Swapping a 5.7 to a 7.4

[Polar_Bus]

Is it possible to swap a current 5.7 to a 7.4 powerplant in my 22 foot Stingray ? Also is 300 hp too much to bolt a 7.4 to an Alpa 1 gen II outdrive? Thanks for your input!
Rich

 

[Don S]

Re: Swapping a 5.7 to a 7.4

300 hp is the MAX hp rating of the Alpha I Gen II drive. Years ago Merc tried using the Alpha behind the 7.4's, didn't take long to stop that, they then invented the Bravo drive.
Even if you wanted to try, you would need a 1.32 Gear ratio, and the Gen II drives don't go that far. 1.47 is lowest. And props won't do it effeciently.

If you want to go to 300 hp, you would be better off to build up the 5.7 than to put the heavier 7.4 with the wrong ratio in it.

 

[Bondo]

Re: Swapping a 5.7 to a 7.4

Ayuh,....

While the Nuts,+ Bolts of it will allow you to do that,...

Even Hot-Rodding the 350 that's there Now, is a recipe for Disaster....

You can Smoke an Alpha with a 260hp,+ a Nasty Right Hand on the throttle....
At 300hp,... It doesn't really take much throttle Abuse,+ you'll be paddling ashore...

 

[TilliamWe]

Re: Swapping a 5.7 to a 7.4

I will slightly disagree with all those above, respectfully. And Alpha 1 Gen 2 is a pretty stout drive. It would handle a 7.4 better than the "old" Alpha 1 (1st generation) would and did. In regards to big blocks in front of Alphas, yes, Mercruiser quit doing it. But my dad's 1988 SeaRay 268 with a 454Magnum didn't break his Alpha One in the 4+ years and 400hours he used it. And that was a big heavy cruiser (it was a 1.5 ratio and he used a 15, yes a 15 pitch prop on it!). More than anything, it's the torque that kills them, not the horsepower. And of course a 7.4 has a lot more torque than a 5.7.
If you didn't hammer the throttle from a standing start and you didn't jump waves, it could last a long time.
Now that said, you own a Stingray, which leads me to believe that you are a quasi speed freak. If Stingray didn't put 7.4s in that model boat from the factory, I wouldn't either.

As far as what it would take, I think it would bolt right in. Might have fuel pump issues, depending on what generation 7.4 you are talking about. Cause at some point, I think they quit putting a place for a mechanical pump on the block.

 

[chiefalen]

Re: Swapping a 5.7 to a 7.4

All that for 40 hp.

 

[Polar_Bus]

Re: Swapping a 5.7 to a 7.4

All that for 40 hp.

The OEM Stingray specs said my '98 came with a 250 hp 5.7L powerplant. That spec was VERY misleading. I now understand that Stingray only used 5.7L Vortec's in the "SX" sport performance boats. My "CS" cuddy cruiser has a somewhat lame 210 hp 5.7L 2bbl carb engine. It's ok as far as power, but out in the New England ocean. a little more balz would be nice.... I will probably just enjoy my boat for what it is. As what I am reading by the experienced boaters here, in my situation, more hp just isn't worth the aggravation. My current 5.7 runs perfect as well.... Thanks for the input!
Rich

 

[chiefalen]

Re: Swapping a 5.7 to a 7.4

You throw a 4 barrel on it relatively inexpensive up grade should give you close to 260hp or am i wrong?

 

[Bondo]

Re: Swapping a 5.7 to a 7.4

Go with an edelbrock marine carb and you will be one happy boater and you'll only spend about $330.

Ayuh,...

That's quoted from a different thread,...
But,...
The answer is the same...
'course,... You'll need an Intake Manifold to go with it...

A 4bbl. Carb will make a Noticable difference for you...

 

[mkast]

Re: Swapping a 5.7 to a 7.4

I did this swap in a 23 footer weighing 2650 dry.
I changed the gear ratio, Alpha 1 Gen 1, to 1.32 :1.
Simple swap, as long as the incoming 454 is complete.
To assist in the overloaded Alpha condition, 330hp 454,
I installed a sternjack and use a two speed Quicksilver prop.
I also installed a drive cooler and synthetic gear lube.
I think that was in 2002, no indications of problems, yet.
What hampered the early Mercruiser installations of big block/Alpha
powerpacks was the weight of the boat. Eight thousand pound boats,
heavy throttle, no drive coolers, no synthetic lube.

 

[krisnowicki]

Re: Swapping a 5.7 to a 7.4

I am thinking about doing the same swap... what intake would you recommend for this swap ( the 2bll to 4bbl)

 

[mkast]

Re: Swapping a 5.7 to a 7.4

Resist the temptation to try and drag horsepower out of the 454/Alpha.
Stock intake (Mercruiser) would be a WISE choice.
Drive is ALREADY overpowered, why install more nails in the coffin?

 

[Polar_Bus]

Re: Swapping a 5.7 to a 7.4

What differs a Holley "marine" carb from a traditional street version?

