Thread: WOT question
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Old 06-07-2020, 05:53 PM   #33
Jmarsh203
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High idle term

Quote:
Originally Posted by DavidM View Post
Checking to see that the engine revs up to its "high idle" speed (wot, no load, in neutral) is to confirm that nothing else is limiting rpms, like not hitting the throttle stop, governor malfunction, etc.

Then with that confirmed you can check the wot rpm under load with a phototach or a phototach calibrated dash tach. If the engine is not reaching the rated wot rpm then the engine is under too much load and it should be reduced by cleaning the bottom or a prop adjustment if it is clean.

David
Sorry David not picking on you specifically but I’m seeing this constantly throughout the form and was using it as an example. Was wondering why do people keep using the term high idle to describe max no load rpm. High idle is either a term used to diagnose a problem with an engine when you cannot get the engine down to its rated idle speed or an option or setting on some diesels to run Small accessories off of a main engine while it’s idiling and not under load such as alternators, air compressors or hydraulic pumps just to name a few since most of them are designed to run most efficiently at a certain rpm which is usually higher then a regular idle but no where near a max no load rpm. Btw Im certified to work on and have gone to specialty schools for Cat, Cummins, Detroit diesel, yanmar and mercury outboards( eh it was free so why not much prefer diesels) . Also when it comes to loading of engines I’ll just use this as an example since The manual for a ford Leighman is the last manual I’ve read and know it off the top of my head. Without getting too deep into things there are generally different ratings for engines such as max no load rpm, max load rpm, and max continuous load rpm. And there may or may not be some sort of overlap between them. A ford Leighman 120 has a max no load rpm of 2650 . It’s max load rpm is 2500rpm at 120hp and it’s max continuous load is 2500 rpm at 114hp, so how you would read that is if you wanted to be able to use max rpm continuously you would need to have it slightly under propped so at 2500 rpm your propeller is only requiring 114hp from your engine. If you were trying to have it use all 120 hp at 2500rpm it should only be run at that load for a specific amount of time without any damage Or accelerated wear to the engine. That time is also generally specified in the manual, which it is not in the ford Leighman manual. Considering that it is not specified I personally would not use max load rpm for more than about 3 hours. That seems to be on the lower end for a time limit on those types of ratings but that’s just my oppinion on that, when it comes to a ford Leighman if an engineer told me less I’d defer to his judgment. Also while on the same thing the max continuous hp rating would be 114 so That would mean if you have the propeller set up to load the engine to 120hp at 2500rpm the max continuous rpm would be at the rpm curve for 114hp.
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