Gear ratio for engine upgrade?

[erikgreen]

Okay, it's a new day and time for my next question

I'm putting a 350 block in a boat that had a 305 block. Additionally, since my old sterndrive was trashed (in pieces) I'm going to use a sterndrive from an even older boat temporarily until I can get a new unit.

The older boat had a Ford V-8 engine, and the sterndrive has 1.5:1 gears. The new engine will have more torque/hp (equal to a Mercruiser 260) and I see a 1.28:1 gear ratio is recommended for it.

I presume (correct me if I'm wrong) that the lower ratio is due to the fact that the new engine produces more torque, and therefore the prop can be spun faster than with the older engine without problems, hence less need for gear reduction?

If that's correct, then I can run the 1.5:1 ratio drive for a bit, and all that will happen is that I'll be getting more torque than I can effectively use out of it, and less prop rpm? That'll also mean my engine will be running at a lower than WOT rpm a lot of the time, which should work but isn't a good idea long term?

In short, how critical is it that I change the gear ratio now instead of waiting?

Erik

 

[Don S]

Re: Gear ratio for engine upgrade?

Assuming you are talking Merc Alpha, 1.5 gear ratio is what you want for any of the small block V8's.

 

[erikgreen]

Re: Gear ratio for engine upgrade?

I am, and that's good to know. I think you're a better authority than most manufacturer's web sites, Don

FYI, I got the 1.28 (1.32) number from the Piranha props site:

http://www.piranha.com/enginelist.p...P=260.0&submit.x=30&submit.y=19&submit=submit

Erik

 

[180shabah]

Re: Gear ratio for engine upgrade?

I thought that ratio (1.3) was only used with big blocks.

 

[Don S]

Re: Gear ratio for engine upgrade?

Piranha doesn't have a clue. If you want to know the proper ratio for a Merc drive to an engine, don't go by some aftermarket prop company with sketchy info at best.
With a SBC v8, you need a 1.5:1 gear ratio.