I'm helping a friend find a 350 Olds engine and I need to find the range of acceptable compression in all 8 cylinders. He's working at getting a "low mileage" '68. My books are useless there.

 

To be honest, the relative differences from cylinder-to-cylinder are far more important than the absolute numbers. Variables like cranking speed and oil coating in the cylinders will cause a dramatic variation in the PSI you get. A leakdown test is usually a better indication of engine health.

 

Thanks Joe. What are the acceptable differentials then, 5% 10%, 14 pounds? and leakdown rate?

 

Completely agree with Joe. For comparison only, here are the results of my 1971 350 cid (120k miles) compression test.

EDIT: I performed the compression test (wet & dry) w/ the engine "cold". I keep forgetting to update my image. You can/should expect a higher compression reading (as the metal expands) w/ an engine at normal operating temperature.

 

Most shop manuals mention around a 20% variance maximum for compression. Most online sources say 10% for compression variance. Online sources say 20% for leak down. A healthy 8 to 1 or 9 to 1 compression Olds 350 with the stock peanut cam, you would want around 150 psi. A 10 to 1 350, closer to 200 psi. Oil pressure is also another great check, tells a lot. I would want to see 10 psi at absolute minimum at hot idle in gear and 40 to 50 psi hot cruise oil pressure with a 10W30. Hopefully you can find a running motor that someone is swapping an LS in its place. Otherwise you are at the seller's mercy. Good luck.

 

Thank you! In my experiences, the seller has an inflated view of their engine, while the buyer has to be pretty trusting. He simply wants to swap in a good runner and not overhaul it. Compression tells a lot and leakdown

 

Good info and thanks for it!

 

Note that a 1971 350 engine would have around 8:1 compression from the factory, whereas a 1968 350 would have 10.25:1 (4bbl version) or 9:1 (2bbl version). So add some to the above numbers for a 4bbl version.

 

I will say 10-15% maximum variation between cylinders with a minimum 120psi. Anything else is suspect. You can start it on the ground; don't need some fancy run stand. Just don't run it long.

 

X2

I had a guy try to sell me an engine by starting it hanging from the hoist !!!

a quick comp test after that showed 1 sub 100PSI cylinder

 

My original shim head gaskets 1976 350, so 7.9 to 1, supposedly from a 90,000 Km from an old man's Delta 88, we have all heard that. He wanted $150, I said $120, he agreed. Pulled the pan, the bearings and crank looked new, cylinders looked nice, no obvious wear or too much glaze. I threw my Performer cam in advanced 2 degrees. It had 140 to 142 psi all 8 cylinders with thr bigger cam. It had 20 hot idle in gear, 25 hot in park and 45 to 50 hot cruise oil pressure with 5W30 oil. I got 7 years of abuse, including many 1/8 mile runs until the second cam went flat, best $120 I ever spent. Unless it can be started, heard running and an oil gauge can be put on, it is a $200 motor, maybe slightly more, depends on the seller. Most guys here are honest. Otherwise he is buying a core.

 

Chuck,
I don't know what your friends budget is but chances of finding a decent used engine that he can just put in the car are slim to none. I do this stuff every single day and every time somebody buys a used engine they end up sorry. Tell him to just find something that is a rebuildable core and go from there.

 

8:1 rated engine that is very good just fine. I have had high mileage engines making 125# that didnt smoke and ran great too

 

We found that out thanks to your warnings! He bought a '73 2 BBL with "102K" miles. 7 of 8 were in the 100's; #8 was the actual #8 cylinder and the reason they quit driving it, puffs of smoke on startup (I suspect the valve guides) but it seemed to be to be a good bet. It looks all original. no aftermarket stuff. Number 8 plug was a little oily. It was a 4 door 88 and all the brackets, pulleys and ancillaries came with it save the A/C compressor. Hopefully the machine shop will verify it is a good one. He paid $800 for all. The oil pain has a kink in it but shouldn't cause an oiling problem. Most likely I will watch over his project, but I am still working to get my own working and it has not been my finest hours,