1988 V20 steplift/305 chevy, mercruiser drive.

U

UBHSTRY

Junior Member
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.
 
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).
 
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.
 
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
 
brisboats said:
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
Click to expand...

And that 454 ALPHA had a custom set of 1.36:1 geras in it built just for the extra torque and it BOMBED!
 
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
 
Ok, could you help me with the location of what would identify exactly which outdrive unit I am working with?
 
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
 
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
 
Forgot to add 1988 puts you square in alpha one generation one category but you never know what someone may have bolted on over the years. External clues are the flat top on the upper gearcase, and a squared off cavitation plate on the lower versus a rounded one. Funky drives are the lowers from 83-somewhere in 85 they will have the newer case with a square cav plate but the earlier internals, smaller shaft and a preload pin. Like Spare said you really won't know what you have until you open it up. Count your teeth in the upper gearset I have seen some 5.0 come with 1:5:1 ratios too.

Brian
 
Ok thanks guys. I am going to take a real close look when I pull the boat back in to drop this engine in.
 
I must say I appriciate the help. Most boards call the newbee nasty names when they are doing something a little different.

By the looks of things, I will for sure need to do something with this outdrive. I surmise it may not last long.
 
Ok, fired the engine up this evening....well, sounds healthy. Going to spark limit it to 4800rpm to possibly 5200rpm, untill I can figure out the drive situation. My biggest fear is the thing pulling up from the hull. I have it strapped down, just in case something goes horribly wrong. Man alive, this thing should be in a car:head: (The gentleman has a really nice 88 7 series BMW I did for him, and keep telling him he should let me put it in that.....lol) Possibly a different drive unit altogether? What does it take to be able to reliably spin an out drive to 6500rpm? What fails? Bearings, or gears? Jet drive possibly? Or am I just off my rocker, and keep it dialed way down? (hm...way, way down)
 
Wowser, we took it out last sunday for a shake down run. Well, its fast, and gets on plane in the blink of an eye. The new holley carb did have a stuck float, so called it a day. Got the new digital ignition in this week, and will be heading out again tommorow to finish the engine up. Then on to the outdrive........ this 383 seems to have been a good choice.

I was going to post pics of the build, and runs but dont seem to be able too yet.
 
The drives don't like more than about 5500 rpm, you can upgrade the lower end by replacing the shift pin with an outboard style(hi performance) screw in shift pin(you'll have to dissasemble the lower). Thats the first thing to go boom. I'd run a twin drive shower to keep the upper cool, and add an external resivoir for the gear lube, bigger the resevoir, the better. I'd also recomend running merc high performance gear lube(its made by bellray), its good stuff, its like climbing lube
 
Just to give you guys an update if you like...

Ok, a decent 383, msd digital ignition, bravo 1 24p 4 blade prop. Shower head, extra oil res. in the outdrive. Putting a temp gauge in this week. Max rpm 5200.

I did turn it to 5500rpm just to see. Its fast. Pulls a skier like no ones business, and cruises at around 25-30mph at what a I thought was a very nice 2900-3400rpm range. It has a safety switch on it, that keeps it at 4000rpm or below. You need to lift the switch and flip it, to take it to 5200rpm. I think the boat could actually use a larger pitch prop.
 
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