www.iFourWinns.com

The DPS will be OK, it's used with the big blocks already . You may, quite probably will, have to change the propellers though and I'm not sure if the drive gear ratio is the same for 5.7 vs 8.1 motors.

Not impossible however boat only spends june throug sept in the (fresh) water and no signs of blisters or anything to indicate water. In fact, now this past week I even managed to get the bilge 100% dry after fixing the last fresh water drip from the heater

Sounds like first thing is a leakdown test to check if performance is indeed reduced in some way.

Thanks, you're probably righht that BBs have a lower final drive ratio - there may be props that can offset the drive ratio "incorrectness". Something to add to the list.

When the wire turns green, it is.

Update: first step taken, I'm on the regions largest firm's waiting list for a free 8.1 motor. As soon as one pops up, I'll be notified and lets see how it goes and when. A few years ago, many BBs were swopped for diesels but the economy just isnt there anymore so the turnover is "low", the prediction was "probably towards next spring but you could get lucky". The good newsis that no physical changes are required to motor mounts or anything, straight plug and play. Some changes likely to electrical harness but minor. A side benefit is that all 8.1s are fresh water cooled and there is an oil cooler, something that is missing on my 5.7.

The engine mount pads will have to be moved because the 8.1 is both longer and wider than the small block, you need to find out of the present mounts are long and wide enough for the engine mounts to sit properly on them.
There are a few other issues:
Rear engine mount/transom plate, are the engine mounts in the same place, are they the same distance apart, if not you have to change that as well
Driveshaft/coupler depth, will the splines of the drive you have fit the coupler that fits the 8.1? They could be different, I would not assume anything
Length of the engine compartment, how much longer is the 8.1 and how much extra room do you have in front of the 5.7?
Height of the fuel system/exhaust system, how close is the FI and risers to the inside of the engine hatch, how tall is a fully dressed 8.1?
Exhaust system, will the manifolds/risers for the 8.1 mate up to the Y pipe for the 5.7, or will you have to swap that too?

I do feel boats this big and heavy should have a big block, but as you can see there are a lot of details, its not just plug and play.

Honestly if I had a good running 5.7 with 450 hrs on it and the boat was just a little slow, NO WAY would I spend that much and do all that work for what? I have a 26 year old 4.3 that's also a little tired and slow, but it starts every time and gets me out to fish. When it gets REALLY tired, then, I'll get a new crate engine and install closed cooling.....

More info is needed:
What is your max properly trimmed out rpm? Is it at manufacturer's specs? If not I'd start there to make improvements. Just as in high performance cars and 4x4 trucks, gear ratio is key to good performance, and more important than anything else you do. In boats gear ratio over all depends on the drive but also on proper propping.

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

All good thoughts and comments.

I've read elsewhere that the mounts are in the same position. Y pipe can be reused but that's it. Drive and coupler is the same. The guys at the shop said the same, unprompted, no leading questions.

Plenty of space around the engine in all directions! I can measure top clearance to be sure. Once I have the engine, double checks will happen! Not the first time I do something others shake their head at

The main reason is higher cruise at the same rpm. Consumption isnt that critical. I just cannot run the current motor harder, it breaks my heart. I have a very strong sense of mechanical empathy and sustained cruising at >3500rpm and like 80% throttle just kills...

The investment will be about $10k and most of that comes back as increased value, the net I'm guessing at about $5k, and thats cool in my book. If I wait longer existing engine will be worth nothing, better now while it's in good shape than later. Plus...the alternative is a serious case of boat swap itch and this operation will relieve that itch.

I wont be doing the job myself (I dont have the equipment to lift the motor and boat isnt stored at home, but I'm competent enough to oversee and ensure quality.