 

[180shabah]

Re: Swapping a 5.7 to a 7.4

What differs a Holley "marine" carb from a traditional street version?

Short version...

 

[180shabah]

Re: Swapping a 5.7 to a 7.4

The OEM Stingray specs said my '98 came with a 250 hp 5.7L powerplant. That spec was VERY misleading. I now understand that Stingray only used 5.7L Vortec's in the "SX" sport performance boats. My "CS" cuddy cruiser has a somewhat lame 210 hp 5.7L 2bbl carb engine. It's ok as far as power, but out in the New England ocean. a little more balz would be nice.... I will probably just enjoy my boat for what it is. As what I am reading by the experienced boaters here, in my situation, more hp just isn't worth the aggravation. My current 5.7 runs perfect as well.... Thanks for the input!
Rich

Are you positive about the rating? Count the number of bolts that attach the intake manifold to the heads, are there 4 or 6 bolts per side?

 

[QC]

Re: Swapping a 5.7 to a 7.4

LOL at shabah's "short version" . . . That could cut a lot of repeat threads much shorter . . .

 

[mkast]

Re: Swapping a 5.7 to a 7.4

What differs a Holley "marine" carb from a traditional street version?

The primary areas of a carburetor that require some sort of modification to meet these requirements include the fuel bowl vent tubes and throttle shafts. The vent tubes of a marine carburetor are bent inwards so that the tube looks like an inverted "J". These tubes oftentimes are referred to as "J" tubes, as a result. The reason for bending the tubes inward is that if flooding should occur, the fuel that would normally come out of the fuel bowl vent tube is rerouted back into the carburetor. I have run across older Holley two barrels that didn't have J tubes from the factory.

Throttle shafts also get special machining attention. Shafts are "grooved" and "slabbed" to prevent fuel from exiting out the throttle shaft if flooding should occur. When a carburetor floods fuel will end up "puddling" on the throttle plates. A non-marine carburetor will allow this fuel to seep out of the throttle shaft ends and onto the manifold. This is not allowed on a marine carburetor because normally the engine is situated in an enclosed bilge where potentially deadly gasoline fuel fumes can accumulate. Throttle shafts that are "grooved" and "slabbed" channel the flooded fuel safely down into the intake manifold. With no raw fuel allowed to puddle on the manifold outside the carburetor, there is no chance of deadly fuel fumes to accumulate in the bilge and no chance of explosion or fire.

 

[chiefalen]

Re: Swapping a 5.7 to a 7.4

Short version in a auto a overflow condition in a carb and be leaked onto the pavement below the auto.

In a boat the fuel would collect in the bilge and a spark can ignite it causing severe damage, to the hull, motor and most important the boat next to yours.

Notice i didn't include the operator of the boat who installed the non marine carb, cause he's not germane to this conversation.

It's also why you need a marine, starter, and alternator to try and prevent sparks in the bilge.

 

[Polar_Bus]

Re: Swapping a 5.7 to a 7.4

The primary areas of a carburetor that require some sort of modification to meet these requirements include the fuel bowl vent tubes and throttle shafts. The vent tubes of a marine carburetor are bent inwards so that the tube looks like an inverted "J". These tubes oftentimes are referred to as "J" tubes, as a result. The reason for bending the tubes inward is that if flooding should occur, the fuel that would normally come out of the fuel bowl vent tube is rerouted back into the carburetor. I have run across older Holley two barrels that didn't have J tubes from the factory.

Throttle shafts also get special machining attention. Shafts are "grooved" and "slabbed" to prevent fuel from exiting out the throttle shaft if flooding should occur. When a carburetor floods fuel will end up "puddling" on the throttle plates. A non-marine carburetor will allow this fuel to seep out of the throttle shaft ends and onto the manifold. This is not allowed on a marine carburetor because normally the engine is situated in an enclosed bilge where potentially deadly gasoline fuel fumes can accumulate. Throttle shafts that are "grooved" and "slabbed" channel the flooded fuel safely down into the intake manifold. With no raw fuel allowed to puddle on the manifold outside the carburetor, there is no chance of deadly fuel fumes to accumulate in the bilge and no chance of explosion or fire.

The "short" version was more educating.... j/k

I've been around Holley and Carter carbs for over 22 years, and always wondered what the true differences were. Your post makes total sense ! Thanks!
Rich

 

[Polar_Bus]

Re: Swapping a 5.7 to a 7.4

Are you positive about the rating? Count the number of bolts that attach the intake manifold to the heads, are there 4 or 6 bolts per side?

6 bolts per side. I don't even think I have a roller cammed 5.7 either. However the good thing about a lame 2bbl 5.7 is the awesome fuel economy. I've had a few seasoned boat passengers compliment my boat being real easy on fuel for a "V8"

 

[chiefalen]

Re: Swapping a 5.7 to a 7.4

The four barrel would be good to if you don't get on it to mush and the secondaries don't open.

Have someone watch the carb and at the point the secondaries open mark the speed and tack in your mind and stay below that, until you want to show off.