By the math at 8.0-8.4 to one it should be about 117 psi. But add in losses cam overlap and what would you expect to get?
A friend is looking at purchasing a exceptionally nice looking car but the compression tests came in at 65 on the low cylinder with the rest being 70-75. The car has a miss at idle on cylinder #2 that does not move with plug or wire swaps.
I suspect that the motor is just worn out. It has probibly seen many hours of run time even though the millage is not that high but I don't know what the measured compression should be
My first car as a teen was a 55 Ford with a 312 in 1962. The original 292 was replaced because bearings, rings and valves usually needed refurbishing/replacing before 100,000 miles. Dusty gravel roads and oil bath air cleaners didn't help.
NOHOME
MegaDork
4/14/22 11:28 a.m.
In reply to dean1484 :
That seems very low to me. Friend did a compression test on the F2 flathead a couple of days back and it was 120 across the board for what I believe is an 8:1 compression engine.
NickD
MegaDork
4/14/22 11:30 a.m.
Not sure of the exact specs, but I certainly wouldn't want to see less than 100psi.
I am going to look at doing a wet test on the cylinders. We will also look at the vacuum at idle. Another thing I will be looking at is timing and lastly, we will probably consider taking the valve covers off and measuring the valve lift to see if there is any cam ware.
Any other things to look at?
NickD said:
Not sure of the exact specs, but I certainly wouldn't want to see less than 100psi.
That was my initial reaction as well.
As a general rule any domestic, overhead valve, v8 should have over 100 PSI on a compression test.
Rock the crack back and forth by the balancer to see how much play there is in the timing chain. If it's more than four or five degrees then it can affect compression. If it's more than ten or so the timing may have jumped.
The Googles told me 155 psi. Of course,that would be for a fresh, new engine.
Agree with others that under 100 ain't makin' it. It it hasn't been driven lately, the Marvel Mystery Oil treatment might loosen the rings enough for them to work better, but your "worn out" assessment is probably about right.
Is the compression gauge accurate? Was the engine warm? Did you loosen the plugs, then fire the engine up for a moment? Was the throttle open?
It's probably worn out, but make sure the data is accurate.
ddavidv
UltimaDork
4/15/22 6:55 a.m.
Here's a video about the Y-block that discusses some of it's problems. May be helpful? https://youtu.be/JI_TBe1qss4
Uncle David (Forum Supporter) said:
The Googles told me 155 psi. Of course,that would be for a fresh, new engine.
Agree with others that under 100 ain't makin' it. It it hasn't been driven lately, the Marvel Mystery Oil treatment might loosen the rings enough for them to work better, but your "worn out" assessment is probably about right.
Google is not good at basic math. 8.4 x 14.696 is 123.44. That is the max pressure it can be for a stock high comp motor for that car.
Streetwiseguy said:
Is the compression gauge accurate? Was the engine warm? Did you loosen the plugs, then fire the engine up for a moment? Was the throttle open?
It's probably worn out, but make sure the data is accurate.
Yes tested
Yes
No I never do that for fear of stripping out a head. Heard about this but never seen anyone do it
and Yes
NOHOME said:
In reply to dean1484 :
That seems very low to me. Friend did a compression test on the F2 flathead a couple of days back and it was 120 across the board for what I believe is an 8:1 compression engine.
That matches what the math said it should be. Thanks!!!!
The car may have sat for a long period of time without running and now the rings are stuck and/or some valves aren't fully closing. It may get better after it runs for a while and the engine goes through some heat cycles.
stuart in mn said:
The car may have sat for a long period of time without running and now the rings are stuck and/or some valves aren't fully closing. It may get better after it runs for a while and the engine goes through some heat cycles.
Good point I don't know the history I will ask.
dean1484 said:
Uncle David (Forum Supporter) said:
The Googles told me 155 psi. Of course,that would be for a fresh, new engine.
Agree with others that under 100 ain't makin' it. It it hasn't been driven lately, the Marvel Mystery Oil treatment might loosen the rings enough for them to work better, but your "worn out" assessment is probably about right.
Google is not good at basic math. 8.4 x 14.696 is 123.44. That is the max pressure it can be for a stock high comp motor for that car.
