1988 V20 steplift/305 chevy, mercruiser drive.

UBHSTRY · #1 04-05-2011, 10:28 PM

I am working on one of these, I have been looking for some information on how much power the drive can handle on these boats. Also, what could be done to them if say, you wanted to stuff some decent power in it. IE: Chevy 383, nice aluminum heads, good intake, roller rockers and cam, and a holley 600. Mild on the cam. But capable of spinning to 65-6800rpm.

THEFERMANATOR · #2 04-05-2011, 10:36 PM

It depends on which drive you have. The early ALPHA units are rated to 300HP at the flywheel, and teh later ALPHA's are rated to 300HP at the propshaft IIRC. The pre-ALPHA drives are a bit weaker by design I believe though. As for high RPM's, they don't take well to that fro mwhat I understand. Theres a guy at screamandfly that mods the ALPHA drives to 400HP or say and 6500 RPM's, but this is for light applications, a V-20 is not considered a light application for an ALPHA drive. 5,000-5,500 is fine on them, and if you plan on going over 260Hp at the flywheel I suggest running the exhaust through the transom and install a drive shower along with running synthetic lube in teh drive as heat is teh enemy with the I/O's. And torque is teh killer for an ALPHA as the gears in them just are not meant to take the abuse(Ibelieve teh ALPHA's use the same basic design as the 3.0L MERCURY 2 stroke outboard lower unit).

spareparts · #3 04-06-2011, 06:15 AM

ferm has it right on the numbers. If you want a reliable package, the 383 might be a bit too much for every day use. Merc sells their own 383(320 hp) and usually puts it in front of a bravo. For what you are doing with a V, a good strong 5.7 would be enough.

step up here · #4 04-06-2011, 06:30 AM

x 2 times,

they are right on the money. i currently own that combo. 1986 vintage!

brisboats · #5 04-06-2011, 07:24 AM

Mercruiser put the alpha one behind the 454 for a time and it was a disaster. The torque of the big block would wipe the upper gearset in hard use. The alpha is good for about 300hp and you cannot run it like an offshore powerboat when you have a hot smallblock even with a drive shower and extra reservoir for the fluid. Car hot rod modifications do not really work all the time for boats, your rpm range is to high.

Brian

THEFERMANATOR · #6 04-06-2011, 05:50 PM

Quote:

Originally Posted by brisboats

Mercruiser put the alpha one behind the 454 for a time and it was a disaster. The torque of the big block would wipe the upper gearset in hard use. The alpha is good for about 300hp and you cannot run it like an offshore powerboat when you have a hot smallblock even with a drive shower and extra reservoir for the fluid. Car hot rod modifications do not really work all the time for boats, your rpm range is to high.

Brian

And that 454 ALPHA had a custom set of 1.36:1 geras in it built just for the extra torque and it BOMBED!

spareparts · #7 04-06-2011, 07:00 PM

The 1.5 HD gears were actually stronger, the 1.32 had too small of a driven gear. I had a customer that ran a 720hp Troy Dennis blower motor in front of an Alpha SS. he changed the lube every time out, never drag started it, rana duel shower and usally only ran other boats from 65 on. We had the 1.5 HD but ran out of prop options, ended up with 1.32 and a 27 Mirage(double cupped and labbed), biggest prop we could find with a rubber hub(pre plastic hub days), boat radared out at 101. I still put a fresh set of upper gears in every spring

UBHSTRY · #8 04-06-2011, 10:29 PM

Ok, could you help me with the location of what would identify exactly which outdrive unit I am working with?

spareparts · #9 04-07-2011, 05:55 AM

88 should be an Alpha 1 generation one. The top cap on the upper unit should have a flat top. Gear ratio should be stamped on the side on one of the decals. You're gear ratio snould be 1.65. IF you happen to pull the upper unit apart, drain the lube and look for small marks on the teeth, they should be timing marks(only on the 1.65), the gears will have the same number of teeth on each gear. Lower unit should have plastic insert type water pick ups vs molded in water holes. The only way to be sure you're lower is an Alpha is when you drop the lower to do a water pump, it shouldn't have a small spring loaded pin on the vertical drive shaft. If you decide to build a 5.7, you can switch to the HD 1.5 gear ratio upper gears, just know it takes specific tools to set up the gears

brisboats · #10 04-07-2011, 07:12 AM

Spare is right the 1:5:1 HD are the gears to have AND you still have to be very very careful. Running the sticks like you are Reggie Fountain will only put lots of money in your mechanic's pocket. Personally I think the 5.0 warmed over would be plenty of motor for a V-20. Making a 300hp 5.7 is easy but with this hull I think you would be reaching very diminishing returns.

Brian