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-   -   RPM at WOT (http://www.wakeworld.com/forum/showthread.php?t=794655)

Bumpass1 07-22-2012 6:23 PM

RPM at WOT
 
I was on the Supra forum and ran across the following and thought it was useful info that everyone could benefit from.

If you wakeboard or wake surf and load your boat heavily it is critical that your boat is propped appropriately to ensure that your engine gives you the kind of life that you expect. It is critical that the engine is able to turn RPM at wide open throttle within the designated range for your engine. The standard 5.7L engine, Assault 325, 330 … the engines that have the flame arrestor at the top of the engine like a carbureted engine … the WOT RPM range is 4600-4800. The premium 5.7L engine, Assault 340 … the engines that have the flame arrestor at the rear of the engine and a decorative plenum on top … the WOT RPM range is 4800-5200. The 6.0L engines … the LS2 and the L96 … the WOT RPM range is 5200-5600.

What you need to do is load the boat the way you would for whatever activity you are loading it for. If that means all standard ballast full, any extra ballast, 3 cases of cold beverages and 8 friends … load it up. Make sure your fuel tank is full also. You want the boat to be the heaviest that it is ever going to be. Now carefully take the boat up to wide open throttle and see what the maximum engine RPM is. Be careful because the added weight might make the boat handle differently at WOT than it does when lightly loaded.

If the RPM for your engine is within the WOT RPM range for the engine you are “GOLDEN”. Go wakeboarding … go surfing … have fun! If the engine turns RPM at WOT it means that it will be properly loaded throughout the RPM range. You don’t always have to run the engine at WOT … in fact I don’t recommend that you run at WOT for extended periods of time … it just has to be able to run within that range at WOT. If the engine is not able to turn in the WOT RPM range for your engine, you have two choices … take some weight out of the boat or change the propeller. Every time you run the boat in those same conditions (overloaded) you are risking the longevity of your engine, and if the engine fails under those conditions it is a good chance the failure would not be covered under warranty even though it may be within the warranty period.

If you can’t tolerate reducing the weight in the boat, you will have to re-prop with a smaller pitch/diameter propeller to reduce the load on the engine. I am not a prop expert so I won’t be able to help you select the correct prop. Fortunately, I have some friends who are experts. You can contact the good folks at ACME (www.acmemarine.com) or OJ (www.ojprops.com). Either of those fine companies will be able to help and make propeller suggestions that will get you where you need to be RPM wise.

Once you are propped for the maximum load in the boat, what is going to happen when you run the boat without the load? Not to worry … running the boat under-propped for the load may have an effect on the WOT performance of the boat … it probably won’t go as fast. You won’t have to worry about over-revving the engine though because all of our fuel injected engines have RPM limiters that will not allow the engine to over-rev. It is much better for your engine to be under-propped than it is to be over-propped. If you don’t like the lightly loaded boat performance with the smaller prop, keep your old prop and use it when you are running lightly loaded and switch to the heavy load prop when you use the boat heavily loaded.

Why is this so important? For example, if your engine is only able to turn 4000 RPM at WOT, the engine is running hot and hard but because it cannot turn at the rated RPM, the water pump is not pumping as much water as it should be or that the engine wants when it is working that hard. Also, under those conditions, the throttle settings are telling the ECM/computer to supply fuel to the engine for WOT operation but because the engine is not turning the appropriate RPM, the engine is over-fueled. It may not be able to burn that excess fuel … so now we start washing oil from the cylinder walls which accelerates cylinder wear. And where does that excess fuel go? Some goes past the rings into the oil which dilutes the oil and reduces its ability to properly lubricate the engine … and some goes out through the exhaust. If the engine has catalytic converters, unburned gasoline in the catalysts creates extra heat in the catalyst and with the diminished water flow because of reduced engine RPM we have a hard time keeping the catalysts and manifolds cool which creates more problems.

Bottom line … it is extremely critical that your engine is able to run within the rated RPM range at Wide Open Throttle. If you run your boat heavily loaded and the engine will not turn within the designated RPM range, you are killing your engine and if it dies under those conditions that is considered abuse and abuse is not covered by warranty.


Larry Engelbert
Indmar Marine Engines

Here is the link to the thread

http://www.supraboats.com/bbs/showth...8410#post78410

Raf1985 07-22-2012 7:53 PM

I was always wondering about this. Does the same apply to surfing? It just seems kinda hard on the engine when the boat is turning high rpms but only going 11mph or so.

shawndoggy 07-22-2012 8:24 PM

Quote:

Originally Posted by Raf1985 (Post 1770093)
I was always wondering about this. Does the same apply to surfing? It just seems kinda hard on the engine when the boat is turning high rpms but only going 11mph or so.

That's not what he's suggesting... he's saying you should be able to approach redline fully weighted with the hammer down. Not that you have to be at redline while surfing.

ralph 07-22-2012 9:03 PM

FYI My merc mechanic told me the same deal 4-5 years ago so I think it is true for all motors also.

Raf1985 07-22-2012 11:55 PM

Quote:

Originally Posted by shawndoggy (Post 1770102)
That's not what he's suggesting... he's saying you should be able to approach redline fully weighted with the hammer down. Not that you have to be at redline while surfing.

I'm just saying that I can weight the boat down for surfing, and still pass the WOT test but it seems like the motor gets really stressed while surfing at low speeds and high RPM.

nitrousbird 07-23-2012 3:11 AM

He posted the same thing on The Malibu Crew.

And several of us asked questions regarding this, as it doesn't make a lot of sense. The ECM should have proper fuel maps for a given load, referencing RPM vs TPS vs MAP sensor inputs, and adjust the injector pulse-width and timing accordingly. If the motor is running rich because of this "overloaded condition", that is the fault of improper tuning IMO, not just overloading the boat.


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