I own an 89 Firebird with a 305. I was wondering what the compression should read in pounds per square inch. I performed the test and came up with 150 psi per cylinder and I was wondering if this is what it is supposed to be, or is it low. Also how long are you suppossed to crank the motor when you are performing the test?

 

as far as cranking compression goes, it can vary from engine to engine, the key to running a compression test is to see if any of the cylinders are below 10% of the 'normal' cylinders, that is, if they all are 150psi +- 15 psi of each other, then the engine is in good shape as far as compression. 150 lbs sounds about right for a 305 engine, with reasonable mileage. If any of the cylinders are far below what the rest read(say, only 70 psi), you can be assured that there is either a bad valve, weak or broken ring(s), or significant cylinder wall scoring. You should only have to crank the engine to get a solid reading, a few revolutions should be fine. As a note, you can tell if the problem is ring/cylinder related, or valve related by performing the test again after squirting some oil into the cylinder. A cylinder that has valve problems will read the same number, whereas an engine w/ mild ring weakness or scoring will jump up in pressure. Hope this helps.

 

That is a good point about the oil compression trick. My friend just bought an old jeep. We noticed that with the oil the compression was up again so now we know it's the piston ring or block .

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, Jon (350 TBI!)
91 Red My website

 

Saus,

Test the engine warm if possible. Remove all the plugs. Charge the battery fully. Hold the throttle open fully. Crank the engine at least four full cycles (eight revolutions). Your compression should be at about the peak reading, and additional cranking probably won't get any more pressure built in the gauge.

150 PSIG is pretty good for a flat-topped piston 305, especially if you tested it cold with the throttle closed.

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
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