Engine Repower Ideas - 4.3 Liter to ?...

Author Message   Jim Miller New member Username: mylze3642 Post Number: 1 Registered: 10-2009 Posted on Thursday, October 29, 2009 - 11:32 pm:    OK Folks - I'm not crazy, and although it may be a mid-life crisis, I'd prefer to think of it as a bit of good old immaturity...I've got a 2005 27' Crest Savannah Pontoon boat with a 4.3 liter Mercruiser/Alpha Drive(about 4,200 lbs.). On a good day with the wind at my back, I can hit 30 mph. I want to go faster. The boat is wonderful, the engine performs flawlessly, but I'm considering re-powering to a v-8, and just don't know which direction is best, and well, cheapest - I'm paying two college tuitions right now, it'll be tough to slide anything major past the budgetmaster. Anyway, the model I have did have the option(and room) for up to a 496HO from the factory. If I wanted to put about 350hp back there, how much should it cost me? I do have a friend who use to work at a marina who would be willing to help me build an engine, but I want to compare that reman engines, or whatever other options may be available. This could be a really neat winter project, just want to find out how much a project like this would really cost - what are all the hidden costs? Thanks.   Brian Brackett Senior Member Username: kainon Post Number: 1012 Registered: 03-2009 Posted on Friday, October 30, 2009 - 01:26 am:    Small block V8 with 350hp - your looking at about 8K with changing the outdrive as the gear ratio is wrong, and you're kinda at the break point for what an Alpha 1 Gen II can handle, I think. Keep your eye on ebay, there are complete re-power packages out there.. if you dropped a 260hp V8 in there you'd be looking at about 40-45, and 350 maybe about 55mph based on you only being able to hit 30hp.. If you want to go fast then go for the 496ho.. but really you may be best off just trading it in for one with a 496, as you'd need a Bravo outdrive to handle the torque.   chiefalen Advanced Member Username: chiefalen Post Number: 314 Registered: 03-2008 Posted on Friday, October 30, 2009 - 06:40 am:    how fast can one go on a pontoon safely? I have more but i go 20-25 mostly 20. Now to your question, a low hour fresh water 5.7L will give you 300+ hp if fuel inj. Carb 5.7 L 260 hp maybe more. Brand new gm 5.7 crate motor 1800-2500 straight from Gm. Then you need the externals, and a drive with the proper gearing, and yes gen 2 will do it, but a bravo is better, with a double prop. Me i would leave it alone or sell it and get what you want with the power package you want.   baytro23 Member Username: baytro23 Post Number: 71 Registered: 11-2006 Posted on Friday, October 30, 2009 - 09:13 am:    Michiganmotorz has a 5.7 (315hp) for $3800. That and a V-8 upper gear ratio (easier to get a used V-8 leg) would be your cheapest and easiest option. Not sure that 100 more hp will be what you are looking for though as far as speed. I repowered my 23' w/a from a worn out 230hp (probably down around 200hp) to a 300hp mercruiser 350 magnum and gained 8mph at 3400rpm's after repropping. It was worth it for me because mine was a dog out of the hole and I was only hoping for a 30mph cruise (went from 24mph to 32mph) and plane time is quick and effortless now.   Kghost Senior Member Username: kghost Post Number: 1058 Registered: 07-2008 Posted on Friday, October 30, 2009 - 01:53 pm:    Well one thing is the wieght difference that you need to be concerned with. Going from a v6 to a bigblock v8 is worth I would guess 300-400 lbs. With a pontoon boat you could probably keep the alpha with the big block. Just no hard accelerating starts. If you really want speed and power with a potoon you should or should have purchased one with a large outboard motor. We used to rig some 24 footers with 150 hp O/B mercs and they went very well. Approx 40 + mph. I also agree with speed concerns while driving a BARGE at high speeds. When you get over 40+ mph on a pontoon and you need to turn sharply you will have your hands full. If you want a faster pontoon style boat you should look into the V hull Deck boats. They are designed with the comforts and speed that the typical pontoon boat owner is not looking for.   gl115 New member Username: gl115 Post Number: 2 Registered: 11-2009 Posted on Monday, November 02, 2009 - 06:16 am:    agree with Kghost, deck boats are great,they take some chop on the water also and run wot around 45-50 mph.   henry S. Member Username: mongo Post Number: 51 Registered: 07-2008 Posted on Monday, November 02, 2009 - 04:22 pm:    Also have to consider what structural changes may have to be made. The boat may have had a 496 option, but it may have been modified significantly to handle the additional thrust and weight - could be suicide without the mods.

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