ILriverboater

I have twin 1991 Merc 5.7's producing 260hp according to Merc. The outdrives are Alpha one gen II are good for up to 300hp so this is my limit. I would like to get 300hp out of both engines at a lower rpm 3500-4000 than the current 260hp at 4200-4600 since I don't cruise in that rpm range (fuel economy). I would like suggestions on the most economical way to get this done. Pulling both motors is not out of the question but keep in mind it cost around $400 to pull a motor. I was thinking that head and exhaust work may do the trick with a possible cam change. But would like to hear from people who may have already been down this road.

Thanks all.

andyt25

Intake and cam would probably get you close to 300 but to be honest I would leave it alone .

endeavour32

iTrader: (4)

I agree with andy, you're only going to gain 2 mph for a lot of work. Not worth it!

Knot 4 Me

If you looked at a dyno sheet when those motors were new, they produced their maximum rated HP at the upper end of their recommended operating range. If you want those 350's putting out 300 HP at 3,500 RPM, they are going to be making more than 300 HP at 4,600 RPM. HP isn't so much your concern here with the Alphas as is torque and the weight of your boat. How many hours are on the motors?

Josh LaFountain

My old boat had a 350 LX in it. I put a set of Edelbrock Aluminum performer rpm heads, a matching intake and EMI thunder manifolds , 650 demon carb, and insted of changing the cam i put a set of 1.6 crane gold roller rockers on it , that changes the cam just enough to notice a difference. and it was real close to 400 hp. The alpha held it fine i just never stabbed it out of the hole. get plained out easy and then you can throttle up. I ran it for 4 years like that and never had a problem.

Josh LaFountain

Here are a few pics of it. It gps a lil over 70. sad to say it was faster than my powerquest is. but it had to be glass out or it would throw you out. LOL.

Josh LaFountain

I agree totally. Mine made all its power at the top end. 4000 and up. i turned a 14 3/4 23p mirage and it was a pig out of the hole. never hurt the alpha tho.

Griff

Less rpms just lungs the engine and makes them work harder. You still have to push the throttle just as far forward and it uses the same amount, if not more fuel.
SBC's make more power and torque with more rpms.

ILriverboater

Josh,
For the price of the parts you describe above ~$3,600 I could get a stroker crate engine. I like your setup just too expensive for me right now.

What about reworking the current heads? Is it possible to get more flow out of the current setup?

ILriverboater

One motor has about 500hrs and the other is a rebuild and only has about 100hrs. The weight of the boat is 10,000lbs. Would a drive shower or another mods to the outdrive increase its HP and torque rating?