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Baseline Cylinder Compression Numbers

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Jonny Quest View Drop Down
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    Posted: September-11-2016 at 12:19pm
This is almost like asking "which oil is best?"

I've done a fair bit of research and the opinions out there seem to be quite varied. Here is the question:

For the main 351W marinized engines (351 Carb, 351 High Output, and GT-40) what would I expect compression test results to be on: 1) new engine; 2: used engine but good condition; and 3) reasonably running engine, but tired and in need of rebuild?

My 351W H.O. has 700 hours on it and cylinder numbers 1 and 5 are at 130 psi and the rest of the cylinders are at 135 psi. I'm trying to get a feel for where the engine stands in the life cycle (as shown by compression tests).

Thanks for your input.

JQ
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Orlando76 Quote  Post ReplyReply Direct Link To This Post Posted: September-11-2016 at 1:25pm
I wouldn't sweat those numbers at all. Have you verified where the difference with 1&5 is coming from? Maybe just do a Gt40P change over?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Blamey Quote  Post ReplyReply Direct Link To This Post Posted: September-11-2016 at 1:34pm
I'm not an expert but I have been trying to learn and read as much as I can.

The absolute numbers don't mean much but I have seen people with numbers over 150. I don't think 135 indicates any issues and the fact the numbers are all so close is really good.

Most likely this tells you nothing about the engine condition other then it's just fine. A leak down may tell you a little more but with those numbers I wouldn't worry about it unless you are having issues.

In all likely hood when something does go wrong it will probably be something unrelated to the current compression numbers. I.e. something that's working fine now will break before the whole engine runs out of steam.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Blamey Quote  Post ReplyReply Direct Link To This Post Posted: September-11-2016 at 1:42pm
As a ball park to sender you question if say over 150 I'd like new 100 to 150 is used but okay and under 100 probably need a rebuild.

The other thing to understand is variables in how you test cause different results. E.g. How often you turn the engine over, The gauge you use. Another reason the absolute number don't mean much.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote flyweed Quote  Post ReplyReply Direct Link To This Post Posted: September-11-2016 at 1:46pm
My 1993 SN, which has 1700 hours on it, checks out at 135PSI on all 8 cylinders.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: September-11-2016 at 1:53pm
Originally posted by Jonny Quest Jonny Quest wrote:

This is almost like asking "which oil is best?"

I've done a fair bit of research and the opinions out there seem to be quite varied. Here is the question:

For the main 351W marinized engines (351 Carb, 351 High Output, and GT-40) what would I expect compression test results to be on: 1) new engine; 2: used engine but good condition; and 3) reasonably running engine, but tired and in need of rebuild?

My 351W H.O. has 700 hours on it and cylinder numbers 1 and 5 are at 130 psi and the rest of the cylinders are at 135 psi. I'm trying to get a feel for where the engine stands in the life cycle (as shown by compression tests).

Thanks for your input.

JQ


Hi JQ

I seem to remember that you are at 4000 ft or so elevation.

If you took these readings at 4000 feet you're reading about 88% of what somebody like Orlando76 would get in the lowlands of Orlando. Goes right along with that 3% HP loss for every 1000 ft of elevation change

He'd have about 153 pounds or so down there

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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: September-11-2016 at 2:12pm
With all 8 measuring within 5 lbs of each other your engine is perfect. If a cylinder was down 20-25 pounds I would be concerned. Keep good oil in it and enjoy the boat.
Nothing wrong with your engine at this point as far as compression goes.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 63 Skier Quote  Post ReplyReply Direct Link To This Post Posted: September-11-2016 at 2:30pm
Originally posted by Jonny Quest Jonny Quest wrote:

My 351W H.O. has 700 hours on it and cylinder numbers 1 and 5 are at 130 psi and the rest of the cylinders are at 135 psi. I'm trying to get a feel for where the engine stands in the life cycle (as shown by compression tests).

Rob, I've got some really bad news for you. Based on those numbers you may have to keep using this engine for another 1,000 to 1,500 hours or more. That might be 10-20 years depending on what kind of hours you put on each season.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dreaming Quote  Post ReplyReply Direct Link To This Post Posted: September-11-2016 at 6:38pm
mine new was 165, but that also depends on the head, stroke, pistons etc that you are using to get to the compression ratio that is intended.     the old motor I replaced was a range of 165 to 95 (HO motor).    I have always been taught that it's not the # itself that makes the most difference in whether a motor is healhty or not, its the difference in range between the cylinders.    If they are within 10%, your motor is pretty healthy.     a tired motor will probably have less compression over all, but until your in the 85 PSI range, or you have a wide range of cylinder pressures, I don't think you should consider replacement.    Run it and spend your money on other things, unless you just have a hankering to build a warmer motor.... then build one on the side, and when your ready to swap, pull it out and sell it to someone who needs a good running engine.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jonny Quest Quote  Post ReplyReply Direct Link To This Post Posted: September-11-2016 at 10:15pm
KenO:

Good point. I didn't take elevation into account when I was looking at the numbers. I'm at 4,200 feet here in Salt Lake City. With 2 at 130 and 6 at 135 psi, all cylinders currently are within 9% -- so I'm not going to get too concerned.

Thanks guys for your responses.

JQ
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