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  What damage will result from over-proping?

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Author Topic:   What damage will result from over-proping?
Arch Autenreith posted 07-16-2003 10:07 AM ET (US)   Profile for Arch Autenreith   Send Email to Arch Autenreith  
Did a search and couldn't find an answer.

Specifically what will happen to an engine that frequently is run at significantly less than factory recommended rpm's @ wot? I saw a post from Sal saying it will shorten it's life also. But why? Will it cause it to overheat? And when I say 'significantly less' I mean that at wot the engine is running 1,000 (or more) rpm's less than recommended.

Sal DiMercurio posted 07-16-2003 02:52 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
You will blow a rod, put a hole in a piston, score the cylinder walls, run hotter then it should, & put "10 years" wear on all moving parts in 1 year.
Like running a to small engine on a very big boat all the time, gotta firewall it where a bigger engine can just lope along with no strain.
Whenever you run any type of engine without letting it get to the correct rpm range, your gonna pay through the wallet big time.
Try putting a 4 cylinder honda engine in a Peterbuilt rig, thats what happens to an engine thats over propped.
Try driving your truck in 3rd gear all the time, up hill, down hill, stop & go, long do you think it will last ?
Arch Autenreith posted 07-16-2003 03:29 PM ET (US)     Profile for Arch Autenreith  Send Email to Arch Autenreith     
Thanks, Sal.

Even more specifically though it all that a result of overheating? I might think so. I do understand they're designed to run at certain wot rpms.

Without a doubt pinning vs. loping is easy to see but pinning @ 5500 vs. 4500 doesn't in itself seem like either is a walk in the park for any engine.

What inherent designs are so specific that causes the engine to wear 10:1 if not run at optimal rpms?

I know I'm over-analyzing this so bear with me.

I guess I should say that friends bought a heavier Nova Scotia-type 21-ish foot runabout last year w/a mid 80's 50 Mercury. They used it for maybe 1 month last year and ran fine when they put it away for the next 10 months. When taken in for servicing over the winter the mechanic said the shaft was frozen. When following them I noticed they ran at wot and never was close to planing and always noticed the engine never ran near max rpms (b/c it never go up on plane).


Curtis Johnson posted 07-16-2003 11:42 PM ET (US)     Profile for Curtis Johnson  Send Email to Curtis Johnson     
Today while cruising at max 4000 rpm instead of 5500, I thought my particular problem may be a blessing. I checked the linkage to the carb and can't find that it won't open all the way. How would you tell?
Clark Roberts posted 07-17-2003 07:05 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
The reason that damage is done to an engine by over-propping (running at wide open throttle and not achieving max rated rpm) is that when throttle is fully advanced so is the timing fully advanced! The spark is therefore coming too quickly for engine speed and detonation (pre-ignition/lean burn) occurs which raises the piston/cyl head temp (holes pistons and scours cyl walls) and creates "knock" (even though you seldom hear it in a two stroke) which results from the spark coming well before piston gets to top or proper firing position and piston is momentarily being forced back down the wrong way which will beat the bearing to death (both wrist pins and crank) and bend/break rods. Some of us old guys refer to this as "laboring" or "lugging" (sp?) an engine (car, boat, etc. no matter) and it is a very damaging condition! Hope the above helps clarify.. happy Whalin'.. Clark.. Spruce Creek Navy
PS> laboring occurs, to some degree, at all rpm when an engine is over-propped so backing off throttle only delays the damage (maybe not...hummmm????)also it is best to prop an engine so that it exceeds max rated rpm slightly when boat is lightly loaded so that engine is not laboring when heavily loaded.. engines with engine control modules (ECMS)are less prone to laboring since the ECM matches/modulates the timing to engine speed/throttle position/load/fuel quality/ambient and cyl head temp... etc. etc. blah, blah, blah
Arch Autenreith posted 07-17-2003 07:12 AM ET (US)     Profile for Arch Autenreith  Send Email to Arch Autenreith     
Very good explaination, Clark. Now it makes sense to me. Thanks.
David Jenkins posted 07-17-2003 09:05 AM ET (US)     Profile for David Jenkins  Send Email to David Jenkins     
Excellent explanation. Thanks, Clark.

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