Sljs3413

Thinking of buying a 90 zr1 the guy was nice enough to run a compression test for me. Car has 42k are these numbers

175, 190, 205, 185, 190, 200, 185, 195

Thanks I'm new to the lt5 and still learning

DanielRicany

Yes.

Sljs3413

The 175 to 205 isn't a problem?

Sljs3413

Anyone else? I want to make sure I get a healthy motor. I've heard no more than 15% different

gbrtng

16 numbers ? I'm confused (again) ...

Sljs3413

That was the numbers I was sent, I guess he did It twice that's what I figured

Sljs3413

Update....I got the numbers corrected, they were just sent twice.

Rkreigh

not bad, which cyl is 175

Sljs3413

I'm not sure. It was the first number so whatever order he did it in is 175 bad?

Sljs3413

Cylinder 1 is 175

lgaff

Carbon buildup can cause the variance. I picked up 15 PSI by cleaning them....there have even been motors diagnosed as bad cylinders that were just victims of carbon buildup

mike100

there's a lot of compression test variation. could be valve seal or worn rings on the low cylinder, but... it could also be off a little more if the engine wasn't warm before the test or if the each cylinder was cranked a different number of engine revolutions. I usually go 5 or 6 turns...not forever until the needle peaks on the gauge.

btw, a poor man's leakdown test is to see how much psi the very first compression stroke yields on the compression gauge. I seem to recall it was 90-100 psi on the LT5.

phrogs

Need to do a leakdown test to see where the leaks are at.

there are plenty of variations to do these test, some say it should be on a hot engine, others say it can be cold.

I always remember being told not to remove the spark plug on a hot aluminum head so I do mine cold.

perhaps it needs a good hard run when its hot to burn out the carbon. Seems all these cars don't get driven correctly and they build up carbon doing the speed limit everywhere.

wsaugenstein

I just bought my 1991 ZR-1 with 37,000 miles a few weeks ago, and all the numbers were between 196 and 201.

Paul Workman

In the first place, you're w/in 15% low-to-high

Second, the mileage is quite low.

Then, how was the test run? For example, was the battery fully charged when the test began?
Was the battery charge brought back to full before conducting subsequent tests on remaining cylinders?
Was the motor hot or cold?

pumping a couple squirts of motor oil into a low cylinder and then doing another test: if the pressure comes up significantly, there may be a problem with the rings, If not, then the valve(s) may be carboned up or the valve seat is in question.

Also, depending on the cam phasing will affect pressure. IIRC, mine picked up about 10 PSI overall from stock phasing to that phasing adjustment as part of the 500+ hp upgrade. (It would balance a nickle on its edge while idling w/ the stock phasing. Not now!)

Also, if the phasing slips one one or more cams, the pressure will be affected. (Some have reported their cam or cams being as much as 4º out of spec., which can affect the overall performance some.)

With as low mileage as you have, I wouldn't get too excited. When you get a chance, run it fairly aggressively for a few hours, let it cool, pull all the plugs, put it on a battery charger to assure the battery is charged uniformly between tests, and redoux the test yourself and see what you come up with!

Let us know what you find.

Hib Halverson

If the compression test is going to make or break the sale, I'd rerun the test, but make sure of the following:

1) Battery in good condition and fully charged
2) All the plugs out
3) Throttle blocked or held wide open
4) Crank engine for four compression strokes on each cylinder

You want no more than 15% between highest and lowest.

GM service information states compression should be 150-200 psi.

Dynomite

I just did a compression check on a 95' and found great compression of 215-220 on all 8 cylinders without TB plates open. All injectors checked out at 12.4 Ohms. Use a bit of antiseize on Header Bolts with the Stage 8 Locks. Also use a bit of antiseize on all spark plugs.


1995 LT5 SPECIFIC TOP END REBUILD TRICKS
A 1995 Top End was rebuilt and best tricks linked here
Post 230 - 1995 LT5 SPECIFIC TOP END REBUILD TRICKS
Post 231 - Clean Up of Valley and Removal Of Crankcase Cover (gasket replaced)
Post 232 - Installation of Rebuilt Starter and New Alternator
Post 233 - Battery Disconnect Switch and Installation of Coils
Post 234 - Installation of New Secondary Port Vacuum System hoses
Post 235 - Installation of Reconditioned Injectors with New "O" Rings
Post 236 - Installation/Relocation of Plug Wires and New Spark Plugs
Post 237 - Blocking TB Coolant
Post 238 - Reconditioning and Installation of Plenum with SS Allen Head Bolts
Post 239 - Addition of Billet Aluminum Power Steering, Water Pump, Belt Tensioner Pulleys
Post 240 - Reconditioned 1995 Engine with New Fluids all around including Differential Drain Installed

On three other 90's and one other 91' ALL Compression Test numbers without TB Plates Open were right around 210-215 ALL within 5% of each other. This using 4 compressions with ALL spark plugs removed and a fully charged battery.



Easy to remove even the Number 8 Spark Plug using one of these Blackhawk 5/8 Spark Plug Swivel Socket 3/8" Drive.



The 15% would be 30 lbs which seems excessively high and the 150 lbs seems very low especially with TB Plates Open. If I got 200 lbs on one cylinder and then 170 lbs on another I would be removing the engine