Lugging my motor...please explain this

[caper_45ca]

Recently I cracked a ring on the # 3 piston in a 1995 90hp Johnson v4.....the top rpm I ever reached was 5300...with an average load.....it was determined the ring failure was due to coking as a result of lugging....It was suggested that the top rpm for my motor should be in the 5800 range....That part I can understand...what I dont get is am I supposed to run the engine at 5800 all the time???....and if not and I can reach planing speeds at 4500 is this not lugging????Powerhead is being rebuilt and I am trying to learn as much as possible b4 I get bk in the water...Im lost any help greatly appreciated....

 

[dolluper]

Re: Lugging my motor...please explain this

Lugging would be extended periods of low idling like trolling without a blast once in a while

 

[ob15]

Re: Lugging my motor...please explain this

Lugging is running you motor with a prop that won't let it achaive the WOT. In most cases we try for 5800. if the motor is proped for 5000 rpm, the combustion temps rise and that's where the rings stick and then fail. Hopefully one the the experts will come around and explain it better but I think that's about it.

 

[jegervais]

Re: Lugging my motor...please explain this

The best analogy one can give for lugging is to relate it to an automobile with a manual transmission. What happens when you drop it into second or third gear and try to go from a complete stop? It bucks & snorts and clangs and rattles (and sometimes stalls if you're not quick enough with the clutch).<br /><br />As far as I know, all the outboard manufacturers have a recommended rpm range for each particular hp/model. Most (not all) are recommended to run in the middle to upper part of that range.<br /><br />For example, if the recommended WOT rpm range is 5000 to 6000 rpm, when running with a normal or average load, you should be running around 5500 rpm. If you run lighter you may pick up 200 or 300 rpm and conversly you may loose 200 or 300 rpm when you have a full load in the boat. This keeps the engine in the strongest part of its operating range with most loads.<br /><br />When your engine is over-propped, the damage occurs when you get the boat on plane or when it's accelerated hard - Remember the car analogy above. <br /><br />Since the 2-stroke doesn't have all the extra moving parts (cam shaft, valves etc) you don't get the audible sounds like the car engine makes - the boat is typically slower to plane or accelerate compared to similar hulls that are propped correctly. The damage adds up and over time, one day it goes "bang".<br /><br />It doesn't mean you have to run @ 5800 rpm all the time. It simply means the "gearing" is correct for the application and load. <br /><br />Hope this helps,<br /><br />-John

 

[walleyehed]

Re: Lugging my motor...please explain this

Let's think in terms of what the position of the throttle plates are in relation to RPM.<br />If over-propped, at 4800RPM the throttle plates are fully open. If we prop for 5800, and drop to 4500-4800, the throttle plates are just over half-way open, Resulting in much lower temps, and much higher fuel economy.<br />The end result of propping for 5800 in this case, would be a higher efficiency throughout the total RPM band.<br />The term "Lugging" is best described as low RPM, High manifold pressure (In aviation) a condition you want to avoid at all costs.<br />We can compare manifold pressure to high pressure combustion. with these temps, raw fuel and oil harden with temp at extreme levels, and the flow-rate behind the rings is low..this is where coking appears most and also does the most damage.

 

[jegervais]

Re: Lugging my motor...please explain this

Good point on the temps and fuel efficiency Kenny.<br /><br />-John

 

[Joe Reeves]

Re: Lugging my motor...please explain this

A prop with too much pitch restricts the engine from obtaining its proper full throttle rpm, usually referred to as "lugging".<br /><br />At full throttle (WOT) a engine for example might have a factory specified limit of say 6000 rpms. A perfect top rpm would be in the range of say 5800 rpm (not to exceed the 6000 rpm limit). However a prop with too much pitch might keep that WOT rpm down to a low (to pick a figure) 4900 rpm (Lugging).<br /><br />At full throttle, the carburetor butterflies are wide open. The spark advance is at the full spark advance setting.... ideal if the rpms are where they should be, but they are not. This sets up a scenario whereas the engine cannot breathe properly and it is impossible for the exhaust to escape completely.<br /><br />The above "Lugging" setup in turn causes a related condition known as "Coking". A build up of a grayish white waxy like substance in the combustion area between the piston top and the cylinder head dome.... a heavy build up! I have seen various engines with this coking build up whereas it would be 1-1/2" to 2" thick, enough that it would be removed with an ice pick.<br /><br />Best to have the proper propeller and a good tachometer.

 

[caper_45ca]

Re: Lugging my motor...please explain this

Well thank you very much for the quick responses..It is great to know that there are so many knowledgeable people available to share their thoughts. For us guys who are very limited on such topics, it is an invaluable resource.<br /><br />Thank You