Thread: Calculating compression ratio

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I have a rebuilt 59ab, bored .125 over, 3.75 in. stroke, egge dome pistons, Edelbrock script heads, H & H cam. The block is factory relieved. Curious as to the compression ratio? This engine has not been started, I am waiting on tires ($) before I go to the muffler shop. Thanks

You'll have to pour everything with a burette and alcohol to know what it is for sure, but in the meantime, figure 8.00:1 from what I remember.

Originally Posted by techinspector1

You'll have to pour everything with a burette and alcohol to know what it is for sure, but in the meantime, figure 8.00:1 from what I remember.

Richard,
I'm curious about the burette process with the flathead. Can you give a high level overview step by step you'd go through?

Originally Posted by rspears

Richard,
I'm curious about the burette process with the flathead. Can you give a high level overview step by step you'd go through?

Heads off, put spark plugs in the heads with Vaseline on the threads and pour them like any other head, using a flat piece of clear Lexan or other suitable material. Put 'em on the bench so that you will be filling from the highest point, allowing air to come to the top.

For doing the piston and valve volume on the block, you'll have to make a fixture that spaces the Lexan up above the piston crown and the valve heads. Cheapest and easiest would be to use a old head gasket. Cut it apart and stack two, three or four pieces on top of each other and glue them together with silicone. If you don't have any old gaskets, NAPA sells singles for under 25 bucks.
http://www.napaonline.com/Search/Det...+50026+2026043
Or, you can bend up a spacer from solder or small-diameter copper tubing or rubber o-ring seal material or anything that you can get to conform to the cylinder and valve area on the block and then glue it to a flat piece of Lexan with a small hole drilled in it for filling your volumes.
Anyway, make your gasket sandwich and glue the Lexan on top of the sandwich with silicone to make a one-piece fixture out of it. Once it's good and set up, place it on any smooth level surface, seal it with Vaseline and pour it full of your colored alcohol measuring fluid. You want to find out what the volume of your fixture is. Let's say it was 78.3 cc's. I'm just picking a figure out of the air. It doesn't matter what the volume is, we just need to know what it is. Now, bring the piston you're going to check to TDC. Seal the valve perimeters with Vaseline. Seal the piston to the bore with Vaseline. Seal the edge of the fixture with Vaseline and seal it to the block. Pour the fixture. Let's say it pours 72.9 cc's. Deduct 72.9 from 78.3 and you find that you have 5.4 cc's of piston crown and valve head volume above the block deck. If this operation pours more than your fixture, then you will add that amount to the head volume, but I'll proceed here as if the piston/valves poured a little less than the fixture volume.

Now, let's say the combustion chambers in the heads poured 63 cc's. I think this is about what some of the Edelbrocks poured. Deduct the 5.4 cc's from the 63 cc's and find an actual volume in the heads of 57.6 cc's.

Using your clean, flat surface again and a flat piece of Lexan with a small hole drilled in it for filling, seal one cylinder of a head gasket to the surface with Vaseline. Seal your Lexan plate to the head gasket surface with Vaseline and pour the head gasket. We'll say one cylinder poured 14.3 cc's.

Now, let's figure the cylinder volume. Let's say the bore and stroke are 3 5/16" X 4 inches, so .7854 times 3.3125 times 3.3125 times 4 times 16.387 = 565 cc's.

OK, so we have....
565 in the cylinder
57.6 in the heads
14.3 in the gasket
All totaled, 636.9 cc's
Now, subtract out the cylinder volume and just add the head and gasket....
57.6 plus 14.3 = 71.9 cc's
Now, divide 71.9 into 636.9 and find a static compression ratio of 8.86:1

you could run it up on TDC use could use petroleum jelly but i use assembly lube . make up a plate out of thick plex and a old head gaskets . on pistons i have sleeve that have a given cc with a stop in them so they repeat so if you know the volume of the given sleeve then put the piston in it fill them subtrack the given number to fill the sleeve with out piston then with piston in the sleeve give you the cc of the piston .on dish tops i level them and fill them with out any flat plate . i fill a lot of heads on the mill i never use a plex cover you spend alot of time trying to get the air bubble out and on bbc the intake valve is over the deck i free hand them and you do enough you can get deadly with out the plate so if you know the head cc you could leve the one bank on the engine stand then you can put sealer/ petroleum jelly around the valves and piston.head gasket fire ring if you have a used gasket the same thickness and crank it down 40 pounds or so and fill till you top it off the spark plug that will give you the cc as well. i done this and knowing the ruff ccs of the head gasket first and the head then cc with the head on. then i know it can not be off to much is you do know the true cc of the head and gasket .to find the cc of a head gasket that s not a true round fire ring i pasted a used head gasket on my formica counter top and cc them

Richard & Pat,
Thanks for going over the details.

wouldn't there be a way to back calculate compression based on psi of a compression test. not as accurate, but a guideline ?