Compression Ratio Help?

[scatpackjimmy]

I have a 72 30 over 360 it has ported J heads with 2.02 1.60 pistons are hyper sterling 405p heads have been milled .10 and Im running a thin head gasket I think .25 or .29 can't remember. Any rough guesses on cr? Thanks

[DartGT66]

The pistons should be around 0.11" beloiw the deck at TDC, and they have a dish of about 8cc. Assuming the heads were around 70 cc bfefire milling and around 67cc with the cut, you should have the CR at about 8.1-8.2:1. Oops, dou you really mean thath the heads were milled 0.10"? I thought they were milled .010". If they wre truely milled 0.1" then you should be around 10:1.

[cudabob496]

how much better is it to have zero deck height, instead, for example, the .11" mentioned above?

[DartGT66]

Typically with open chamber heads it doesn't matter much. Either you should have a relaitvely tight quench, about .040", or a lot of room between the chamber and piston top = over 0.12".

[dirttrackracer]

I have the same pistons in a stock un decked block they are .054" in the hole.

[DartGT66]

There are variations, I calculated it with the advertised compression height and assumed the block height was 9.60". 1.576"+ (3.58"/2) + 6.123" = 9.489". Like with anything considering these not so accurate 30+ year old products, the only way to know is to measure.

[Cudadrag]

Originally Posted by DartGT66

Like with anything considering these not so accurate 30+ year old products, the only way to know is to measure.

This is very true. I was suprised after assembling my 1974 400 block. I was expecting about a 11.5:1 - 11.7:1 compression after purchasing rods, pistons and having the heads setup. After assembly the pistons were closer to zero deck than expected. I had the block squared with very little shaving needed. But really did not measure the deck height assuming it was near stock. Well someone in the past had to have shaved the block in their attempt for a build. (This would explain why the block squared up so easily) My comp measures about 12.7:1. (with a .027" head gasket that was also purchased before assembly) Being I was planning on using E-85 fuel this was no problem thus it was a pleasent surprise. However, I also got lucky that the block was not shaved too much. (The B1 heads are closed chamber thus if over zero deck I would have been screwed) So I guess what I'm saying, assume nothing!

[scatpackjimmy]

Update I got new pistons. I now have kb 107's that are suppose to be 9.8 to 1 with 72 cc heads I am running the thin gasket I think .o29 or .025 can't remember ? The heads have been milled .010 also so Im thinking around 10.0 to 1 now? Should be better.

[scatpackjimmy]

By the way I have the pistons mentioned for sale already pressed onto the rods with hastings moly rings for 250.00 if anyone is interested give me a shout.