What do the leak down numbers mean with 95psi applied?? 1957 301 v8
- Thread starter Rusty Muffler
- Start date
read this and draw your own conclusions
Rusty Muffler - you realize you have to test each cylinder at it's TDC position - to insure the valves are closed. Or take the valve covers off and loosen the bolts holding the rocker arm shaft so that all valves are closed, no need to have to turn the crank to get TDC for each cylinder. But be aware that the pistons will move, to BDC when air is applied. There are various schools of thought as to which position is best to do a leakdown test.
You are trying to diagnose some sort of engine problem. Was it a rough idle?
Did you measure engine vacuum while idling? Have you adjusted the idle mixture and set the timing correctly for idle?
I think today's ethanol gas is not helpful for these engines. I would try running high-octane ethanol-free gas during your tests, but if you're in California all your gas might contain ethanol now.
Also, for what-ever engine oil you're using, make sure it is high in zinc (ZDDP). It's necessary to prevent cam wear.
You are trying to diagnose some sort of engine problem. Was it a rough idle?
Did you measure engine vacuum while idling? Have you adjusted the idle mixture and set the timing correctly for idle?
I think today's ethanol gas is not helpful for these engines. I would try running high-octane ethanol-free gas during your tests, but if you're in California all your gas might contain ethanol now.
Also, for what-ever engine oil you're using, make sure it is high in zinc (ZDDP). It's necessary to prevent cam wear.
Last edited:
Justin Plant
Well-Known Member
With just 100 miles on your engine you may not have enough "break in" time on it. The rings may not have seated just yet. Drive it, put a few hundred miles on it. Go from there. If you can't drive it, jack up the *** end start it and let it run. Couldn't hurt.
Rusty Muffler
Active Member
It has one thousand miles now.
70bigblockdodge
Old Man with a Hat
Have you ever floored it, WOT. Did you baby it for the first 500 or whatever miles?
Drive it like you stole it. Get those rings loaded heavy against the cross hatch in the cylinder. Get the ring settled in the ring lands. Anything over 10% is worn out. Obviously yours is not worn out (new) unless you had like .040 ring gap it should be fine in some more miles.
As my usual reply to new something issues, just go drive the damn thing.
Drive it like you stole it. Get those rings loaded heavy against the cross hatch in the cylinder. Get the ring settled in the ring lands. Anything over 10% is worn out. Obviously yours is not worn out (new) unless you had like .040 ring gap it should be fine in some more miles.
As my usual reply to new something issues, just go drive the damn thing.
Isin't the rule-of-thumb that you use a "special" motor oil (break-in oil?) for the first 500 miles on a rebuilt engine, then change filter and oil to regular from that point on? Perhaps this is even more important for flat-tappet engines?
Finsinthemirror
Member
Hey Rusty, I feel bad about your issues with your poly. I tried to PM you and can't. You have that feature turned off or something. You need to talk to Gary Pavlovich. He is one of the few poly gurus and knows them inside and out. I tried to give you his email in private but couldn't. I won't post an email in public so.. figure out how to PM me. If there is anyone who can help you, it's him.
Jakter
Well-Known Member
65furyrewind
New Member
65furyrewind
New Member
Sorry for previous blank reply. Hit the wrong button
I’ve had experience w using differential compression testers on aircraft engines and it may be relevant (or not) to cars. One thing that affects readings is the size of the orifice between the high and low side. In aviation there are two orifice sizes, .040 for engines with less than a 5” bore and .060 for over 5”. It definitely affects readings. I understand that the tester for autos may have a smaller bore than these but I don’t know for certain. It’s important to have the correct tester for the job. Also its best performed the check on a warm engine and it was more concerning if the leakage was past a valve then if by the rings.
Differential compression used to be the holy grail of cylinder health in aviation but that has become less so recently. Maybe it because Continental motors did an engine test in which they filed ring gaps so large that the different compression read 0/80 and the engine still made rated horsepower! Differential compression is still used as a diagnostic procedure, but it’s definitely not the strict go, no go that it use to be for rejecting a cylinder as not serviceable.
I’ve had experience w using differential compression testers on aircraft engines and it may be relevant (or not) to cars. One thing that affects readings is the size of the orifice between the high and low side. In aviation there are two orifice sizes, .040 for engines with less than a 5” bore and .060 for over 5”. It definitely affects readings. I understand that the tester for autos may have a smaller bore than these but I don’t know for certain. It’s important to have the correct tester for the job. Also its best performed the check on a warm engine and it was more concerning if the leakage was past a valve then if by the rings.
Differential compression used to be the holy grail of cylinder health in aviation but that has become less so recently. Maybe it because Continental motors did an engine test in which they filed ring gaps so large that the different compression read 0/80 and the engine still made rated horsepower! Differential compression is still used as a diagnostic procedure, but it’s definitely not the strict go, no go that it use to be for rejecting a cylinder as not serviceable.
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