Merc 5.7 - 260hp
 

So gonna start off for apologizing as this isn't a "offshore boat" but a direct drive, barefoot boat (Sanger). Doing a bunch of web searches and all of them have led me to this site, cant find a whole lot of engine build info on other boat sites...

Anyways...

Boat currently has a 94' Merc 5.7 260hp carburated motor in it. Barely pulling me at 40mph when barefooting and 3 people in the boat, with a single passenger, nobody in tow I got 44.6mph on GPS; I need more speed! I know the hull is capable as the same hull with a 400+hp motor will pull a footer 45mph. That being said I want to do soem work on my motor. The motor was freshened up after a overheat (didnt change the impellor), about 400 hours ago (900 hrs on the hull). I was thinking about changing intakes and heads to vortec's and going with a bigger cam, something like a comp XM 270HR-12. Is that too much lift for vortec heads? I am wanting to get it around 350hp (obviously I wouldn't complain if I got more). Am I looking for trouble throwing this combo or parts onto a bottom end that has 400 hours on it? Looking to do this project in the winter, but wanted to start getting parts now.

The other, more costly option (dont really want to go this route) would be to build a new motor, was thinking a 377 (400 block with a 350 crank). Was thinking a destroked motor would last longer at higher RPM running, and give good power. Opinions????

Stock vortec heads don't like more than .480 lift from what I remember, but having a machine shop cut down the top of the valve guide for more retainer clearance would be easy for them and it really should be done in order to run the camshaft that should be installed in the engine.

Vortec heads, that cam, and a good intake like an air gap with a nice 650/750 Holley should make about 400HP depending on the exhaust setup.

For those that dont wanna look them up, those cam specs are 226/236 @ .050 .480/.489 lift with a 112 LSA with a 110 Intake Centerline.

Was looking at exhausts, and marine exhausts are expensive! Would stick with the stock merc exhaust, will there be a problem with reversion with that cam and a stock exhaust? Also, with that cam, should I stick with a dual plane intake or go towards a single?

I think you would gain more useable marine power by putting the 400 crank in your 350 and making a 383. Maybe a 383 crate motor with better parts, save or sell yours. Either way you should probably go with aftermarket exhaust.

If you change the exhaust, I would lean towards the single plane but if you're not going to upgrade I think it wouldn't be as beneficial.

Maybe look through the swap shop/classifieds and pick up something a little better manifold wise than what you have?

I agree that if you're going to change engines to make the 383 and not the 377. In a boat you want/need the torque to get moving.

375 horse can be made with vortec heads and correct setup. More is possible especially if you go 383 (with vortecs). Not an expert, but PM me and I will give you my build detail, it's not mine own but has worked very well for me.