Finally got around to compression testing the 360 on my ‘76 Highboy. The results:

1)50
2)90
3-8) 88-93

Altitude is 9,000’.

called a local shop and he said to test it cold. Just read a great thread here that said test should be performed hot, but most folks don’t like to burn their hands so they do it cold.

After reading the aforementioned thread it seems that: 1) I may want to bite the bullet and run the test on a warm/hot engine and see if makes a difference. 2) the fact that 7/8 cylinders were consistently low indicates a general degradation of the engine internals and 3) I’m probably looking at bore, pistons, rings.

The other thread mentioned conducting a leak down test and also a vacuum test. My question is whether it’s worth the effort doing these latter tests, given the results of the compression test.

Opinions and advice welcome and appreciated.

Mac

 

Those are terrible numbers, does it even run? Mosquito fogger? You did remove all the plugs, correct? I'm sure that's the case, but...

I'd want to look at everything. Has it just been brought out of a long slumber? What is the history on the engine? Is it burning oil excessively?

9,000 feet is getting up there a little but good compression should be 120+ psi I'd think. What I'm getting at, is do the numbers you're getting jibe with the general engine condition and other typical traits of a high mileage engine with pooched rings/bores? An excessively worn/stretched timing chain would kill compression too.

 

Um that's below a well used lawn mower engine (seriously). I would verify with a leak down test. That engine has to burn some serious oil.

 

I’ve owned it for 6 yrs. I drive it every couple of months. No idea really if it was cared for or not. It doesn’t run particularly rough, but it has very little power. it puffs white/blue smoke on startup which goes away indicating a valve issue (I think). The 1406 Edelbrock 1406 is relatively new but is dire need of jetting for this altitude.

I rebuilt the front end last summer, successfully I might add, which was incredibly satisfying as I’d never done anything close to as complex. So based on advice here I’ll tackle whatever needs to be done engine wise.

Ted, you were helping the guy in the other thread I’ve been reading. He has a 73 F100. Did I do the comp. test incorrectly by doing it cold?

Mac

 

It does burn oil, and also leaks it from the rear mains. So exactly how much it burns is in question.

 

When doing the test, did you hold the throttle all the way open? Rubber bands or whatever to hold it open to allow full air intake basically.
And the bit about taking all the plugs out is to let the starter really crank the engine at a decent speed. And speaking of that, did it? Spin fast I mean?

Good luck. Definitely low numbers, but the difference between 50 and near 90 in the rest is a telling thing as well.

Paul

 

Paul,

I did remove all the plugs to start, but I did NOT open the throttle all the way. The battery is not new but I keep
a tender on it and it was to spin the engine fairly fast.

Between the throttle not being open and the engine being cold, I’m guessing I should repeat the test.

 

Here is what the 1976 Ford truck shop manual says about doing a compression test:
Have the correct viscosity oil in the engine.
Have the battery fully charged.
Perform the test with a normally warmed engine.
Remove all spark plugs.
Open the throttle fully.
Crank engine for at least 5 compression strokes for each cylinder.
All pressures should be within 75% of the highest cylinder.
On low cylinders add 1 tablespoon of engine oil to the top of the pistons. Retest.
Improvement indicates ring problem; no improvement indicates valve problem.

While all your numbers seem to be a bit on the low side, #1 appears to be the most significant problem. The manual makes no accommodation for altitude, though that might be connected to the overall low values. You'll have to do more investigation to determine the real problem with #1. Could be rings, valves, blown gasket, or crack. Before head removal, check the valve action at the rockers just in case it's a cam problem.

 

With what he has posted I don't think warm motor will bring the numbers up and the same with the throttle open, low number are low numbers.

Now we do not know the miles on the motor but the owner said he does not know the condition of the motor other than it smokes on start up (bad valve guides) burns & leaks oil.
With all that I would pull motor for a rebuild / replacement. Rear main leak motor should be pulled, heads got to come off for a rebuild guides so why not rebuild?
My .02
Dave ----

 

Yes, every Tune-Up manual since about 1910 says to warm up the engine first. If you really want to do it right, it's actually the oil temperature you're looking at, not coolant. Idling no matter how fast or how long won't do that. Good highway run, under load.

But as mentioned it isn't going to make that much difference here. I don't know what the typical psi change might be cold vs. hot test. 5 or maybe 10 psi, but I dunno?

I've always generally looked at it from the standpoint if the cold engine compression numbers are good to excellent (i.e. within spec) and, they are even across all cylinders, the fact that the compression numbers would only improve if the engine is hot means I'm not going to worry about it.

If they are marginal then maybe another test is warranted with engine fully warmed up. But the results still have to be looked at in terms of other factors. Oil consumption, you mention it's really down on power, and smokes like a chimney etc. Sounds to me like an overhaul is in the cards.

 

Thanks for the insight and the text from the Ford shop manual. I don’t have one and I probably should.

I’ve decided to do a leak down test, if for no other reason than I’ve never done one and I want to. Also, I’ll try the warm engine, tablespoon of oil approach on one of the cylinders (the easiest one to reach) and see if makes any difference at all.

It’ll be a bit as I’m only up here about every month or so.

Thanks again.

 

Absolutely.
While I'm kind of with the others regarding the changes expected from a warm engine (letting rings and pistons expand, and engine oil coat the cylinder walls and such) I'm fully on board with the open throttle making a notable difference.
Warming up the engine could make from zero difference, to perhaps 15% difference I would think. But opening the throttle could conceivably bring your numbers up 25% or more. I would also expect numbers to be lower than normal at 9,000 feet, but don't know by exactly how much. Maybe not much at all, since the dynamic here is the piston compressing the intake charge.
However, as air is less dense at altitude, as is measurable air pressure, that less dense initial volume won't compress to as high a dynamic pressure, and is starting from a lower point anyway I would think.
But I'm not a scientist or engine dynamicist, so no idea if that really becomes a thing. We know the differences altitude causes with engine vacuum, and vacuum can be correlated with compression. It's not going to be exact, due to cam design, but it should be consistent, percentage-wise.
So if you lose 15% of vacuum at that altitude, you maybe lose 15% compression at that altitude? Just making that up on the spot here, so by no means use that as a rule. Just curious and wanted to put it out there for argument's sake.

I've seen members here quote the expected vacuum change per 1,000' of elevation, so maybe someone can post that up as well. Just for a data point.

Paul

 

I found this same chart across 4 different automotive forums. I take no credit or blame if its wrong lol. But here is the factors for different altitudes

Altitude Factor
500 0.987
1500 0.960
2500 0.933
3500 0.907
4500 0.880
5500 0.853
6500 0.826
7500 0.800
8500 0.773

 

As a rule of thumb an engine loses 3% of it's horsepower for every 1,000 feet of altitude so at 9,000 feet you're already down 27% on power.
It's a combination of lower atmospheric pressure and less oxygen content in each cubic foot of air. That's where turbochargers come in handy.

 

I was able to retest the compression yesterday using the Ford recommended procedures; engine was at operation temp and throttle was wide open.

Old results:

1) 50
2) 90
3-8) 88-93

New results:
1) 70
2) 110
3) 110
4) 70* - can't be sure I got the gauge all the way in.
5) 111
6) 98
7) 110
8) 105

I'm basically throwing out #4 since I'm not confident I had the gauge properly seated. What does the general improvement on all but #1 indicate? I'm at a bit of a loss here (not unusual for me).

Mac J