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96 5.8 Compression Numbers

27K views 24 replies 11 participants last post by  Daves 88' 
#1 ·
Hey fellas.

I am working on finalizing a deal for a 5.8 out of a 96 Bronco that is said to only have 85k on it. The place I am buying it from runs a compression check and leak down test on it before we finalize the deal, and they give it a 6 month warranty.

I called today and was told that the engine tested compression at 110 dry with a 10% variance. I know very little about the motors on these trucks (learning) and have never heard of a "dry" compression check. So, can someone give me some feedback on this process and the 110 result? What is a "dry" compression check? Guessing that is without oil, but can that be done safely?

Thanks...
 
#2 ·
dry compression check is without putting oil in the combustion chamber. it is safe, as this is how your engine functions normally.

a wet comp check is putting oil in the combustion chambers, it's a backyard way to see if a low compression reading is because of worn rings, or valves. the oil helps seal the rings if they are worn.

i'm not 100% sure on what the PSI numbers should be for a 351, from what i understand it's more crucial that they are all balanced numbers within 10% from highest to lowest.

FWIW, last comp check i did was on a 20 year old 300 straight six....and got 145-150psi across the board.

someone else should pop in with actual specs, i hope.
 
#3 ·
dry compression check is without putting oil in the combustion chamber. it is safe, as this is how your engine functions normally.

a wet comp check is putting oil in the combustion chambers, it's a backyard way to see if a low compression reading is because of worn rings, or valves. the oil helps seal the rings if they are worn.
Ahhhh... makes sense!!
 
#5 ·
all i have to add at this point is +1 for this guy's ^^^ user name. :rofl:
 
#6 ·
Hmmm... what I seem to be finding online is about 170psi for the desired amount on a 351, but they arent saying whether its dry or wet... and it sounds like the dry is the baseline, then you do the wet test to determine what the problem may be (rings etc). So, its sounding to me like 110 is low...

Found this:

Ford 351 Compression Specification

Q. What is normal compression on a Ford 351m400 for each cylinder? It's a 1978 2bbl changed to an Edlebrock 4bbl intake and Holley 650 carburetor?
Ford 351 Compression Specification

A. Minimum compression is 100 psi @ 200 rpm. The lowest cylinder reading should not be less than 80% of the highest reading. Perform the compression test with engine at normal operating temperature, spark plugs removed and throttle wide open.

Normally I look for about 170 psi to 180 psi.
Here: http://autorepair.about.com/library/a/1f/bl286f.htm

And a few other references as well... granted thats for a 78 so the numbers may be totally different...
 
#8 ·
If they're a reputable shop, I'm sure they did, but ask to make sure those numbers they got were after the engine was good and hot, and had been run for a little while. From my experience, a cold engine will definitely give you lower numbers, and an inaccurate reading.

Also, ask if they cranked it long enough. Some places will say "5 cranks is what it takes" and even if the cylinder isn't full of compression, they'll stop cranking. Again, this gives a low reading. Sometimes it can take more cranks of the engine for the compression to peak out if, say, the battery is tired or something, etc.

In my experience, 110 is pretty low. As with Unforgiven, the last three engines I've done were 300s. One's 33 years old, one was 30, and one's 27. All of them were around 150 - 155 in each cylinder.
My brother did his 351 a while back and even though I don't remember the exact numbers, I remember they were pretty close to what mine were.

The benefit of having all of your cylinders within a certain % of each other is that they'll all get the same air/fuel burn. If you have cylinders with all different ranges of compression, and give them the same amount of air and fuel, with the same spark intensity, and at the same time (which the engine is going to do), some cylinders are going to run rich, some are going to run lean, some are going to run right on. This creates a very inconsistent engine that's going to wear itself out quickly, foul plugs (rich), and burn valves (lean). That's why the consistency between cylinders is important.

If the compression is low (even if it's nice and consistent), the engine's going to run well, but it won't have as much power as an engine with higher compression. This is often where you'll hear people say their engine runs nice, but they can tell it's "tired".
 
#9 ·
The benefit of having all of your cylinders within a certain % of each other is that they'll all get the same air/fuel burn. If you have cylinders with all different ranges of compression, and give them the same amount of air and fuel, with the same spark intensity, and at the same time (which the engine is going to do), some cylinders are going to run rich, some are going to run lean, some are going to run right on. This creates a very inconsistent engine that's going to wear itself out quickly, foul plugs (rich), and burn valves (lean). That's why the consistency between cylinders is important.

I did not know this. learn something everyday. Thanks AbandonedBronco. :thumbup
 
#12 ·
I compression checked my 91' 351 with about 88k on it a couple months ago. If I remember correctly I pulled right around 125 +/- 4 or 5 lbs across the board. This was a dry check on a cold engine. My engine seems to run great in opinion. What I've heard is that my numbers were affected by the altitude (I'm at about 4500 ft) I'm not completely sure if there is any truth to this?
 
#14 ·
if you would read, he said at about 4500 feet.
 
#19 ·
don, are drunk or something?
 
#21 ·
i have 94 with a 351. a couple weekends ago, i compression tested my 180,000 mile engine, dry (w/o putting oil in the cylinder prior). I read 165 psi +/- 5 psi per cylinder. for a 351 with 180k, this sounded good per everything i read, however what really sounded good was the fact that they were all really close to each other. if you're reading low on all cylinders, you probably have a valve/crank timing issue. if you're reading low on some cylinders, but not on others--you prolly need to walk away. imho.
 
#24 ·
Thanks guys. They are supposed to pull the motor (it was still in the truck) and do another compression test on it and call me with the numbers. I may go down and have them do it again with me observing. Between the feedback I have received here and the info I have found using the google machine, I am leaning towards walking away.

When I talked to the guy on the phone, he had to get his yard guy on the hand held and ask him about the numbers, and they sounded like they really werent sure wtf the were talking about (as in, what motor those numbers were for exactly) so I am keeping my fingers crossed that they were just really unorganized and quoting the wrong motor or something like that. lol...

But, if not and the numbers are really that low, I think I will have to say no thanks. Again, I am not looking to buy and rebuild immediately. I was hoping to buy, swap in and drive for a few years before I had to worry about rebuilding...
 
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