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Mercruiser 305 to 383 engine swap

J

jrgiust

Regular Contributor
"I have a 1991 20 ft mariah i

"I have a 1991 20 ft mariah i currently have a 305 mercruiser 200 hp engine, want to swap to a 383 mag mercruiser 350 hp problem is, 383 is set up for a bravo outdrive, i have an alfa. The engine came with a sea pump, how do I hook up the water? is there a idler pully instead of the sea pump? or do i take the impellar out of the outdrive? or do I get a reducer line and just hook it up with the impellar in the outdrive? and also the 383 came with, quick couplers on the power steering lines, and my other one doesnt have quick couplers, is there fittings i shoul get? please help am VERY pressed for time here..."
 
"jeff;
Being pressed for time

"jeff;
Being pressed for time when working on a boat is not a good combination IMO.
You can order the quick connector fittings from Merc, but I am not sure of availability. My replacement 383 came with standard screw fittings, so I did not have that problem.

I suggest you take off the belt driven pump and its mounts, and use the impeller pump in the drive. This is how I am running mine and it runs cool. Switch to serpentine belt no 57-865615-004. That's the one used for Alpha configured V-8's with PS and no belt driven pump.

You can run the water inlet hose straight from the inner transom plate fitting to the inlet of the PS cooler on the back of the engine stbd side just below the distributor.

If your engine has the 10 pin barrel electrical plug, that should mate up perfectly. Your gauges and alarms should all work fine.

I just put a 383 MAG MPI in my boat with an Alpha II leg last year and its working fine. The 383 is rated at 350 shaft hp. The Alpha II is rated for up to 300 hp, so I'm a little cautious, but so far so good.

One other thing: Your engine would have come with a Bravo drive shift cable bracket. You will need to switch that to your Alpha bracket. You will need a new interrupt switch which is an NC plunger style. You can't use the older style with the roller and metal blade, which is an NO switch. The switch has to be ordered separately.

The Bravo ECM does not recognize the Alpha interrupt signal to pause the ignition, so you have to wire the new NC interrupt switch directly to the coil driver circuit. Wire it in series into the pink/wht wire. This is just beside the coil on the back top port side of the engine.

If you don't do this the ignition will not interrupt properly, and shifting out of gear will be a b*tch.

Let me know if you need any other info. I can send you pix of mine.

Good luck, Rod"
 
"Hey Rod, my interrupt switch

"Hey Rod, my interrupt switch just came in. How do you mount the plunger style switch to the roller style bracket? Does it work with the old style cam on the bracket also?"
 
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