I had a 305 small block with 3000 hours on it without a rebuild. Still ran great when I sold it.

So to answer your question, Longer than you think.

If you run a larger blower like I suggested with low boost, you can leave the internals alone. Your motor is already set up nicely with forged pistons, etc,. You'll see about 100 to 120 HP gain, which translates to approx 10 MPH or high 70's boat speed. The advantages are obviously greater speed, takeoff torque, and of course you'll peg the "cool factor" meter! Most of the time, the engines suffer no effect from the superchagers because they are not making boost at cruise speeds. The disadvantges are reduced engine life, increased fuel consumption, frequently fouled spark plugs, and sometimes idle problems. Although, with 3 pounds of boost this shouldn't be an issue. I absolutely love superchargers and would not run a performance boat without them. There is no other upgrade that can come remotely close to a blower in terms of horsepower gain, PERIOD... I'm on my third blower motor boat, so I have some experience with these... Bob

Might consider head gaskets due to the age of the motor.

would a wieland 177 blower anygood not shure i spelled it right

Cig20,
You might consider leaving the stock 420's and raising the X and using a 4-blade Bravo 1 prop. I am assuming this is a 20' Donzi. If so, both the blower or a raised X would likely require mod's to the hatch/hatches. You would also be stretching the limits of the TRS past 500-600 HP. I hear that the TRS parts are also getting pretty scarce.
Ben

A blower would be a nice easy way to add power to the Merc 420, but it's the TRS drive that would be the weak link. The Merc 420 has 4-bolt main saddles and an all forged rotating assembly which makes it strong and reliable.

A small, mild blower with LOW BOOST may be safe for your application, but when you notice the power difference from what you had before, you may become a little too confindent one day and BOOM...there goes your TRS drive. Especially, while coming out of the hole too strong or the fatigue on the TRS from the added torque that is inherit of a blower will eventually sneak up on you one day when you least expect it.

it is a 22

I would go for it. What's the worse that can happen? You blow an engine or outdrive, so what? Go online and buy another one and be back on the water by the weekend! The chances of you blowing two of these in a season is quite remote. To answer your earlier question, I would shy away from the 177 blower as it's too small and will produce too much intake heat.

so is a bm250 the way to go or a ???
i have a extra drive