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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.
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.
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.
Last edited by Vintage Chief; December 15th, 2021 at 02:24 PM.
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.
Last edited by olds 307 and 403; December 15th, 2021 at 02:31 PM.
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
Completely agree with Joe. For comparison only, here are the results of my 1971 350 cid (120k miles) compression test.
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.
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.
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.
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,
December 15th, 2021, 12:54 PM
