Somebody please correct me if I'm wrong, But I believe there is a bigger difference than 40HP between an old carb 260HP/350 and a new 300HP/350magMPI.
A 260 carb engine is 260 at the crank. A 350magMPI is like 325HP at the crank. That is more like a 65HP increase. I think you will gain 400-500rpm with the same prop which should give you about 3mph... You will love the MPI engine, well worth $2000!!!
Good Luck,

voodoo
 

what kind of boat is that in your avatar,sure looks neat,Darin

Velocity 280, thanks!

justin vip
 

your original motor 350 260 hp you said 56 mph at 4,500 let me know if I am wrong but isnt wot range on that motor 4,800? Darin

Everyone has told me differently about what RPM I was supposed to be running with the original 260HP motor. Trimmed out I would only hit 4600. I never tried another prop on it all this year. Next year, with the new motor, I will be trying some diff. props. I have access to some diff. ones to try, and I think that will be my best bet.

Voodoo, I'll have to grab your GPS someday and go screw around on the Mississippi! Couldn't figure out how you knew I had a "100HP" Firecat!!! (until I saw your pic. of your boat!) I'm hoping that I was propped wrong last year so that I can see 60MPH this year. If nothing else, I will like the MPI vS the carb....

Thanks guys!!

Justin

Houston Prop, I will be calling you in the early spring...thanks for the info.

My '81 260/350 says 4200-4600RPM's on the valve cover. I've ran it as high as 5100 with small props, but I'm sure that's not good for the motor.

Brian

AGREED!!!! Mine says the same thing, although it has been known to turn a little closer to 5000 at times, I am plannin on swappin the 19 for a 21 soon(before I start breakin things). I have finally got the motor runnin right, and was able to pull 5000 an Sat. with 5 adults, and a full tank, 23 foot Wellcraft, Saltwater, Light chop in the ICW.

Do you know the weight of the boat??? Try here

http://www.mercurymarine.com/prop_selector

Guys a 260/350 will not make more power by spinning it over 4600rpm. Also, by over-reving the engine will lean out. I learned the hard way by melting pistons...

If you change the cam, intake, and re-jet the carb. You can turn your 260 into a 300...

Happy Boating,