Truck engine
- Thread starter jester
- Start date
I believe 7.75:1. 
Tough question, to answer, Jester. I think it depends on engine history or how it was maintained. I had small chevy's with 80 lbs. in almost all cylinders that ran pretty decent, but used oil. I also had a couple with over 100 lbs. that ran like mechanical a**holes.
I think it depends on engine history or how it was maintained. I had small chevy's with 80 lbs. in almost all cylinders that ran pretty decent, but used oil. I also had a couple with over 100 lbs. that ran like mechanical a**holes.
Stick62
Well Known Member
It seems that that motor is 7.75:1 compression. That is what you need to know, to know accurately how much compression you should have. An 11:1 motor for example would be higher.
Prob can look up the spec somewhere on this site, but I dont know where or how.... haven't played with that much.
How much do you have, and how much variance are you getting from cylinder to cylinder? Is one down way farther down than the rest?
You can also do a "wet" compression test on each cylinder..... after you test it dry, put some oil in and check it again.... little old school, but I don't know what your workin with.
Also......if you have a leakdown tester, leak the motor if you think you have a problem.
Prob can look up the spec somewhere on this site, but I dont know where or how.... haven't played with that much.
How much do you have, and how much variance are you getting from cylinder to cylinder? Is one down way farther down than the rest?
You can also do a "wet" compression test on each cylinder..... after you test it dry, put some oil in and check it again.... little old school, but I don't know what your workin with.
Also......if you have a leakdown tester, leak the motor if you think you have a problem.
OK, let me rephrase the question again... If I had a one cylinder engine , say with a 4.135 bore and a stroke of say 3" how may lbs psi of compression would be required to cause internal combustion . If it has ,say 50psi do you think it would fire. would it need 80 or 100 psi. I have this truck engine with 50 lbs psi in all cylinders . My friend say that it might be from sitting and that it might improve once I get it started. I don't think it will start with 50 psi. Anyone have any ideas?
larry t
Well Known Member
If the compression is low because it has been sitting, you can pull the plugs, put a couple of squirts of oil in each cylinder, spin the engine over a few times and then check the compression again. If the rings were dry or the cylinders washed down, this will seal the rings enough to bring the compression back up. If it doesn't, you've got more trouble than an engine that has been sitting.
Larry T
Richard/SIA
Well Known Member
My experience, it won't run. Might pop occasionally, but not run.
Not enough power per cylinder to keep it turning over for the next cylinder to fire once you let off the starter.
If I'm wrong I bet the engine is REALLY loose and has virtually no oil pressure.
You have a core to rebuild.
Not enough power per cylinder to keep it turning over for the next cylinder to fire once you let off the starter.
If I'm wrong I bet the engine is REALLY loose and has virtually no oil pressure.
You have a core to rebuild.
I don't think it will run either. I do remember reading something about your question many years ago. I seem to recall that the answer isn't simple(what is anymore) like it takes a certain psi for internal combustion to occur. Maybe someone with a clear head can remember
My bet is on tripower to tell us this one!
robert
My bet is on tripower to tell us this one!robert
Actually, the oil; pressure was suprisingly good for what I was looking at. The engine came out of a local veggie delivery truck . The odometer had 38000. Could be 138 or 238, Who knows. The brake and clutch peddle pads were almost perfect which made think it was a low mileage engine . I know that if I install a manual trans and push it at 40 mph if wil probably run but that's alot of work for a slim chance. I'm going to do the oil and the leak down tests before I decide,Not enough power per cylinder to keep it turning over for the next cylinder to fire once you let off the starter.
If I'm wrong I bet the engine is REALLY loose and has virtually no oil pressure.
You have a core to rebuild.
Thanks for the help guys
Happy new year to all
If it's a low milage engine thats been sitting for a long period of time, chances are that if you lube the cylinders enough and turn it over on the starter with the plugs out, install new plugs, make sure it's getting good spark and fuel, maybe mix a little Marvel mystery oil with the gas and give it a whirl the rings would reseat and you would have a pretty good engine.
My thinking would be that as long as you have gas, air, and a spark the engine will fire. It may not have any power and you may get a lot of blow by, but it should fire. I know I have cranked a lot of engines which have been setting in junk yards for years and as long as you have those three ingredients, they will fire. They may not continue to run, but unless the rings are completely stuck, they will eventually run.
Poocho
Poocho
Hi Put Marvelous Mystery Oil Into The Cylinders -CRANKCASE And The Gas -after Pulling All The Plugs- Spin The Engine -to Lube And Loosen The Rings-it Is A Thin Oil And Will Get Into Places-- Thicker Oils Can Not-and As A Result Loosen Up The Rings Quicker- My Dads Airplanes Compression Was Not Coming Up To The Annuals Recommendations-the Mechanic Used Mystery Oil -and The Compression Did Come Up -to Pass The Test----once Running Check Out A Product Called-engine Restorer- (any Auto Store ) It Has Teflon In It And When Added To Motor Oil -will Equal Cylinder Compression Across -all Cylinders-by Smoothing The Cylinder Walls-i Saw Tests Run On The Stuff And Watched Cylinders With Uneven Compression----all Equal Out- Used It On A Van With 100,000 Miles-with Good Results--it Is Recommended For Any Engine Over 50,000 Miles- To Improve Smoothness And Horse Power -one Fellow Said It Stopped His Rear Bearing Seal From Leaking -i Am A Consultant On Chevy 409s-(SUPER STOCK RECORD HOLDER ) 64 Cadillac Collector Antiques And Chrysler Mini Vans- on Several Web Sites- The Mystery Oil Is A Common Solution To Getting A Sitting Engine Back Up And Running -the Engine Restorer -is My Own -good Experience-- Just My 2 Cents-- Happy New Year --regards ---tony
Depends on whether you're taling about cranking pressure or static pressure as in a leak down compression test. Minimum cranking pressure on the passenger car 348 with hydraulic lifters is 160 psi for tune up purposes and no more than 20 psi variation between cylinders. I would expect the lower compression truck engine to be somewhat lower, maybe 150psi like the 283 or 130 psi like the L-6. Static compression , as in leakdown check, is normally based on 80 psi constant air into the cylinder at TDC on compression stroke with the resultant pressure difference caused by leaking rings, valves, head gasket, etc. depending on how loose it is. Again there should not be a significant difference between cylinders but a reading of 60 psi on leakdown wouldn't be too significant as long as they are all within a certain percentage of each other.
Oil it up, stake the valves if you get a low reading on a cyl and check it again. Alow reading between 2 adjacent cyls could be a head gasket blown out between the two.
Oil it up, stake the valves if you get a low reading on a cyl and check it again. Alow reading between 2 adjacent cyls could be a head gasket blown out between the two.