Merc 7.4 LX (TBI) potential?
 

Howdy folks, new guy here.

I'm looking at an older boat that was repowered recently (last 10 years) with a 7.4 LX and a Bravo One. The boat is nothing special, but the engine and drive are what I'm looking for to repower my current boat.

I know this Merc is rated at a measly 300 horsepower and I'd like to do a mild performance build, say 500 NA horsepower. It was suggested (by a member of Classic Offshore Boat Restoration on Facebook) that I ask here.

What are the drawbacks to the Gen VI block, regarding performance? I've read they're a slightly thin casting, boring 0.125" isn't possible, but I don't think I'd need an 1/8" bore to make decent power. How are the heads on these? Ample for 500 horsepower, or should I be looking for better heads? I've read some conflicting info, but it seems that these engines have a cast crank. Is throttle body injection sufficient?

Help me out, if you would please. I've built a few hot rods in my time and have had a handful of boats, but never really gone this far in the boating world.

Welcome. You didn't mention what boat you're trying to repower, but I'll give it a crack.

The block is good. The roller lifter retainers are good. Valve and front covers. And that's about it.

If you're gung-ho and can get it for next to nothing, you could bore it 030, throw a decent forged rotating assembly in the block (4.25 stroke), decent heads of your choice, something in the neighborhood of a cam to support the heads, good intake and carb (fuel injection will add considerably - the TBI on the 7.4L is of no use), and finally, exhaust that will support the power and not drown the engine in water from reversion. $10-12k, maybe a little less if you can get some deals. Plus there's increased capacity oil cooler, plumbing, fuel pump (most Gen VI blocks do not have a mechanical pump provision). This all adds up quickly.

People here have some pretty decent stuff for sale for a helluva lot less if you start asking. I'd go that route.

If you want something within the 500 HP range, Mike Mello (Sick Stinger) has a 40 hour HP 500 for sale over on Facebook for $8500. He'd be able to set you up with whatever you needed.

Thanks TomZ, much appreciated info and thanks for the reply.

I tried to include a photo of my boat but can't until I have 10 posts here. My boat doesn't necessarily "fit" here as it's a day cruiser and not really an offshore boat, but it's what I want and I will be making a few Catalina trips in it.

The boat is a 26' 1986 Sleek Ambassador. It has a Merc 260 horse SBC and pre-Alpha drive. I have no idea if the engine runs or if the drive is in usable condition.

The long-term goal is to have a reliable 500 horse boat that can run 70 MPH but cruise all day at Lake Powell, Lake Mead and Havasu. I'd love to go faster but 70 is about the limitation of the hull.

The short-term goal is stuff the 7.4 and Bravo in and get it running for the summer, then pull and build them next winter. Of course I have a rotten bulkhead so I'll be redoing the bulkheads, stringers and transom.

I should also mention that I want to build the engine myself, so I'll be looking for parts rather than "take outs" or such. I do know for sure that I'll need exhaust manifolds to exit the transom soon... no way I can stand to have the exhaust exit the drive on a boat like this, LOL.

that drive will have to go too.... a 500 hp big block wll eat a prealpha for a mid morning snack. but its an easy swap

I completely agree. My original post said that the boat I'm looking at taking the 7.4 out of also has a Bravo One attached.

The Bravo conversion is really the largest factor in the whole process, as the pre-Alpha is bound to scatter sooner than later with some power. I'm not terribly concerned with what I chose as the engine as I have a 425 horse SBC and a 525 horse LS in the garage, but for the sake of simplicity I think I'd just transplant the 7.4 and Bravo from the donor boat. No figuring out what does and doesn't convert, searching for parts, etc.

This is post #10 for me. Will it allow a pic yet?

That was an obvious no. Post 11...

https://cimg8.ibsrv.net/gimg/www.off...2eb663115c.jpg

The glass and gel are in really nice shape. Interior is gutted (everything was rotten). Again, stringers, bulkheads and transom will be replaced. It will get a color change and full interior in the near future.

Are you sure about roller lifters? I didn't see a year mentioned and my 1996 MPI has the high wear rate solid lifter system.

Depends on the year. Can be a Gen V or VI. Gen 5 would be flat tappets; Gen VI, roller with the resulting hardware. My assumption was a Gen VI. The block itself was more a focus than anything because it would be a 4-bolt.

Your 96 I’m guessing is a Gen V, right? Stamped steel timing cover?

Its a good engine in stock form but really not a good candidate to try and build to 500hp. Its probably a 2 bolt block and has cast internals and has peanut port heads

I appreciate all of the input fellas.

There’s potential in a 454 tbi engine ?

Someone please correct me if I'm mistaken, but I believe the 7.4LX used the same intake, throttle body and components and the 454MAG and 502MAG EFI's. With that being said, it certainly has way more potential than a typical 7.4 MPI?

Yes, same intake as the Mag MPIs, but I think the bottom end was not like the Mags, it was like the 7.4 MPI, relatively weak if power added.

I have that motor, 7.4 lx mpi. Its a gen 6 roller, peanut heads cast crank and little rods vst....etc....

So I think I'm going to buy the engine and drive. Before it goes into the Sleekcraft I'll pull the pan and heads and take inventory of what's there. If it's a 2-bolt main I'll plan to have my machine shop drill for a 4 bolt main, splayed if possible. I truly appreciate all of your help.

Good to know about the rollers and heads, thanks! I'll start watching for a set of good heads to make it flow when I rebuild.

I don‘t believe that it shares the same intake etc as the MPI. If we’re talking the same engine, it has a throttle body injection setup on top of a flat intake. There is also a 7.4 MPI that is similar to the MPI but it has a different t intake and isn’t performance oriented like a 454/502 Magnum MPI (these have forged bottom ends/big rods).

I believe this is correct. Regardless, unless is was a Merc "blue" intake system I'd probably be changing it up anyway. And yes, I believe the crank of the 300/330 horse engines is cast, not forged. I'll be replacing that when I rebuild as well.

The block should be a 4-bolt. If it’s a Gen VI then you have a good foundation for stuff later. It’ll probably run good in your boat now (once you get all the glasswork done).