I need help achieving my compression ratio (9.5-1)
The bore will be 4.020 with the stock stroke 3.622. I want to use a thin head gasket and Im undecided on heads but they will be anywhere from 68-72cc.

So how deep of a dish need to be in the pistons and what head gaskets do you recommend?

 

Go to wallaceracingcalculator.com and play with the compression calculator until you come up with the figure you want

 

Never use the head gasket to manipulate compression. That's what heads and pistons are for.

Use the gasket that gets your quench as close to 0.040" as possible. If your quench is much under that, you run the risk of head/piston contact. If it's much greater than that, you'll get detonation and poor performance.

 

Find out how much the pistons come out of the hole, or in the hole

That will dictate which thickness gasket to run to reach optimal quench..

You really can't find an accurate CR calculation without knowing all of the specs of the engine.

Bore size
Stroke
Rod length
Piston comp height
Deck height
Head gasket bore
Head gasket thickness
Head CC size

This is off the top of my head, so you only know the bore size and that you want to be stock stroke? Anymore info?

If nobody beats me to it I'll reply after work with a engine gameplan for you to get started

 

here's 1 example I did quick, you can tweak it based off which rod/piston you choose

Bore 4.020''
Stroke 3.622''
Rod Length 6.125''
Piston Comp. Height 1.315''
Piston Dome cc -8.00 cc
Head Comb. Chamber cc 72 cc stock 317 heads
Head Gasket Thickness 0.053'' GM gasket
Block Deck Height 9.240''
Cylinders 8
Head Gasket Bore 4.060''

this is roughly 9.47:1 - 9.5:1 static CR

this is based off using a typical 6.125 aftermarket rod set and typical aftermarket dished pistons (diamond, mahle, etc...) in the bore you want...

 

That's what heads and pistons are for.

 

Bore 4.020''
Stroke 3.622''
Rod Length 6.125''
Piston Comp. Height ?
Piston Dome cc ? cc
Head Comb. Chamber cc 64 cc
Head Gasket Thickness 0.052'' GM gasket
Block Deck Height 9.240" (stock 03 6.0?)
Cylinders 8
Head Gasket Bore 4.060''

Thats about all I know, I need to know how much of a dish will get me to 9.5:1?

 

Gm gaskets are .052-.053


But just based on what you posted above you would need a -18cc dish to achieve 9.53:1 CR

Now this is based off your specs you gave..

You should try and find a head gasket .030 bigger than the cylinder bore!

Double check the piston comp height on whichever piston your looking at

 

Yeah your right, I just went through my messages. The gaskets are .052 and 4.060 I changed the info above to match.

 

-15cc piston will give you 9.59:1 CR
-16cc piston will give you 9.49:1 CR

 

Ok, thanks for the help.
Just curious about this one but how much would a machine shop typically shave off the deck height to make sure it's flat and how much would that affect everything?

 

A good machine shop will shave off just enough to make the deck flat and parallel. How much that is depends on the block they're working on.

It changes the piston deck height, which requires a different head gasket.

 

Yeah after they deck the block, whoever assembles the shortblock should measure how much the pistons come out of the hole..

Then pick the head gasket for however much quench area you prefer..

 

I come up with the same numbers Burken posted in post #10, which is assuming the piston is .011 above the deck. Your calculated piston compression height would be 1.304.