WOT rpm range

[jimmbo]

If the Manufacturer specifies that an engine has a recommended WOT rpm range of 4500 to 5500 why do some posters on these forums insist on proping the engine to turn 6000+? They drone on and on about overproping, but never about the stresses of overrevving. Since the manufacturer states 4500 is the lower end of the range, anything above that to 5500 should be fine. Correct? Also going a given top speed at WOT, shouldn't an engine that attains that speed in the lower end of the range get better fuel economy than one that is propped for the high end of the range. I ask cause I believe that at WOT an engine turning 4500 rpm is pulling less air in than one turning 5500 rpm, and since fuel is metered proportionaly to the air, less fuel should be burned. In addition, since horsepower drops off on either side of the rated rpm, under propping will result in less horsepower being produced while frictional losses are rapidly rising.

 

[JB]

Re: WOT rpm range

The "perfect" rpm at WOT is the rpm at which the engine makes its HP rating. That is the rpm at which it works most efficiently at WOT.<br /><br />For example, an engine rated 150HP at 5,000rpm will typically have an operating range from 4500 to 5500.<br /><br />We load and balance our boats differently at different times. We therefore want to prop it so that it reaches 5500rpm with a light load toward the stern but can still reach or exceed 4500 when heavily loaded and perhaps a bit bow heavy.<br /><br />When we have more extreme ranges of loading and balance we might prop the boat to run 5,000 with a typical load but be capable of 6000 with the lightest load. That doesn't mean we are going to go out and run her at 6,000 all afternoon.

 

[imported_JD__]

Re: WOT rpm range

jimmbo,<br />I got the same impression. Sure, engines will rev up more than the manufacturer recommends but I think its best to err on the side of cautiion. Good point.<br />JD

 

[Dhadley]

Re: WOT rpm range

Over-reving. Remember we are generally talking about 2 stroke motors that run on needle bearings and roller bearings. No babbitt bearings, no push rods, no valves, no lifters, no springs, no...well, you get the idea.<br /><br />Experience has shown us that as long as there is a load on the engine these motors can achieve extremely high rpm fairly easy and live a long time.<br /><br />Lets assume we are talking about a carb'd V6 crossflow. Stock port timing and stock port area. Stock carbs and jetting, timing, heads, no rev limiter in the pack, stock ignition etc. We know the stock reeds will start to flutter in the low to mid 6000 rpm range. Usually at somewhere around 6200 to 6400 the motor just wont rev (breathe) any more. Install Boyesen reeds and you have the ability to get the same motor to 7000+.<br /><br />A stock V6 looper with the limiter removed will hit 7200 or so easily. Thats where the stator gives up. Install a CDI race stator and 9000 is no problem. <br /><br />As for the set up rpm's, we're looking to set up a given recreational motor so it will have the longest service life possible. It has to do with combustion temps. An overpropped motor will have combustion temps above normal design parameters. Abnormally high combustion temps promote coking and piston swelling. Coking causes stuck rings that can lead to broken rings. Pistons are designed to swell to a given point (those design parameters again). Abnormally high combustion temps lead to swelling past that point (sticking) and can remove the hardness in a piston resulting in wrist pin pound-out.<br /><br />Sorry about the long post but you have some very good questions and this is just the tip of the info. There's a lot more to it and it all ties together with hull design, balance, prop design, hydrodynamics and so on.