Originally posted by Chipmaker
I would think horsepower is horsepower. (proovided that both manufacturers rated them the same) The big difference is the rpm range or power band in which the motor gets the rated horsepower. I would bet that the B & S engine would swing a higher pitch prop without getting lugged down than a comparable 2 stroke would, so you should be able to pitch the prop up on the B & S to improve speed, and run it at lower rpms due to it having more torque. Most 2 strokes need to be run wide open as thats about where their max power band is at. The real nice thing is to be able to slow it down to slow speeds without bogging down and choking up and such like a lot of 2 stroke motors do.
The same indiviudal that I know that has a few outboards adated to B & S engines also has a troling motor made from a weedeater......he just substituted a trolling motor prop for the cutter head on a Poulan brand weedeater, and it works great. Much noiser than the typical bat powered troling motors, but then this thing probably did not cost him one cent to make as he is even more frugal than I am. As frugal as I am I still like for whatever I cobble together to have at least a sense of not looking all that much homebrew if possible.
You're right on a lot of your thinking here. The B&S would need more prop than than the original 2-stroke would to achieve similar top end speeds. The problem is that it would be harder to get the boat on plane with a larger prop.
I'm not the greatest at explaning things. In fact I kinda suck at it, but here goes. Horsepower is just a number derived from torque.
It's actually torque multiplied by r.p.m divided by 5252. With this you can figure that an engine with 15 horsepower at 3750 actually has more torque at that r.p.m. than an engine with 15 at 5200 has at 5200. Logic would tell you that 15 at 3750 would be better. The thing about horsepower though is that it's based on r.p.m. Most outboards run a gear reduction somewhere between 1.6 to 1 and 2 to 1. When you put torque through a gear reduction, it multiplies the amount of torque by the gear ratio. Example: 20ft.lbs. with a 2 to 1 ratio equals 40ft.lbs. Once again 15 at 3750 looks better. The difference is that the slower winding engine would need a higher gear (lower numericly) to achieve the same top speed as the higher winding engine, or it would need more pich on the prop. Putting a bigger prop on the engine is effectively changing that gear ratio, just like putting taller tires on a car. This means that the higher you make your power, the more torque you will have where it counts because you can use a lower gear (higher numerically) or a smaller prop. The lower the final gear ratio, the faster you will come out of the hole and get on plane.
I'm a mechanic in a marina, and I see a lot of 2 and 4-stroke outboard engines. I can tell you one thing. Nobody makes a 4-stroke outboard that will compete with a comparible hp 2-stroke.
It's not even close.
I'm not saying that setting a briggs up on your outboard won't work. It's just not going to have the same performance as the 2-stroke. I understand the reasoning for wanting to do it, and it makes sense. If you're just looking to get through some muddy water and hunt ducks, it should work. If you're looking for serious speed, I don't think it would do well. I would actually keep the prop that was originally on the engine to take advantage of the gear ratio, and just not worry much about top speed.
