Merc 5.7L vs Merc 383 - The Hull Truth

RantinRaven · 01-25-2009, 08:12 PM

Time to repower my 99 Sportcraft 251 with a Merc 5.7L (carb) and Bravo III. For the best value for the money, I’m leaning towards a Merc 383 MPI 325 HP package with a Bravo III. Both motors have a 1 year warranty and with the MPI, I should pick up 22% better fuel economy.

Anyone have any pros or cons on this set up?

1bayouboy · 01-25-2009, 08:43 PM

Only downside to the BIII is the $1000 prop sets. There are some good deals on Merc reman 383s. I repowered my 24ft Sailfish (really a Sportcraft) with the 350MAG Horizon for the FWC. You should see 2.5MPG cruise around 28MPH.

captainkid · 01-26-2009, 12:15 AM

If the 383s were rebored, they will have incredible torque. I have a 2000 33ft Rinker Fiesta Vee and dual rebored 383s with bravo threes and they push her in upwards of 50 knots. If they are rebored, I believe your performance would be increased.

yottyboy · 01-26-2009, 05:47 AM

If you go with a 383, be sure it is built for marine use and not just a stroked 350. Some builds use the short 400 rods instead of the longer 350 rods. You may also find that the engine won't rev over 5500. The engine's longevity is directly proportional to the build quality. Ask which parts were used...is it a genuine GM build?

As you know, a 383 is a standard 350 block with a 400 crank, modified to fit. These can be built to produce well over 700 hp with the right intake and head. With supercharging you could add another 150-200. That's big block power coming from a small block. Nowadays there are plenty of piston manufacturers as well as aftermarket cranks and rods specifically tailored for a 383 build. Caveat emptor.

billinstuart · 01-26-2009, 05:58 AM

Long rods do increase power(torque) and reduce the side load on the pistons. Either a 377 or 383 with 5.7" rods (Stock 350) or longer is the ticket. The vortec heads are great because of the higher compression without the tendency to ping.

It's not the bore that makes the difference..it's the stroker crank. A cast crank is perfect for this application. Stock PM factory rods are no problem. Keep the rpms at 3500 or so with a mild cam and they will run forever. Make SURE to use a roller lifter cam!!

RantinRaven · 01-26-2009, 06:41 PM

I couldn't see the responses from work, but I had the Marine Power Service Rep I’m working with today read them over. He said the thing to keep in mind is the 383 Stroker motor is a Merc Remanufactured small-block 6.2 liter engine, that has 350 crankshaft horsepower and 325 propshaft horsepower.

http://sites.mercurymarine.com/porta..._schema=PORTAL

I use the boat primarily for offshore and bay fishing and liked what I read about the increased gas mileage on the Sailfish (I already have 24” duo props) because ever since I started using E-10, I’m down to 1.8 mpg with the 5.7L. Not to forget that I’ve had my carb rebuilt twice. As far as the increased speed, it’ll be nice when the water smoothes out and I want to really get some where fast.

It just hit me, how is this motor on a slow troll for stripers or faster troll for tuna?

billinstuart · 01-26-2009, 07:04 PM

The horsepower rating is meaningless. That only tells you what MAXIMUM power is at top rpm, which isn't given. You want TORQUE at cruise rpm, and preferably a torque to rpm curve. From this you COMPUTE horsepower. Looks like typical 377 (6.2 liter)..standard bore, 5.7" PM factory rods, 3.75" stroke, roller cam, multiport fuel injection. Bored 0.030" over gives a 383. At cruise with the correct cam, the bigger motor will produce more torque. Fuel injection is more tolerant of the crappy ethanol laced gas, since it is sprayed at 55psi or so from the injectors. Get the closed cooling, which will allow the engine to operate at a higher temperature.