305 to 350, change prop?

[MBAKER]

Looking for some advice here..just finished up repowering a 2 barrel 305(valve issues) with 4 barrel 350 in a 2000 Maxum 2100SR with 1.47 Alpha 1. How much difference in prop pitch will this require, all else being the same. Just general guide line will help get me started. With the 305 we ran a 17" pitch prop and would get into the proper range at WOT but im not sure if the engine has been a little weak all along anyway. Now with the new engine I assume it will need more prop, but not sure if 2" would be enough, or more? I can leave the 17 on it to try, but Im trying to round up a 19 and maybe 21 to try.


I know its a hard question to answer, but if anybody running something similar has a suggestion I would appreciate it. Id like to go out with something close, and not mess with swapping props several times is all.

 

[Bondo]

Re: 305 to 350, change prop?

Ayuh,.... I'm gonna Guess it might pull the 21", from what you've said....

 

[emilsr]

Re: 305 to 350, change prop?

Can't say for sure.....but....

FWIW, it will depend on the rpm range of the 350. A few years ago, a friend and I had identical boats. Mine had the 5.0mpi, his had the 350MAG. We wound up running the exact same props; turning 4800rpm on my rig and 5100 on his.

Test before you go prop shopping; you may not have to do anything.

 

[MBAKER]

Re: 305 to 350, change prop?

It should be a pretty even comparison from engine to engine, just stock cammed vortec 350 from ad sheets Ive seen I think it would be about the same curve, but the 350 making about 75# more TQ and 100 more HP.

I thought at least a 19 would be in order, but wasnt sure about a 21. The mercury prop selector using weight for calculation keeps giving me something like a 15 or 16 and only like 4800 WOT, which doesnt seem anywhere close. Until I get it back to the water I guess its just a shot in the dark anyway.

 

[JaseBosto]

Re: 305 to 350, change prop?

Put it on the water and find out....

 

[MBAKER]

Re: 305 to 350, change prop?

weather permitting, I hope to this weekend....

 

[scott8058]

Re: 305 to 350, change prop?

In my experience it depends on whether u use aluminum or stainless, my 350 over revs my 21 aluminum and gonna get try a 23 this yr, but even tho it over rev'd my 19 stainless it put a different load on the motor and the bigger aluminum actually runs thru the rpm range much smoother. but if i were gonna purchase and spend the money id go with a 21

 

[Mi duckdown]

Re: 305 to 350, change prop?

To be honest with changes you are making.??? Need to get it running than get back with true RPMS and GPS speed. everything is else is pure speculation.

 

[MBAKER]

Re: 305 to 350, change prop?

Well we made it out. WOW what a difference from the 305. Anyway, im not sure i could tell much more defintite other than it needs more prop\, which i figured. It would jump right up out of the water and take off then it would hit 4800 and still have 1/8 or more throttle to go all without any trimming. Granted it was light load of fuel, and only 3 adults, but Id say a 19 is in order at the very least. I think it would pull a 21, but im leaning towards the 19 just to keep it freed up with heavier load, and hot humid summer air. Plus for what we do we use the bottom end more than the top ayway.