Info:
85 Four Winns Santara 21.5 cuddy cabin boat.
Engine: 5.7 GM.
Drive: Alpha w/drive shower.

The bottom end is a 350 Goodrich motor. I have upgraded to GM performance heads. 64cc, 200cc intake runners, 2.02 intake, 1.90 exhaust. The cam is a comp marine 491/490 grind. I have a weiand single plane manifold and have changed to a 750 edlebrock carb.( fat jetted ). I am running the Mallory MAL9-26000 marine system from ebasicpower.com. My issue is I did most of these upgrades this year and have not gained any top end speed? I am turning with a 14x17 prop 4600rpm @ 47MPH. I prop up to a 14.25x19 and loose 400-500 rpm and no speed gain. I prop up to a 14.25x21 and loose 400-500 rpm and gain no speed. I would think I would loose 200 rpm with the prop changes and gain some speed. Over all the 17 gives me the best hole shot and best top end. I'm willing to sacrifice some hole shot for top end but I don't see it. I would think I gained 50-75 HP by changing the heads and cam. The original heads were the stock Goodrich iron 76cc head. And I am checking speed through GPS.

From my understanding the distributor has a TA of 28 degrees. Is that limiting me on the high end? I see most performance SBCs are running 30-34 degrees TA.

Any ideas??/

Thanks Mike.

What Dist are you running. And yes thats prob killing you power and fuel economy. Any good 5.7 should make 275-300 hp. That should be 55+ on that boat.

Chris

Two things:

The hull is limited due to displacemt

The drive is buried nice and deep.

I threw a bunch of power into a 1975, 18' searay and I hit a speed wall because of those two reasons.

I am running the Mallory MAL9-26000 marine system from ebasicpower.com.

Are you sure you have a good understanding of how your advance operates?

I would put 18 to 20 degrees in at idle (initial timing) and have a full 34 degrees or so all in by 2200 rpm or so.
The boat will be happier at the dock with more initial timing.

18-20? I think I have it set at 8-10 right now at an idle.

Yes, 18 to 20 initial.
You will have to limit your advance to 14 to 16 to get your desired total.

Got ya, makes sense. So starting at 8-10 I may only be hitting low 20s for TA.

The single plane intake is hurting your setup not helping. Sub 5000 rpm SBC's need a Dual plane. I did a back to back test on an old motor years ago. RPM Air Gap to Vic Jr, same day, conditions, swapping to the Vic Jr lost me 3 gps mph top end and ALOT of midrange. This was on a 355ci 10-1 AFR head, 226/230 roller, with dry exhaust spinning 5800-6000 Rpms. Needless to say I went right back to the Rpm air gap. Your giving up a lot of power everywhere thru the rpm band with the single plane.

Verify 34 degrees advance at WOT. This is what you need. Not familiar with that systems advance setup.
If you are now at 28*, your leaving a lot on the table.

Changing power and props is not a quick fix. A lot of testing and time is required to dial it in. Turning only 4600, you are not in the power and, hence when you add pitch and drop rpms you do not gain speed. Prop for 5000-5200 I would think with that setup. What type of prop?

I personally think that unless you plan to go down a waterfall the speeds you want may not be in the cards. That is a lot of boat and considering the size of the engine - you maybe at the limit around 50 mph. That's only my opinion. When you start messing with props, effecting engine rpm's to a maximum- you may get the speed up but are really taxing the engine, drive etc. At the end of the day you may have one heck of a pile of broken engine/drive stuffs. I know in our home mama wouldn't be too happy!!!

3pointstar