Baseline Cylinder Compression Numbers
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=39900
Printed Date: October-07-2024 at 4:22am
Topic: Baseline Cylinder Compression Numbers
Posted By: Jonny Quest
Subject: Baseline Cylinder Compression Numbers
Date Posted: September-11-2016 at 12:19pm
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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 ------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
Replies:
Posted By: Orlando76
Date Posted: September-11-2016 at 1:25pm
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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? ------------- Please support The Cystic Fibrosis Foundation 1976 Ski Nautique 351 Escort 1993 Ski Nautique purple and black 351 HO PCM |
Posted By: Blamey
Date Posted: September-11-2016 at 1:34pm
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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. ------------- 96 Super Sport Previously: 95 Sport Nautique, 1980 Ski Supreme |
Posted By: Blamey
Date Posted: September-11-2016 at 1:42pm
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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. ------------- 96 Super Sport Previously: 95 Sport Nautique, 1980 Ski Supreme |
Posted By: flyweed
Date Posted: September-11-2016 at 1:46pm
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My 1993 SN, which has 1700 hours on it, checks out at 135PSI on all 8 cylinders. ------------- '93 Ski Nautique NWZ, Air Boom Tower Drink Tonight..for tomorrow We Ride! |
Posted By: KENO
Date Posted: September-11-2016 at 1:53pm
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 KenO |
Jonny Quest wrote:Posted By: MrMcD
Date Posted: September-11-2016 at 2:12pm
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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. |
Posted By: 63 Skier
Date Posted: September-11-2016 at 2:30pm
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. ------------- '63 American Skier - '98 Sport Nautique |
Posted By: Dreaming
Date Posted: September-11-2016 at 6:38pm
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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. |
Posted By: Jonny Quest
Date Posted: September-11-2016 at 10:15pm
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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 ------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |