Determining Compression Ratio

Is there anyway to determine approximately what the compression ratio is on an engine by putting a compression tester on it and doing some sort of equation with how much PSI you get on the gauge?

 

nope. Too many variables. Cylinder leak down, valve timing, valve overlap, valve leakage.

 

you can get a real rough idea by dividing the cranking pressure by the local air pressure (which will be 14.7 psi or less)

 

in Houston I'm at about sea level. Well I am going to look at a running 350/350 combo in a 69 camaro. The guy said the block and engine have not been touched since he bought it. He his going to a date coded block. Anyhow he said it does not smoke and it would do a one wheel peel on that non posi camaro. What else can I look and listen for?

 

Now if you had the engine on a stand and had it turned just right you could fill a cylinder with a graduated cylinder like the ones we used in chemistry cl*** marked off in milliliters. Measure how much volumn it takes to fill the cylinder head completely up to the bottom of the spark plug hole with the piston at TDC. Then roll the crank till the same piston is at BDC and see how much more it takes to fill it again. Divide the large volumn by the small volumn and you would get the static compression ratio. Of course you would have to make sure that the spark plug hole was the highest point so there would not be any air trapped inside.

 

You can use the cranking compression to get an idea of how pump gas friendly the motor might be. There are ALOT of factors that effect this number, but for most street oriented engines the general rule of thumb is that if the cranking compression is under 200PSI it should run on pump premium. Most performance oriented motors with camshafts of reasonable duration and realistic static compression ratios will generate between 165 and 200 psi while cranking. If your over 200psi there's a good chance it'll need race gas.

-Bigchief.

 

Lobucrod is telling you the straight stuff.

 

I was told it does not smoke. I would like to pull a valve cover and get the casting numbers off the heads, but I might get some looks if I ask to do that. If it runs nicely do you think it is worth the $700 the guy is asking which includes a Turbo 350? I guess I could always through a cam and some summit 2.02 heads on it and see where that takes me.

 

if you get some looks trying to figure out what you're laying out $700 for, then move on.....

 

I agree. Anyone not liking when you look at what they're selling is trying to hide something.

They could be trying to give you ****py type heads that are prone to cracking for all you know.

 

I went and saw the motor, which is still in the 69 camaro. It ran real smooth, didn't knock or smoke, didn't stumble when given gas. I am just clueless as to what kind of heads it has because I did not pull the valve covers. The trans must be rebuilt recently, because the fluid is so clean(Not using the trans anyhow but for resale purposes) A very mild build from what I could tell, or course I am going to a solid flat cam anyhow, and a set of heads could really waken it up, aslong as the bottom end is strong.