Hey,

Can anyone tell me what the stock compression in PSI would be for my 1988 305 TPI? I'm planning to do the test today, just want to compair to stock.

Thanks

 

There's too many variables to give you one "number" and say that if it's not that, it's bad.

But... expect somewhere between 165 - 185 psi.

More important than the actual "number", is whether all 8 are THE SAME, within 5-10 psi. As long as all of them are more than 150 or so but less than 200, and they're within about 10 psi, they're fine.

Don't forget: remove the fuel pump fuse (or relay), unplug the 12V wires from the distributor, remove all 8 spark plugs, and block the throttle wide open. Make sure you have a good hot battery in it, such that the engine spins over at the same speed on the 8th cyl as it did on the 1st (charge it from time to time if necessary). Do the test with the engine at a consistent temperature; if it's hot, or even warm, when you start but cold on the last few, those last ones will read WAY lower than the first. All of these things will help keep you from having "mystery" variations introduced by extraneous factors OTHER THAN the actual compression, which will confound your results and make them useless.

 

Thanks,

All I needed was a normal stock reading, what the compression is when new. I can diagnose from there. Although I didn't think about removing the fuel pump fuse.

 

There is NO SUCH number. Even if you find one in a factory service manual for NEW cars, it won't necessarily apply to an OLD motor. That is, the "number" can be SUBSTANTIALLY different, and there still not be a problem. IN fact it probably will be.

There's too many variables for there to be one such number, and it to be valid. Wrong question to ask.

Don't worry about the "number" you get. Now that you know how to execute the test properly, look for the numbers to be consistent; NOT some one particular "number". That's the wrong way to attempt to interpret the results.

 

Agreed, that the numbers are consistent is the most important factor. But with some kind of idea what the test would be on a new engine will give me a starting point.

The chances that are the cylinders are within 10% I think would be unuassual as the engine has alot of miles on it. The test is to give me an idea what work is required so that I can get "all my ducks in a row", so to speak.

Thanks again for your help.

 

Put it this way. GM didn't put stock compression numbers in either of my 2 Helms manuals. All that's there is what sofa has said.