RPM's

[rbh]

Why does some 260hp mercs spin higher than others. The tag on my motors say not to exceed 4700 rpm, and some guys are spinning theirs into the lower/mid fives?

 

[Bondo]

Re: RPM's

Ayuh,... Mostly it's got to do with Vintage, or Induction...

 

[Don S]

Re: RPM's

and some guys are spinning theirs into the lower/mid fives?

Some of the newer Merc and Volvo engines with MPI and roller lifters have a max of 5000 to 5100 rpm, I don't know of any SBC engines running mid 5's, and none of the 260hp engines ever went that high.

 

[rbh]

Re: RPM's

Thank you gentleman,
interesting to read sometimes that the guys sometimes hit the 5000+ mark when they plane out the leg, I am thinking now that they may be going a little to far.

 

[Bondo]

Re: RPM's

interesting to read sometimes that the guys sometimes hit the 5000+ mark when they plane out the leg, I am thinking now that they may be going a little to far.

Ayuh,... If you mean trimming out the Drive,.... Yes, it's known as prop blow-out...or ventilation...

 

[Don S]

Re: RPM's

Possibly they are way underpropped on engines without MPI, as they have rev limiters in them. But just getting higer reves doesn't mean anything, if they are under propped, then they are loosing top end speed too.

 

[45Auto]

Re: RPM's

Why does some 260hp mercs spin higher than others. The tag on my motors say not to exceed 4700 rpm, and some guys are spinning theirs into the lower/mid fives?

Yours will spin way past 5000 RPM also if you put smaller props on them. Just because some people are spinning their's that high doesn't mean it's recommended by the factory. I believe the factory recommendation for most 260's was 4600-4800 RPM, and they make max power at around 4700 RPM. As Don said, underpropping them so they spin past the point of max power will actually give you less top speed. Your boat's fastest point will be where it is turning the biggest props it can and still deliver maximum HP. Turning the engines faster on a smaller prop is equivelent to driving your car down the freeway in 1st gear. The RPM will be very high, it'll be way past the point of max power, but it will be much slower than it would at lower revs in a higher gear.

 

[CheapboatKev]

Re: RPM's

And I will state the obvious..

Spinning an undersized prop into that upper rpm arena will cause excessive and premature wear & tear to your engine & drive, with as stated NO increase in actual speed.

Sounds like a lose/lose, unless your a shop!

 

[rbh]

Re: RPM's

Yours will spin way past 5000 RPM also if you put smaller props on them. Just because some people are spinning their's that high doesn't mean it's recommended by the factory. I believe the factory recommendation for most 260's was 4600-4800 RPM, and they make max power at around 4700 RPM. As Don said, underpropping them so they spin past the point of max power will actually give you less top speed. Your boat's fastest point will be where it is turning the biggest props it can and still deliver maximum HP. Turning the engines faster on a smaller prop is equivelent to driving your car down the freeway in 1st gear. The RPM will be very high, it'll be way past the point of max power, but it will be much slower than it would at lower revs in a higher gear.

Probably the same boat and motors as yours, I can not get it over 4200 rpm
even trimming up the leg.

 

[45Auto]

Re: RPM's

Do you know what props are on your boat, Rob?

My '87 Sundancer 300 with the 260's has 19" pitch High Fives that will allow it to rev to 5200 RPM if I would let it. I keep forgetting to take the GPS out on it (it doesn't have a speedometer from the factory - no hole for it on the dash - does your 270 have one?) so I don't know what kind of speed I'm getting. I plan to put a fuel flow gauge on it this spring, then mess with the props a little to try to optimize the cruise efficiency.

I guess it's possible that with the boat heavily loaded the 19" pitch props may be in the 4800 RPM range. I've never had more than 3 people on it and both tanks (twin 75 gallons) have never been more than half full - usually around 1/4. Another 850 pounds of gas and 1000 pounds of people and supplies might put it right where it should be.