Johnny Jackets Posted April 9, 2021 Share Posted April 9, 2021 (edited) Please advise what the compression readings for a 1948 Pontiac 8 cyl Streamliner should be. Edited April 9, 2021 by Johnny Jackets Initial post not specific enough (see edit history) Link to comment Share on other sites More sharing options...
The 55er Posted April 9, 2021 Share Posted April 9, 2021 This is from a 1953 Chilton's Manual Pontiac Engine Tune-up Specifications page. 1 Link to comment Share on other sites More sharing options...
RussJagoau Posted April 13, 2021 Share Posted April 13, 2021 Its going to be tough to get 1000 rpm when you are cranking on the 6V starter so those compression figures are somewhat optimistic.!! It depends on the C.R of the engine but if its standard Compression ratio with a standard camshaft grind you should see approx 105 to 120 psig at operating temp with all the plugs removed and the throttle held wide open.... Its not so much the ultimate pressure but the variance between the cylinders to give you good cylinder balance and a smooth running engine. 1 Link to comment Share on other sites More sharing options...
Grimy Posted April 13, 2021 Share Posted April 13, 2021 3 minutes ago, RussJagoau said: Its going to be tough to get 1000 rpm when you are cranking on the 6V starter so those compression figures are somewhat optimistic.!! It depends on the C.R of the engine but if its standard Compression ratio with a standard camshaft grind you should see approx 105 to 120 psig at operating temp with all the plugs removed and the throttle held wide open.... Its not so much the ultimate pressure but the variance between the cylinders to give you good cylinder balance and a smooth running engine. Agreed! One would have to start an engine with each of eight plugs removed, one at a time, and then raise engine speed beyond idle to 1,000 rpm. A more useful Rule of Thumb is to multiply the compression ratio x 14.7 psig (at sea level) and use the product as a benchmark. For example, 6.5 c.r. x 14.7 psig = approx. 95 psi at cranking speed. Link to comment Share on other sites More sharing options...
PhilAndrews Posted April 23, 2021 Share Posted April 23, 2021 With all the plugs out, throttle wide open, cranking on the starter my '51 reads 105-115psi average, with the best about 125psi. That's about 150rpm according to the tachometer. Phil Link to comment Share on other sites More sharing options...
Grimy Posted April 23, 2021 Share Posted April 23, 2021 I'd call it GOOD and go enjoy the car! Afterthought for @PhilAndrews: I see you have a 12-volt coil--are you running 12V throughout? 1 Link to comment Share on other sites More sharing options...
PhilAndrews Posted April 23, 2021 Share Posted April 23, 2021 You spotted that, huh? Yes, the previous owner attempted to convert the car to 12V, burned all the gauges, radio etc up. So, I rewired the car, I re-wound the gauges for 12V, converted the guts of the radio to 12V, clock etc. It's easier if I need parts when out on the highway, not many places have any 6V stuff in stock, but mid 60's 12V parts are plentiful still. Phil Link to comment Share on other sites More sharing options...
Grimy Posted April 23, 2021 Share Posted April 23, 2021 16 minutes ago, PhilAndrews said: You spotted that, huh? Yes, the previous owner attempted to convert the car to 12V, burned all the gauges, radio etc up. So, I rewired the car, I re-wound the gauges for 12V, converted the guts of the radio to 12V, clock etc. It's easier if I need parts when out on the highway, not many places have any 6V stuff in stock, but mid 60's 12V parts are plentiful still. Got it, thanks. I haven't closely followed your adventures but see your posts from time to time. Congrats on making the best of a difficult situation! 1 Link to comment Share on other sites More sharing options...
PhilAndrews Posted April 23, 2021 Share Posted April 23, 2021 2 minutes ago, Grimy said: Got it, thanks. I haven't closely followed your adventures but see your posts from time to time. Congrats on making the best of a difficult situation! It's been fun! They're well engineered vehicles, with repairable parts. Link to comment Share on other sites More sharing options...
Grimy Posted April 23, 2021 Share Posted April 23, 2021 1 minute ago, PhilAndrews said: It's been fun! They're well engineered vehicles, with repairable parts. Agreed! I had a 1950 Pontiac sedan for a few years. Very solid and reliable. 2 Link to comment Share on other sites More sharing options...
Bloo Posted April 23, 2021 Share Posted April 23, 2021 As RussJagoau already mentioned, do not get hung up on absolute pressure, particularly if you are just trying to ascertain overall wear of the engine. Evenness between cylinders is what it is all about. That will tell you if the engine can most likely be tuned to run well. No two mechanics will come up with the same absolute numbers. A leakdown test will tell you more about the condition of the engine with regard to miles, ring wear, etc. but even that is probably no good if the car has been sitting around a long time. There will be crud on the valve faces, rings that stick, etc. This all makes the engine look worse than it actually is. 2 Link to comment Share on other sites More sharing options...
PhilAndrews Posted April 23, 2021 Share Posted April 23, 2021 1 hour ago, Bloo said: Evenness between cylinders is what it is all about. That will tell you if the engine can most likely be tuned to run well. Agreed here- #3 on mine is 105psi- the rest are 115-120 average. If you see the pressures within 10 psi of min to max it'll run nicely. You can hear #3 on mine with a gentle THUB each exhaust stroke but it's not enough to warrant doing anything drastic. Link to comment Share on other sites More sharing options...
Bloo Posted April 23, 2021 Share Posted April 23, 2021 20 minutes ago, PhilAndrews said: You can hear #3 on mine with a gentle THUB each exhaust stroke I wouldn't be concerned about 105 either, but I'm pretty sure you should not be hearing that. When you get time I suggest you nail down which cylinder does that (might not be the one with 105lb!) and then recheck the valve adjustment on that cylinder, ohm test the plug wire, see if there could be a vacuum leak near the intake port, etc. Link to comment Share on other sites More sharing options...
PhilAndrews Posted April 23, 2021 Share Posted April 23, 2021 (edited) 3 minutes ago, Bloo said: I wouldn't be concerned about 105 either, but I'm pretty sure you should not be hearing that. When you get time I suggest you nail down which cylinder does that (might not be the one with 105lb!) and then recheck the valve adjustment on that cylinder, ohm test the plug wire, see if there could be a vacuum leak near the intake port, etc. I got a lifter with a chewed up adjustment thread that likes to shift over a thousand miles or so, it opens up. It's got a gentle click to it right now,and I think it needs lapping in more on the intake valve but I really don't feel like pulling the head off right now... Edited April 23, 2021 by PhilAndrews (see edit history) 1 Link to comment Share on other sites More sharing options...
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