Compression test

Fireman450

Fireman450

Daring to be diffrent!!
i have a guys truck I am working on. Came in missing, popping, and blowing oil out the dip stick tube. Blue smoke out the exahaust. So I ran a compression test and found cyls 1/3/5 low(To me) and slow to respond, 2/4/6 were 50 to 75 psi higher. lowest was 370psi, Highest was 475psi.

What i need to findout is what is normal compresstion, and when to reccamend a rebuild?

Thanks
Lynn
 
i dont know the normal compression but the numbers you got are to far apart. if i were to recomend a rebuild , it would be now.
 
Do your self a favor and toss the comp. gauge and leak it down instead, then you will not only know what was hurt but also whats going on with the valve job while the head is off.
 
stock600 said:
Do your self a favor and toss the comp. gauge and leak it down instead, then you will not only know what was hurt but also whats going on with the valve job while the head is off.
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Care to elaborate on this? How does one properly perform a leak down test?

Thanks
Lynn
 
put about 120 - 150lbs of shop air on each cylinder with a gauge and ball valve inline. air the cylinder up and how fast the air leaks off. if you have the intake and exhaust off, you can listen for air bypassing the valves. i always use about 800psi of nitrogen on big engines. if you have a nitrogen bottle around and can plumb it up, i would use it.
 
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Knuckledragger said:
put about 120 - 150lbs of shop air on each cylinder with a gauge and ball valve inline. air the cylinder up and how fast the air leaks off. if you have the intake and exhaust off, you can listen for air bypassing the valves. i always use about 800psi of nitrogen on big engines. if you have a nitrogen bottle around and can plumb it up, i would use it.
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The amount of pressure is subjective, IIRC the aviation industry takes their measurements at 80 psi. A lot of people use 100 psi because then whatever you read on a gauge would be the percentage.
 
mondtster said:
The amount of pressure is subjective, IIRC the aviation industry takes their measurements at 80 psi. A lot of people use 100 psi because then whatever you read on a gauge would be the percentage.
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x2, Use 100# just for that reason. Set Lft. gauge up at 100# and read the Rt. gauge. Say it reads 93#.... then that cylinder is leaking 7%, that simple. Then you can easly determine where that 7% leak is so you can fix it while apart. My engines after a freshing with conventional rings and a modest ring gap are ussualy in the 93-96% range, I butted a ring last weekend and slightly scored #2 cylinder; just that brought that cylinder down to 73%. Compression test is next to useless in my opinion, but for what its worth mine pump in the range of 550psi, cylinder condition and camshaft play a role in what it will pump in how many strokes....
 
point takin guys. i use a higher pressure to simulate a little bit of an actual cylinder pressure. in fireman450's case there is the possibilty of more than just a cylinder/ring failure. doing this i can see if the air is going past the valves, head gasket, or the rings. once the pressure equalizes i shut the valve and see how long it takes to drop a certain amount of pressure. it works great on big engines. it may not be accurate for small ones
 
Knuckledragger said:
point takin guys. i use a higher pressure to simulate a little bit of an actual cylinder pressure. in fireman450's case there is the possibilty of more than just a cylinder/ring failure. doing this i can see if the air is going past the valves, head gasket, or the rings. once the pressure equalizes i shut the valve and see how long it takes to drop a certain amount of pressure. it works great on big engines. it may not be accurate for small ones
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I wasn't trying to say you were doing it wrong, if that's what you're thinking, just that there are several ways to do the test.

As with a compression test, the leak down test results should be taken with a grain of salt. The results will vary somewhat depending on how the engine is assembled as was inferenced earlier. The last SBF nitrous motor I put together would have looked like it was completely worn out if someone who didn't know how it was built were to leak it down.

Both compression and leak down tests have their place and I use both on a regular basis. I usually start with a leak down test and proceed from there depending on what I find.
 
Thanks for the info guys.
Brandon, I hear ya on the compression, mine would do the same thing on my puller.
Im just tryn to figure out where the line is for a rebuild. If I tear it down, it will get a full treatment.
Cyl PSI
1 380
2 420
3 370
4 475
5 400
6 450

Ill do a leak down test and see for sure. I know that IMHO the compression test says its shot..lol and I think there is way to much varaition in psi between cyls.

Lynn
 
Sorry, my last post was less than helpful. :hehe:

If I had to take a wild stab in the dark, 1,3,5 have light scoring on the cylinder walls. Tyler is correct with ~500psi static numbers.
 
What fitting do you guys use in the injector bores when doing a leak down? I'm thinking setting my regulator in my air compressor to 100psi with the compressor air valve open, closing the compressor air valve, and then observing the pressure drop at the regulator gauge. Sound feasible?
 
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