The math is more complicated than that. Here's a link to a page that describes the formula Clicky
In reply to APEowner :
I will have to read that more carefully however my quick skim of it found some things that don't make sense to me. So a re read is needed.
If it matters to you I have patens on compression data acquisition equipment and copyrighted software to analyze the data. I have personally tested and data logged several hundred different engines over the last 15-20 years and I have never had a motor make more pressure than what the static compression ratio calculates to in PSI. But I will re read that link.
Now that being said when you get in to higher rpm's fluid dynamics take over and both the evacuation of the cylinder due to scavenging as well as the stacking effect in the intake caused by valves closing in other cylinders that send a pressure wave back through the intake will change things enough to impact the timing and octane needs of a motor but we are not looking at that end of things in a compression test on a motor being being turned by a starter.
I retested the motor today using my good equipment not the Sears's set my friend has and got basically the same results. Added oil to the cylinders and the compression jumped up to spec (121psi) in all but one cylinder. That one cylinder was the worst of the bunch and only came up to about 105 psi (from 60). A bore scope look in that one revealed some bad scratches in the cylinder walls of that cylinder.
So bottom line is the motor i think is just worn out. I suspect that the supposedly rebuilt motor was nothing more than a rattle can rebuild as it looks nice and shows well. Another thing I noted was there were no cross hatching of the bores and I could see a ridge on the bore with the bore scope.
No affiliation. Mummert makes performance parts for the Y-block if your friend is interested in doing some updates.
tester (Forum Supporter) said:
No affiliation. Mummert makes performance parts for the Y-block if your friend is interested in doing some updates.
I will definitely pass that along!!! Thanks.
Something else that I really liked was the exhaust sound of this motor. I don't know if it is the firing order or what but it is just such a cool sounding exhaust.
dean1484 said:
Uncle David (Forum Supporter) said:
The Googles told me 155 psi. Of course,that would be for a fresh, new engine.
Agree with others that under 100 ain't makin' it. It it hasn't been driven lately, the Marvel Mystery Oil treatment might loosen the rings enough for them to work better, but your "worn out" assessment is probably about right.
Google is not good at basic math. 8.4 x 14.696 is 123.44. That is the max pressure it can be for a stock high comp motor for that car.
The act of compressing the air heats it, which also raises pressure. If you could get a theoretical 8.4:1 compression, with perfect volumetric efficiency, closing the valve at BDC, etc.., and then can hold it there until it cools back down to ambient temperature, it would be 123-odd psi, but when it is hot from being compressed, it will be right on up there.
Pete. (l33t FS) said:
dean1484 said:
Uncle David (Forum Supporter) said:
The Googles told me 155 psi. Of course,that would be for a fresh, new engine.
Agree with others that under 100 ain't makin' it. It it hasn't been driven lately, the Marvel Mystery Oil treatment might loosen the rings enough for them to work better, but your "worn out" assessment is probably about right.
Google is not good at basic math. 8.4 x 14.696 is 123.44. That is the max pressure it can be for a stock high comp motor for that car.
The act of compressing the air heats it, which also raises pressure. If you could get a theoretical 8.4:1 compression, with perfect volumetric efficiency, closing the valve at BDC, etc.., and then can hold it there until it cools back down to ambient temperature, it would be 123-odd psi, but when it is hot from being compressed, it will be right on up there.
Ok that makes sense. I have never actually seen it in testing. I would assume that this would increase as the compression increases. Would this be somewhat negated by heat absorption of the metal around it when compressed in the cylinder combined with heat absorbed by the air in the intake tract effectively making the air less dense. I would assume there would be some cancelation here Ok I now have a new rabbit hole to go down. :-)
Like I said I need to re read that link there looks to be a lot of good info in it.
In reply to dean1484 :
Yup.
This is a lot of the reason why limited engines (restrictor or compression) run what we street drivers would think of as insanely high coolant temps - less compression heat lost to the water jacket. Higher TDC pressure means less ignition advance needed for rapid combustion means less negative work done means more power to the flywheel.
Keeping an engine happy at 250+F coolant temp is another matter.
