Outboard HP rating?

[brw0513]

In the near future I intend to buy a new outboard for my 3.8m Markham Whaler. I really like this boat, but it has proven to be a lot of boat to get out of the hole and onto the plane in a reasonable amount of time - at least for the existing 35HP Evinrude (1982 vintage).

I have done some elementary calculations and believe I need a 45HP outboard. Of course that means either a 40HP model or the 50HP model since there is nothing in between. I guess if my calculations were wrong it could mean two things. Either I buy the 40HP and rue the fact I should have bought the 50HP, or buy the 50HP and rue the fact that I only needed the 40HP.

When I look at two-stroke specifications, I see (Tohatsu, Yamaha and Mercury so far) the 40HP models and 50HP models have the same no. of cylinders and displacement and weigh the same. I suspect the power head is the same. So what turns a 40HP into a 50HP?

Is it different compression ratios or different carburation or different exhaust or different ignition? Or is it just some form of restriction easily removed after the warranty period?

To ask the question another way, how do you turn a 40HP into a 50HP after the warranty period?

Any advice greatly appreciated.

[Shane]

From what I have been told by manufacturers it is just a limiter, ie 40hp ETec can be remapped to a 50hp ETec to go to a 60hp but requires additional exhaust system.

[Rosie]

Often it's just the carbit intake but theres also prop differance on some models. All depends which ones though.

[Rossco32]

The Merc & Tohatsu are the same engines (Both beingbuilt by Tohatsu).

The difference between the 40 & 50 Hp models are:

Carbs are smaller on the 40
Different length exhaust 'stack' inside the leg
Different WOT ignition timing
Slightlgy higher max rpm for the 50hp model.

But why bother? If you run the 40hp to near the top of it's max rpm range it will be pumpimg out well over 40hp anyhow.....

That old 82 model was rated 35 hp at the head, all the new motors are rated at the prop.

That old motor would be like a modern 25hp in reality...

[brw0513]

Rossco32,

Thanks for the good info.