Is this 7.4 to a 5.7 swap a good idea?

lilboatsman

lilboatsman

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Apr 7, 2007
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I've got a 1990 Arriva 2450 with a Mercruiser 7.4L in it that is most likely shot (double checking this week). It's coupled to a Bravo III drive. I know a guy who wants the drive and engine, so would it be fine to swap in a 5.7 Mag that I know of with an alpha drive?

I think the alpha and bravo drives pretty much have the same transom cutout, but will the 5.7 put out enough power for the boat? Or will it seem really sluggish? Would it help that much if I kept the Bravo drive while still swapping in the 5.7 since the Bravo drive is geared differently and takes a larger prop? (I know that in this case I would have to change out the part of the drive that bolts up to the small block motor)

Thanks to anyone who can provide some insight. :confused:
 
H

HT32BSX115

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Re: Is this 7.4 to a 5.7 swap a good idea?

Howdy,

You can probably stick with the Bravo with the 5.7L engine but the ratio will probably be all wrong. You probably have a 1.81:1 or a 2.0 ratio drive. It should say on the side of your drive.

I would NOT trade a Bravo III for any single prop drive. It made a huge difference in mine going from the similar hp 460 King Cobra to the 7.4L Bravo III. Mine might be a little slower top speed (3mph) with the Bravo III but the hole shot is FAR better. (who runs around at top speed all the time anyway?)

That's a fairly heavy boat. If you trade the 7.4L engine for a 5.7/Alpha you probably will not be very happy with the performance. Going from a 454 Bravo III to a SBC Alpha, it'll seem like a real dog out of the hole by comparison.

If you want to put a "new " engine to replace the 7.4L , a good choice might be 6.2l (383) MPI or a 5.7MPI (300hp) small block.


Cheers,

Rick
 
lilboatsman

lilboatsman

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Re: Is this 7.4 to a 5.7 swap a good idea?

I took a quick look and I was wrong, it's a Bravo One on it right now. Looked a little for a gear ratio but couldn't see anything. Probably have to clean it up a little.

I meant that I was thinking about putting in a 5.7 MPI (5.7 magnum?) I hadn't thought about a 6.2. Either way, I'm not sure what the difference is between a Bravo one and three.
 
642mx

642mx

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Re: Is this 7.4 to a 5.7 swap a good idea?

There is no way I would ever go to a less horsepower, smaller engine. If your engine is shot, rebuild it and add some horsepower while you do it.
 
Bondo

Bondo

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Re: Is this 7.4 to a 5.7 swap a good idea?

642mx said:
There is no way I would ever go to a less horsepower, smaller engine. If your engine is shot, rebuild it and add some horsepower while you do it.
Click to expand...

Ayuh,....

Even with Gas where it is today,...
I can't feature going from a BBC to a SBC,... Not even a Stroker motor.....
I'd think it would be a Major Disappointment....
 
lilboatsman

lilboatsman

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Re: Is this 7.4 to a 5.7 swap a good idea?

A 6.2 is still a big block, correct? Or is that a small block? I've heard that the 6.2 is supposed to be the replacement for the 7.4 that isn't being produced new anymore.
 
H

HT32BSX115

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Re: Is this 7.4 to a 5.7 swap a good idea?

lilboatsman said:
A 6.2 is still a big block, correct? Or is that a small block? I've heard that the 6.2 is supposed to be the replacement for the 7.4 that isn't being produced new anymore.
Click to expand...

In a word, no. It's just a SBC that "stroked"

I think your boat would run fine with a 6.2L engine. I think however your hole shot would be less with the 6.2 over a good running 7.4.

I think your best bet would be to rebuild your 454 or buy a long block engine.


I am VERY happy with the 7.4L I put in the Liberator. It runs great. I am STILL looking for a 502 though!!! Only way I'd pull it! If i do I'll probably build one with aluminum heads and manifolds.


I meant that I was thinking about putting in a 5.7 MPI (5.7 magnum?) I hadn't thought about a 6.2. Either way, I'm not sure what the difference is between a Bravo one and three.
Click to expand...

There's a pretty big difference between a Bravo I and Bravo III
One of them is the amount of "prop" or blade area in the water.
bravoIII2.gif


More power is ALWAYS better than less.....
 
T

TilliamWe

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Re: Is this 7.4 to a 5.7 swap a good idea?

HT32BSX115 said:
6.2l (383) MPI or a 5.7MPI (300hp) small block.
Click to expand...


I agree with Rick's first post, but I want to point out that the 6.2 Mercruiser is 377 cu inches, not 383. A 6.2 is a stroked 5.7. Now Mercruiser does offer the 383MPI which is a stroked and bored (.030 over) 5.7. It is a factory remanufactured engine. Comes with a Mercruiser warranty and everything. My dad install one in my sister's 1990 model Chaparral 2350 cuddy, in place of a 454 Magnum. He kept the Bravo 1. The 383 performs about the same as her old 454, but that 454 was a little tired. I don't think I'd swap to anything less than that 383MPI. I also wouldn't switch to an Alpha drive, in place of any Bravo drive. Especially not when you are gonna go north of 300hp in a 24' boat.
Rebuild the 454 if it's rebuildable, that's the best way to go. A 383MPI from Mercruiser is right at $7000!
 
W

wca_tim

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Re: Is this 7.4 to a 5.7 swap a good idea?

Bond-o said:
Ayuh,.....

'ell,.... What's 6" amongest Friends, Anyways,..??..?? :D
Click to expand...

that probably depends on whos drivin, doesn't it?


even a relatively mild 383 will make as much or more horsepower and torque as the original 454.. and weigh less... but a mild performance bigblock will definately be a step up in performance.
 
lilboatsman

lilboatsman

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Re: Is this 7.4 to a 5.7 swap a good idea?

ugh, I won't really know what to do until I do something I guess. Maybe I'll find a nice used 383, but most likely I'll end up rebuilding the big block.

Thanks everyone for your help.
 
jaxnjil

jaxnjil

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Re: Is this 7.4 to a 5.7 swap a good idea?

HT32BSX115 said:
MY BAD!!:p
Click to expand...

you and me both:confused:


allso there is a reason the gm 400 sb's didnt take over the gm big block back in the day. i to thought he was talking about a stroked 350 or 383:cool:
 
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