25:1

Medium dome pistons 9.75:1--10:1

Again, all ballpark figures

Ditto George by me.

You have to cc them and then I would install .020" thick shim headgaskets to hold quench tight regardless of pistons used to come up w/ the CR. you want.

pdq67

Does not matter car or truck. For rough planning just use a 119 chamber. Use the provided calculator or go to Flatlander.com and use the piston selection charts for each brand and compare.
As others have told you... to be exact you would have to get your heads cc'd at a shop.