here's what i did,,, 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(machining 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 separation 112, RPM range 1500-5500. Will use emi manifolds. The boat ran 66 on GPS @4900, about 40 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 anyone's honest input on this.... first engine is about half complete....RESULTS reprogrammed at Full Throttle Marine by Bob Lloyd,,, 400hp@5600 430tq@4200 ,,,, i did open the ports on the cast iron vortec heads and used stock size valves ,,,, Bob told me i could get a little more with a bigger Air-Valve or as some call Throttle Body,,,, Boat ran 73 on GPS with about 80 gal fuel [ATTACH=CONFIG]507131[/ATTACH][ATTACH=CONFIG]507132[/ATTACH]

[ATTACH=CONFIG]507226[/ATTACH][ATTACH=CONFIG]507226[/ATTACH]

[ATTACH=CONFIG]507227[/ATTACH][ATTACH=CONFIG]507227[/ATTACH]

Great job you picked up 7 mph with 100 hp per side.

Wannabe

Thanks for sharing your experience. It sure is enlightening how much power it takes to move a boat at those speeds. Did you try different props? AB

hey abmotorman,,,, in the middle of trying different props now ,,,, my down fall right now in the throttle bodies, they are restricting airflow at higher RPMs ,,,, going from three blade mirage plus 23s to 25s shows a 300-400 drop in RPM ,,,, i know that is suppose to be the norm, the thing is the engines still pull very hard from the lower RPMs to 4600/4700 ,,,, the boat loves the 25s, it pick up an average 8mph cruising speed ,,,,, 23s came on the boat and where in very good shape. i had them tuned and polished, the prop guy put a lil more up in the blades,, said it should drop about 400RPMs and in return raise the bow to create more speed ,,, all it did was lower RPM and slow top speed ,,,,, took them back to have extra cup taken back out, also brought the 25s to have them tuned,,, he told me one prop was out balance, he put the magic touch to these, polished all up with correct dimensions,,,, this is where i sit, waiting to try both sets after tuned and polished ,,, here's numbers before props worked ,,, there's about 6ft of the boat in the water when trimmed out with both props,,,,,
23 mirage plus 25 mirage plus
MPH RPM MPH RPM
30 2600 33 2600
40 3000 42 3000
47 3500 51 3500
54 4000 59 4000
60 4500 67 4500
67 5000 70 4800
73 5400

When your computer was "adjusted" did he raise the fuel shut off / rev limiter? These are designed to shut fuel at around 5200RPM??? Currently, I have stock Bravo I 24p prop's and run 66 at 4600RPM's. My old labbed B1 24's ran 70 (+or- 1) at 4900. Need to get these Air/fuel gauges in before the end of the season or this become next years project. I plan on tuning my own computers but want a stock baseline first. Thanks for sharing your experiences.

yeah, he set limiter at 5600, told me it doesn't make that much more horsepower between 5400 and 5800.... in short they will turn 5800, 5600 is rev limiter, 5400 i use as WOT , i feel more comfy keeping 5200 and lower

I run mirage + 25's spinning in.... Best I have seen is 76 at 5200. But that was once. I can run 73-74 almost all the time @ 5000-5100.

has anyone tried Holley Commander 950 Multipoint Fuel Injection System on 383 small blocks