What is the propper amount of compression in a 2.8??? I read somewhere that 100psi is normal. Is this true?

I'm taking out my motor in the next few days and if the engine has good compression i'll leave the bottom end alone. If its week then I guess its rebuild time. I was hopeing to save some cash and fix my clutch and oil leaks, etc. I live on gravel and don't want to put a load of money into this car. I'll do that with an 88 I want to buy later down the road.

mike

100 PSI is a little on the low side, but what's more important is the variance between the cylinder that has the highest compression vs. the lowest one. It shouldn't be more than 15 PSI or so.

My tired V6 has one cylinder that's 160 PSI, and one that's 90 PSI. It still runs great tho.

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Smokin Joe Fission
1985 Fiero SE 2.8

Perhaps I shouldn't have taken on a Fiero that the original owner said he liked to work on, especially after it spent some time 'submerged'. At least the car was free.

the stupid book that I have says 100psi, and just had to double check it to make sur I read it right.

mike

I have tested both my GT's in the past (about 5 months ago) and my 86GT was about 150 with the lowest reading at about 145 (cylinder #4) the car has 105,000 miles (I was told by a few people that this is great compression for this type of milage). I also tested my 87 GT and it gets about 135psi on all cylinders car has 136,000 miles.

The 100psi you mention is a base minimum # I believe!

Remember when testing compression to crank car at WOT (wide open throttle) for about 3 to 4 cranks.

If you have some low readings squirt alittle oil into the low cylinders and repeat the process your #'s should increase dramatically which means a topend rebuild is in order! If the topend is rebuilt I would suggest rebuilding the lowend aswell because with a fresh topend and a weak low end the low end could fail because of the increased compression from the topend rebuild!

Just my .02!

Good Luck!

Edited because I forgot to post milage!
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Keith F.

[This message has been edited by camon (edited 02-17-2004).]

Now if i'm not mistaken, at first if you have poor compression and you add oil and see it get better this is due to the rings. On the flip side if you add oil and see no change this is a valve problem. I know this is an old school of thought, just throwing things out there.

thanks

mike

quote Originally posted by mikey127: Now if i'm not mistaken, at first if you have poor compression and you add oil and see it get better this is due to the rings. On the flip side if you add oil and see no change this is a valve problem. I know this is an old school of thought, just throwing things out there. thanks mike

Yup, oil in the cylinders seals bad rings, raising compression. The best way to test for bad/worn valves is with a leak-down test with an air compressor and a couple of air pressure guages.

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Yep and pull the ecm fuses so you don't flood the engine. That could make your c/r seem lower than it is. I believe it should have 145 psi +or - 10 percent difference between worst and best readings. Good luck.

Phil

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87 FIERO GT 2.8 5spd

GM auto tech for 27 years. Specializing in electrical and computer problems. Now on workers comp. and it looks like I will be unable to return to work as a tech.