Compression Check Test Results... Counsel Needed

[Fishman]

My very good friend is looking at purchasing a 1974 L82 4-speed vert and asked me a few questions that I could not answer. Please offer some advice as I think I want him to get a Vette as bad as he wants to get one!

He took the car in for an inspection before making any sort of hasty decision (unlike my purchasing decisions). Overall the car got very high marks from the person with a few concerns about the original engine. My friend just sent me the following in an e-mail and we would both value your opinions. Here is what he wrote…
___________________________________________________________
1974 L82 matching numbers no mods
Original 4-bbl / intake / valve covers / air cleaner / manifolds / exhaust

Compression test results:
(From the drivers seat to the front)
Left 1: 100
Left 2: 90
Left 3: 80
Left 4: 80
Right 1: 75
Right 2: 70
Right 3: 60
Right 4: 60

The mechanic says +/- 10 is typical using non-computerized testing

The car has been in storage approx 1-3 yrs

Mechanic recommends valve job and maybe complete overhaul / rebuild

Other:
Car drives excellent
Pulls strong to redline
No smoke at start or any running speeds
Slight oil buildup on back of pan and bell housing (rear main seal?)
No leaks hitting the floor
____________________________________________________


As always… thanking you in advance for your replies.

Brian (& Mike)

 

[photovette]

#6 & #8 cylinders (your right 3 & 4) could be damaged from the EFE valve sticking.Those numbers look pretty low overall--I wouldn't be happy if it were my motor.

 

[bradfordsvettes]

you need to do a wet and dry test to make a correct evaluation or you could use a leak down tester.that would be the only way to an accurate reading and an evaluation

 

[JohnZ]

If the test was done correctly (warm engine, throttle blocked wide open), the numbers are about 50% of what they should be; a leakdown test will show where the compression is going (valves or rings or both). Sounds like a very tired engine in need of an overhaul.

 

[black_81_vette]

what kind of cam is in it? a big duration cam kills cranking psi

 

[74L82]

i'm planning to have another compr test done by local vette expert (shouldve gone there first)...car runs too good for those numbers to be correct. thanks for your feedback

 

[Fishman]

i'm planning to have another compr test done by local vette expert (shouldve gone there first)...car runs too good for those numbers to be correct. thanks for your feedback

Hey Mike (74L82)... glad you joined us! Welcome to the

Now I don't have to be the middle man :L

 

[bobs77vet]

i'm planning to have another compr test done by local vette expert (shouldve gone there first)...car runs too good for those numbers to be correct. thanks for your feedback

have them put some oil in the cylinders and repeat the test this will show if the rings are bad or if it is the valves...

 

[74L82]

i'm taking note of everyone's suggestions to take with me when our local expert checks everything out...i'm sure he's gonna love that !
thanks for all the great feedback...i'll let everyone know what we find out...

 

[Hib Halverson]

Was this compression test done with:
The engine at operating temperature?
Throttle blocked wide open?
3 revs per cylinder?

Also, what's the mileage on the car?

 

[74L82]

Was this compression test done with:
The engine at operating temperature?
Throttle blocked wide open?
3 revs per cylinder?

Also, what's the mileage on the car?

i wasnt there for the test and cant answer your questions...engine certainly wasnt cold as it was only in the shop for 2 hrs (i drove it there). i am having another test done by reputable corvette shop instead of inexpensive novice friend of friend who had a lift etc...what the first inspection accomplished is the car is overall very sound and apparently matching numbers but needs further motor eval

mileage unknown...speedo/odo are inop...indicates 12k...multiple opinions based upon car's condition are 112K...
car definitely sat for extended time...cobwebbing throughout
recent updates include: battery / alt / calipers / pads
any suggestions on tracking vehicle history ? its a california title

having owned a pristine and trouble-free 75 L82 4 sp for 10 years (NCM Corvette Caravan participant) I am amazed at how great this 74 drives and how strong the motor "appears"...

thanks for your feeback

 

[craig32]

With two extremely low cylinders side by side, there is a possibility of a blown head gasket between the two. Also seems kind of odd that the numbers go down in succession - did you do the left bank front to back, then the right the same way? Maybe a faulty gague?? As for the oil buildup on the back of the pan, I'm thinking if you had a rear main leak there would be oil on the ground (did you check after you ran it?) possibly lots of it. Just my opinion, I could be wrong.
Craig

 

[74L82]

With two extremely low cylinders side by side, there is a possibility of a blown head gasket between the two. Also seems kind of odd that the numbers go down in succession - did you do the left bank front to back, then the right the same way? Maybe a faulty gague?? As for the oil buildup on the back of the pan, I'm thinking if you had a rear main leak there would be oil on the ground (did you check after you ran it?) possibly lots of it. Just my opinion, I could be wrong.
Craig

i wasnt there for the test so cant answer your questions. i'm having another test done by vette expert asap
no leak after it was run pretty hard (15 miles @ 65-70mph)
thanks