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Swap 43L V6 for a 383 MPI V8

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rod_stewart

Outstanding Contributor
"I have ordered a 383 MPI Merc

"I have ordered a 383 MPI Mercruiser drop in package. I am planning on re-gearing my Alpha drive (1996) to 1.47 and re-using it, realizing that it is only rated for 300 hp and I will need to be a little cautious with the throttle. The new engine comes with a Bravo seawater pump, so I will remove and bypass the Alpha impeller in the leg.
The question is: if I blow the Alpha up and need to replace it with a Bravo 1, will I need to change the transom inner and outer plates on the hull to accept the Bravo?
Thanks for any help/insite.
Rod"
 
"if You use a different water

"if You use a different water pickup (not from drive). leave alpha pump in. cut or remove hose at gimbal housing, and cap fitting in boat. upper drive needs water for cooling. You will need to set up a Y connector to run on flusher. what is HP. of 383?"
 
"Scott;
I am not quite follow

"Scott;
I am not quite following what you mean. How do I go about cooling the upper drive? Can I leave the Alpha pump in and use it to pump water to the belt driven Bravo pump suction? Or do I hook it into the belt driven pump's outlet to the engine cooling system?
This engine is a Merc remanufactured small block chevy 6.2L and Merc rates it at 350 hp. This must be crank hp. You can find it on their website under reman engines. It is not cheap.
I figure that as long as I am installing a small block V-8 I might as well go for the best power that I can get. The 383 only adds about 120 lbs over the V-6 weight, but I am adding about 170 hp.
Rod"
 
"I'm aware of the 383. jus

"I'm aware of the 383. just did't know H.P. offhand. You are definately overpowering the alpha. don't expect long life from it. since engine is out, You could install thru-hull, or transom mount water pickup for new engine. leave pump in drive and remove hose, drive to gimbal housing. drive pump will cool upper.(when on plane, it's out of water and gets hot) this setup will work fine. drawback is, when running on flusher, water must be supplied to both pickups. You can make up a flusher with a y-connector, short hose, and two flushers. (keep it in the boat) good luck on Your project. caution on the throttle will make it live longer, but it's hard to ignore that extra horsepower."
 
"I don't see any reason yo

"I don't see any reason you could't have the belt driven pump take a suction on the discharge from the alpha impeller... that keeps you from having to cut another hole in your boat... the drive gets the cooling it needs, the engine gets the cooling it needs, everybody's happy.
You may even consider not using the belt driven pump. Maybe try it without and see how it goes. If you end up needing it, it wouldn't be a very difficult plumbing problem to pipe it in.
My 5.7L FWC with an Alpha stayed plenty cool with just the sterndrive pump, in the Gulf of Maine though. I'm curious to see how it does now that I'm in VA, with 20 or 30 degree warmer water this summer."
 
"Troy, Scott; thanks for your

"Troy, Scott; thanks for your input. Great ideas.
I am thinking I will try feeding the belt driven pump from the Alpha leg pump and see how that works. That way I can avoid multiple flush hookups and holes in the hull. The Alpha pump I recall puts out about 20 GPM, so volume should not be an issue.
The issue of overpowering the Alpha does cause some concern. I am planning to put a drive shower on, plus the lake is at 1400 ft ASL; so the actual power will already be down by about 5% from full sea level power. Besides I only hit WOT about twice a season anyway, so we will see how it lives.
If I wind up having to retrofit a Bravo drive, can I use the same engine bellhousing, or is it different from the Alpha?
Thanks,
Rod"
 
"Just based on the fact that y

"Just based on the fact that you ackowledge there are limits to the capabilities of man and machine, unlike many we hear about, I don't expect you'll have a huge problem with 'overpowering' the drive.
Like everything else mechanical, there is some conservatism factored into the HP rating of the drive. I wouldn't make a habbit of eating into that conservatism, but if you're nice to it, it'll be nice to you."
 
"re: bravo drive, You would ha

"re: bravo drive, You would have to replace entire transom assy. shower is a good idea. alpha pump should cool the engine O.K. belt driven is there because it's designed to mate with bravo."
 
"Thanks again guys.
Scott, I

"Thanks again guys.
Scott, I guess having 2 pumps is redundant, so maybe I'll just remove the engine driven pump and use a different belt, like that used on the 5.7L Alpha configuration.
I'll keep the pump around and if I ever have to switch to a Bravo drive, it will be simple to reconnect and plumb it up."
 
Rod: You said earlier in this

Rod: You said earlier in this thread:
"I am thinking I will try feeding the belt driven pump from the Alpha leg pump and see how that works. That way I can avoid multiple flush hookups and holes in the hull."

My thought is that redundancy is not a bad idea--I like the combination of both pumps. Think of it as cheap insurance on your $$$ investment. Guy
 
"Nothing wrong with redundancy

"Nothing wrong with redundancy... the nuclear Navy has been thriving on it for decades.

One question:
Would the failure of either pump (connected in series) prevent the other pump from supplying water to the engine?

My first thought is no, if the impeller for either pump goes, there should be a straight path through that pump. But, are there any other modes of failure (common or uncommon) that are worth considering?"
 
"The concensus here seems to b

"The concensus here seems to be that hooking both pumps up in series is not a bad idea, so I will go with that. The good news is that the new 383 package includes a low cooling water pressure alarm sensor, so if something goes wrong, at least I should get an indication before catrastrophic failure!
Thanks guys.
Rod"
 
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