compresson test results
Thread Starter
Burning Brakes
Joined: Dec 2002
Posts: 1,088
Likes: 0
From: Lakeland fl
compresson test results
HELP !, I have a known poor condition 350 in my 85 L98 TPI. It is completely stock as far as I know. It smokes a little when it runs (at start-up, idle and cruise). So today I decided to do a compression test just to see which cylinders were bad. To my surprise, they all tested between 58 and 62 PSI while cranking. Now Ive never tested a stock smog type motor before but this seems very low! My 71 Nova 355 (which I know has a higher comp. ratio) checks at 180psi. By comparison I'd say the rings are all completely shot, BUT the rule of thumb is "as long as theyre within 10%(of each other)" which they are. Is this motor just that low compression and the stock cam limiting its cranking pressure THAT MUCH????
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Re: compresson test results (SBNova)
Hmm, doesn't sound right to me. My stock LT1 numbers were probably close to double that. There isn't THAT much difference between an LT1 and an L98 motor. I would say that you are loosing cylinder pressure somewhere.
What is very helpful is to do a leakdown test, it gives you a little more info than a compression test and you should be able to hear if it's leaking out the valves or around the rings. :cheers:
What is very helpful is to do a leakdown test, it gives you a little more info than a compression test and you should be able to hear if it's leaking out the valves or around the rings. :cheers:
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Re: compresson test results (SBNova)
Assuming it was a proper test with all the plugs removed, throttle blades open, strong battery, etc. that is horribly low. My 1984, 150,000 mile, quart every 200 miles, wheezer, has compression from a low of 165 to a high of 175. Smoke at start up and jumping on the gas after deceleration, indicates leaking valve guides. Cruise and/or steady state oil burning points to rings. Mine is scheduled for replacement. I hope yours was a "bad" test. Good luck, and...
RACE ON!!!
RACE ON!!!
Thread Starter
Burning Brakes
Joined: Dec 2002
Posts: 1,088
Likes: 0
From: Lakeland fl
Re: compresson test results (flyersfan1088)
funny thing is- it really runs great! No comparison to my Nova of course, but it starts right up, idles and drives well! Seems peppy, but I drive a 400+ HP car every day, so a 230 whatever HP car just doesnt seem like much anyway. I think I'll try to check the calibration of my compression tester. 60 psi doesnt seem right to me either. Thanks for the replies
Former Vendor
Joined: Jun 2001
Posts: 7,204
Likes: 6
From: St Marys PA
Re: compresson test results (SBNova)
Do yourself a favor and do a leakdown test...I did my compression about 5-8 times trying to figure out where my power went with my 350 and everytime it was ~160psi +/- 2 psi....Did the leakdown and knew in a about 15 minutes something was amiss. Pulled the engine apart and every top ring was busted in two. I will NEVER trust a compression check.
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Re: compresson test results (SBNova)
I don't think that's even possible. I'd think with cylinder pressures that low, the gas would never burn, or burn all the way. It definitely would not run well, if at all. Have you tried again with another guage or spark plug hole fitting? If the numbers are that low and consistent, it almost seems like a rebuild gone wrong, or maybe cam timing being wayyy off. It just seems doubtful that broken rings would still be that consistent.
You did pull all the plugs and turn the motor with the starter right?
I had between 180 and 190 psi on my stock '85. As the engine cooled, the numbers dropped noticeably, (I did three tests)
You did pull all the plugs and turn the motor with the starter right?
I had between 180 and 190 psi on my stock '85. As the engine cooled, the numbers dropped noticeably, (I did three tests)
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Melting Slicks
Joined: Jan 2002
Posts: 2,784
Likes: 3
From: Atlanta GA
Re: compresson test results (ski_dwn_it)
Do yourself a favor and do a leakdown test...I did my compression about 5-8 times trying to figure out where my power went with my 350 and everytime it was ~160psi +/- 2 psi....Did the leakdown and knew in a about 15 minutes something was amiss. Pulled the engine apart and every top ring was busted in two. I will NEVER trust a compression check.