Compression test results

christian

christian

Senior Member
As some of you may know, I'm trying to get to the bottom of a fluttery idle vacuum reading. I ran a compression test, please take a look and let me know what you think is on order.

All tests were with throttle wide open, values start from cylinder 1 through 4

Snyders 5.5 head

Hot engine
97.5
92.5
85
90

Cold engine
95
90
82.5
87.5

Cold engine with a squirting of motor oil
97.5
102
90
102.5

time for rings ??
 
I'd do a leak down test to see where it is leaking, paying
extra attention to number 3 cylinder.

Bob
 
I had a simmilr experience with my Tudor standard engine. Going from #1 to #4 cylinder in all cases: Compression Test 60,58,30,60 Leak test air pressure, 100%, 100%, 10%, 100% When I removed the head I found the exhaust valve head for #3 was warped (cupped) so only two opposite sides of the valve closed on the head.
So the answer was todo acomplete valve joj. The head goes on next week. Grant
 
My valves all look pretty good, thinking about making some kind of rubbery dome I can place on top of each valve, with an opening on top I can put gas in, this way I can see if fuel can leak past the valve. If it is a good seal and the fluid doesn't leak in, I'm guessing I can rule out valves
 
Serious blow by, yet excellent power. Could hit 65mph in the heavy fordor. Also off idle stumble, vacuum readings at idle showing quick bouncing between 16in and 23in.
 
a leak down test would have been the way to go for a first step. if you have serious blow by i would think you have ring and or cylinder wall trouble..time for a rebuild....
 
My initial plan was to just hone the cylinders, throw in new rings, lap valves and throw in new springs for good measure, but with these sizable ridges at the top, it may just be a waste of my time. Guess it needs to be bored out. Time to find a builder I guess. May as well pour babbit while I'm at it. Yikes.... This is gonna cost $$$
 
here is what you need:::just had this delivered
 

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I think before I went all out on this engine I would lap the valves paying close attention to cyl #3. This could be done for the price of a head gasket and valve chamber gasket. FWIW
Paul in CT
 
I agree Paul, but I would think at the very least, rings would be needed too, since the compression numbers came up so much from squirting oil in them.
 
You should be able to push the piston to the side and look at the top ring gap if you look straight down on the piston edge. This will give you some idea of ring wear.
 
I removed the valve springs, hand cranked the motor and checked the upmost position of each valve, they are had slight lateral play. I know without numbers its very subjective, but is a minute amount of lateral movement acceptable in valves or should they be tight enough in the guides that they move freely up and down yet no lateral movement?
 
1930 coupe said:
Most vacuum gages will flutter on a model A engine because of the very slow idle speed. If the engine is running good, don't worry about it.
Click to expand...

I have only used a vacuum gauge on any car a few times in my career and that was a waste of time. There are other ways to diagnose things more accurately and quicker. I only use my vacuum gauge for the other scale of low fuel pressure system testing ..
 
Atmospheric pressure is 14.7 pounds so your compression test might have been expected to return these numbers:
standard 4.22 compression X 14.7 = 62 pounds
police head 5.22 compression X 14.7 = 77 pounds

With the 95 to 102 pounds you measured you would have a compression ration of about 6.5 to 1 to 6.7 to 1. No wonder it goes fast. If you aren't rattling or scraping put some of that "worn engine" stuff from the auto store and let er rip!
 
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