Sitting here reading this thread and learning a lot. I too am going through this tough time with engines. I have a 29ft. Scarab, twin Vortec 5.7's, MPI fuel delivery, advertised at 300HP, 1.5 Bravo 1's, 23p Mirage Plus... One engine shattered an exhaust valve. Did tons of checking, asking, reading, and pretty sure the cause was from a riser gasket leaking.... To fix the problem ..... Going back as 383's .. parts are: Eagle rotating 400 steel crank, forged 5.7" rods., dished hyper .030 over pistons, vortec heads with match port(machineing valve guide down), holley MPI single plane intake (2000-7000), full roller rockers, comp cam XM270HR (int-.495, exh-.503, @.050 int-218 exh-224, int centerline 110, lobe seperation 112, RPM range 1500-5500. Will use stainless marine or emi manifolds. The boat ran 68 on GPS @4900, 100 gals. of fuel 2 people.... dont know what my results will be... I am going to get them dyno , one shop can do prop dyno, the other is engine dyno. I am more concerned with being dialed in right (fuel, timing), but will like to know HP/T ...... I would really like anyones honest input on this.... first engine is about half complete....

that's alot of cam you're going to need long risers possibly dry pipes I've been going through reversion issues on my 383s with the same cam I had to extend the risers back another 9 inches

With the hyper pistons make sure you follow the manufacturers recommendations for piston to wall and ring end gap clearances for marine engines . It is especially critical with these pistons .

ok a decent crank and s h i t pistons...better do a better research..you cant tune that 555 pcm so what are you goint to do their?? if i were you i would try and copy the merc 383 engines this way you have acess to a tuned pcm for your application..you will also want to use 6'' rods..

Merc 6.2's use 5.7 rods & hyper pistons. Scorpions do also. Yes, 6.0 rods give a better rod angle and of course forged pistons are better, but for his application this build should work fine. I don't think you're far off on your build, just don't want to spin it to high RPM. No way should that cam cause reversion, even with stock exhaust. If I remember from our conversation, you have a Mefi 3 computer so tuning is not a problem. Just for the record I can dyno the engine in or out of the boat, with full wet exhaust. I also have a riser for the stainless exhaust with an 02 bung. If you opt for the EMIs be sure and get an 02 bung welded in.

Bob Lloyd
Full Throttle Marine

Hey Bob ,, where is your shop ? I remember talking ti one person with a prop dyno, shop is I beleive north of Austin. Would that be you ? what are your thoughts on EMI's

What are your thoughts on HP/Tq

Yes that is me. West of Austin.

I've never done any testing on the EMI, but they should be better than stock.

I think you'll be close to 400HP with that intake, but the cam is a little small for a 383. You might run 1.6 rockers and help it out a little .

he must have older engines then because the 2001 and up 5.7 mpi's have the 555 pcm.

That is correct. If he has the ECM555 then they'll have to go to Whipple.