RMP Range vs performance and engine life

[sabastianunf]

I was just wondering what affect over reving an 1978 120 hp mercruiser sterndrive by 100-200 rpm over manufacturer reccomendation would have on performance and engine life? I simply want to get the best hole shot possible but don't want to kill the engine by over reving or loose performance by getting outside of power band. Any input is greatly appriciated.

 

[QC]

Re: RMP Range vs performance and engine life

Well if you want the hole shot and you are worried about engine life just don't exceed 4600 RPM . . . If your priorities are those two, then why go the 100 - 200 over? I will say that I have pushed RPM over the years at times and I have also never had an engine failure . . .

Oh, and even though you are exceeding the WOT RPM range it does not necessarily mean you have exceeded good engine performance, however, if you propped for 200 RPM Less, your boat would probably have a faster top speed . . .

 

[sabastianunf]

Re: RMP Range vs performance and engine life

Manufacturer rec is 3900-4300. Do you still think that I could run it up to 4600 without any damage? Also as you run more rpm's does that decrease the load and/or wear on the engine?

 

[QC]

Re: RMP Range vs performance and engine life

Well I am picking up 4000 - 4600. Where you getting the 3900 - 4300? My source is not reliable. Anybody else know?

The answer to your last question is kinda yes, and usually no . . . Higher RPM decreases actual torque and pressure etc. for the same horsepower load. However you need to think of RPM kinda like distance. When running higher RPM the pistons are traveling more distance up and down per any time measurement. Soooo, the engine's wear is accelerated at higher RPM. The real killer is very heavy load at very high RPM, everything is being worked hard in that example and, yes life will be shorter . . . Oh, oh, and higher RPM can cause valve float which is really bad . . .

BTW, I think you are over thinking this. Find the right prop for the hole shot you're looking for and keep the RPM to some limit (you can add a little slop, but not much. 100 - 200 should not be a big deal) and you should be fine. Just don't run there for extended periods . . .

 

[sabastianunf]

Re: RMP Range vs performance and engine life

Thanks for the insight - I think I will prop it to run a little over to get a better hole shot but keep it in range with the throttle.

The RPM range I quoted is what is on the valve covers and is from mercruiser. However I have heard that they increased this range on some of the later models.

Also- I like your responce Kinda Yes but usually no! Sounds like something I would say.

Thanks again.

 

[QC]

Re: RMP Range vs performance and engine life

Thanks for the insight - I think I will prop it to run a little over to get a better hole shot but keep it in range with the throttle.

No prah dude, that is exactly what you shoud do if hole shot is numero uno . . .