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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...
 

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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.
 

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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!!
 

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On my '96, was between 180-190psi on all cylinders, not sure if it was wet or dry (done by a third party for the seller, so take it with a large grain of salt)
 

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all i have to add at this point is +1 for this guy's ^^^ user name. :rofl:
 

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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...
 

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Anyone else have any other experience/feedback on this? Any of you guys with experience building or rebuilding engines?
 

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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".
 

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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
 

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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.
Thanks AB.

Well, thats part of the problem... its coming from a wrecked rig and so I am sure they are not warming it up in any way, other than the motor is probably around 130 degrees just from sitting outside in the AZ heat. Lol...

The odometer on the truck reads 85k, and by the looks of the Bronco, I believe it. Other than it was crashed, the thing was pristine. I bet the owner shed a tear after that accident. So, the mileage makes me feel confident, but the 110 kinda has me wondering now. I think I would be okay with 150 or even 140, but 110 seemed low.

I dont really want to have to buy this THEN rebuild it. I was stoked because I thought with the low mileage, I could swap it in, run if for 3 or 4 years then rebuild it when I had the cash...

They are supposed to pull it, do a leakdown on it and compression test it again. I told him to record all of the cylinders and let me know. He did say there was less than a 10% variance on all of the cylinders, but if 110 was the highest, that puts us down to 100 with the variance.



I did not know this. learn something everyday. Thanks AbandonedBronco. :thumbup
X2
 

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Did the results of the leak down test point indicate any problems with the engine?

Alll I can offer is that my wornout 8:1 compression ratio engine did about 140 all around, so 110 seems low for a low mileage, modern, naturally aspirated engine.
 

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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?
 

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Altitude will effect the cr numbers. What elevation are u at?
 

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Not sure how much altitude should effect the numbers but I'm not to worried about it. The engine still seems to be running pretty stout.
 

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just a link I found from googling and it gives a pile of info for altitude and compression tests.
 

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if my math is right by that link your roughly 125 psi @ sea level. kinda low in my opinion.
 

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don, are drunk or something?
 
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