SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: beerjet on September 23, 2007, 01:17:40 AM
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I busted out a gauge today since I was doing some other wrenching and all the cylinders ranged from the lowest being 120 and the highest being almost 160 . I did this since I remembered changing the timing chain and gears a while back when I had the motor apart and the chain had a ton of slack in it and I asumed that was caused from lots of hours . Does all this coincide ?
Good or bad , whats the deal . This is something I've never done or had to deal with .
-beerjet-
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Throttle blocked wide open, all spark plugs removed, is first step. All cylinders should be within about 10 psi of each other. Squirt a bit of oil in the low reading holes to see if ring wear or valve leakage is a factor. A leakdown test will tell you more info. Don't forget that cam profile can (will) affect cranking pressure.
In your case, I would say there is a bit much variation between cylinders as indicated by the readings.
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Throttle blocked wide open, all spark plugs removed, is first step.
I hadn't tried it with the carb wide open . I'll do it again today .
Squirt a bit of oil in the low reading holes to see if ring wear or valve leakage is a factor.
After crancking it over a few times , we realized the oil pressure being built up was making a diffrence so I'll keep this in mind today also .
A leakdown test will tell you more info.
As soon as I figure out what a "leak down test" is and how its performed , I'll be sure to do it .
Don't forget that cam profile can (will) affect cranking pressure.
Thats half of my problem . This is the motor that was in the boat when I bought it so it's kind of like my old player days , just stabbin in the dark ! ;D
In your case, I would say there is a bit much variation between cylinders as indicated by the readings.
Now lets say they are not all with in 10 , what is that telling me ?
Thanks a ton Dave !
-beerjet-
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Is it that they should not be more then 10psi or is it 10% of each other? I have herd both from a multiple of diffrent people.
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It's telling you that some cylinders don't have as much power. Maybe the rings are gapped differently, maybe the valves don't seal as well, maybe the that bore is a little bigger, etc. Will it matter? Probably not unless you are racing competitively.
30 PSI range is normal for a non blueprinted engine, and is probably more common than most would like to admit (at least from what I've seen). Obviously, the closer you get to being all even, the more power you make. I wouldn't consider the cylinder "dead" until it is less than 80 or so. When it is there, that indicates a problem that needs to be repaired / rebuilt. Right now, it's ok (IMHO - but you get what you pay for). If squirting oil in the cylinder doesn't help, the "leak" is valve / headgasket related.
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Is it that they should not be more then 10psi or is it 10% of each other? I have herd both from a multiple of diffrent people.
10-15% variance for a newly built engine (non blueprinted). That's the target anyway.
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Awsome ! Thanks . I do race it and as slow as it is , I need every bit of power .
-beerjet-
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most recent numbers off the top of my head. '95 Mercruiser 502 EFI in a 22' Daytona. 385-400 hours. Lowest was 120. Don't remember what the highest was. Bob Teague said the numbers were a few points lower than a brand new Mercruiser 502 EFI would be pumping.