7.4L Mpi 310HP

[boatsnhoes]

Hello all,

I’m looking at a 1998 Lavey Craft 21xcs with the 7.4mpi 310hp motor. I was hoping to find one with the 360hp but this boat is super clean and I’m thinking of pulling the trigger. I’d also like to go over 70mph

The boat I’m looking at is attached!

Is this a sufficient power plant for the boat?
Can you do minor modifications without hurting the engine?
What’s its weak points?

Any help is much appreciated. I own 1993 Lavey Sebring 20.8 and will be selling. It has a 200 black max outboard on it…

Thanks!

 

[Shlbyntro]

they are great workhorse engines but not speed demons. you will probably see low 60s with that engine in that boat in stock form with the right prop. The heads are the limiting factor on HP and run out of breath relatively quick. I did a mild build on one in my 247XS and it pushes it into the low 60s

 

[boatsnhoes]

they are great workhorse engines but not speed demons. you will probably see low 60s with that engine in that boat in stock form with the right prop. The heads are the limiting factor on HP and run out of breath relatively quick. I did a mild build on one in my 247XS and it pushes it into the low 60s

What did you do for a mild build up?

 

[Shlbyntro]

high compression pistons, exhaust, cam, and tune for 93 Octane.

 

[boatsnhoes]

high compression pistons, exhaust, cam, and tune for 93 Octane.

Okay yeah that is pretty mild. I’ve read you can get away with a pro charger on these if done correctly.

 

[Shlbyntro]

Okay yeah that is pretty mild. I’ve read you can get away with a pro charger on these if done correctly.

ya, they will accept low boost. I'd keep it at 7psi or below.

 

[Dalton]

Okay yeah that is pretty mild. I’ve read you can get away with a pro charger on these if done correctly.

what are the hours on it? when it's time to rebuild it's not hard to get near 500hp with basic stuff

 

[Hydroman55]

Not the fastest but will last forever trouble free. 1,500 plus hours and insurance is cheaper.

 

[OCMerrill]

I have one in my 21' Essex Sterling 7.4 LX MPI. I have about 815 hrs however I cant speak for performance because I have a Bravo 3 Drive. With that I can run just at 60 at 4600 RPM. but duo props are great for fuel economy but not the best for top speed performance.

These engines are in a ton of boats and like stated I am also expecting 1500 hrs.

 

[LazyLavey]

Perfect conditions (like minimize as much weight as possible) my 2000 21 XTSKi can reach 70.. 454 Mag, other wise it's 65ish...23", B-1 4 blade, labbed by Bronson Hill

My friends 2000 21 XCS can break 75ish with a 502 Mag...(.he definitely doesn't have to try as hard)... 26", B-1 4 blade, also labbed by Bronson Hill

 

[boatsnhoes]

what are the hours on it? when it's time to rebuild it's not hard to get near 500hp with basic stuff

The guys hasn’t had it hooked up for hours so I’m not 100% sure on that.

 

[boatsnhoes]

Not the fastest but will last forever trouble free. 1,500 plus hours and insurance is cheaper.

I didn’t think of the insurance gig good note.

 

[boatsnhoes]

Perfect conditions (like minimize as much weight as possible) my 2000 21 XTSKi can reach 70.. 454 Mag, other wise it's 65ish...23", B-1 4 blade, labbed by Bronson Hill

My friends 2000 21 XCS can break 75ish with a 502 Mag...(.he definitely doesn't have to try as hard)... 26", B-1 4 blade, also labbed by Bronson Hill

So you think this setup with right prop should run 65 ish? Since you have first hand experience

 

[RogerThat99]

So you think this setup with right prop should run 65 ish? Since you have first hand experience

I think the 454 Mag is 385 HP, so not a direct comparison.

I have a stock 7.4 MPI (except CMI exhaust) in my 22 Velocity. I have been told it is a 4 bolt main and 330 HP. It is a pretty efficient hull. I can get 70-72 in perfect conditions, but generally 69-70. My boat is a v-hull, about 3,500 lbs (empty) and has a pad type bottom (Steve Stepp bottom).

 

[LazyLavey]

So you think this setup with right prop should run 65 ish? Since you have first hand experience

I think so... My friend stores the anchor, chain, ice box (full big one) and EZ up under the deck and still gets the hull loose... it's incredible how evident the stepped V is.....

He opted for no seats under the deck, just a foam pad

 

[Ramjetfish]

21' Howard running a 22P 4 blade mirage I get it to about 67MPH on the 7.4. Boat runs great, just rebuilt the carburetor. However it is a lighter boat with a good V-Hull specifically made for going fast. I do plan on rebuilding the engine and making about 500HP. It has 570 hrs on it and still runs like new. Motor is original.