So Santa was good to me this year and I have a bunch of shiny new parts for the LXR. This is a nice change from last year, which I spent catching up on a decade of deferred maintenance by the previous owner. I documented most of that in this thread:
http://www.donzi.net/forums/showthread.php?67047-Adventures-in-deferred-maintenance

To recap, I ended up installing a new SEI outdrive, gimbal bearing, drive shower, cap/rotor/wires/plugs, and GLM aluminum manifolds & risers. Just for fun added a set of 4" Corsa exhaust diverters. The boat sounds pretty good to me (ie obnoxious to my wife) and I think that this is about the least restrictive I can get without going to a super high dollar marine exhaust (which I won't do).

Once all of that was complete I could get some baseline performance numbers for the boat:

Boat - 1998 Donzi 21 LXR bowrider
Weight - approx 3600 wet
LOA - 21', single step, 8' beam
Power - Merc 5.7 Vortec 2bbl 250hp. Same roller cam & internals as the 300+ hp 350MPI.
Drive - SEI 116 (Merc Alpha Gen 2), 1.47 gears, Simrek drive shower
Prop - Merc Vengeance 19P
WOT - 51.3 GPS @4400

Tried every possible combination of trim. Theoretically it shouldn't take much trim with the step as it should run fairly flat/level once the hull is aired out. Ended up settling on 1/3 - 1/4 trim per the gauge as the sweet spot with this setup. Gives 4-5 mph over neutral trim. Any higher and it blows out.
IMG_0192.jpg
For any law enforcement professionals perusing this corner of the internet, that is clearly a can of diet Sprite sitting in the cupholder. No beers were harmed in the posting of this thread.

A couple of issues:

1. Boat is starving for air and fuel above 3.5k rpm. Sounds like a shop vac under the hatch - the 2bbl and tiny intake and flame arrestor are acting like a NASCAR restrictor plate. This is a roller cam vortec SBC and should want to wind up a little more.

2. Prop. The Vengeance is a big round elephant eared budget SS prop. It has enough rake that it wants to lift the bow, which is not what this hull needs. Also tends to ventilate/cavitate quite a bit at takeoff and at higher speeds. Not optimum for a stepped hull. See pic below - this is WOT with 4 people and a bunch of gear, but you can definitely see that the stern is digging in a little too much and not taking full advantage of the step.
Running.jpg

The fix for issue #1 is some extra power. I have an Edelbrock Performer RPM high-rise dual-plane intake, a 1" phenolic carb spacer, a big stinking flame arrestor, and an Edelbrock 1409 600cfm 4bbl and all the required plumbing in the shop (thanks Mrs. DH!!). I debated going to a 750 vacuum carb based on some discussions on here, but finally settled on the 600 per Randy "BigGrizzly" C. He recommended the 600 for small blocks and ran them on his own fleet, so that was good enough for me.

There is a place online that sells a kit with the same parts I bought and dyno'd the '98-99 Vortec motor at 307hp after the carb & intake swap. I pieced it together mostly from Jegs and CP Performance and came in $100 cheaper than the kit.

For issue #2 I am tracking down a 22P Bravo 1 4-blade prop. This was the stock prop from the factory for the Z22 (same hull) and small block 22ZX (similar hull). This prop should provide good stern lift at WOT and excellent mid-range cruise. Also better planing and handling vs. the 3-blade. If the B1 falls through I might take a look at Hydromotive. Bottom line, I want to get the hull running flat and utilize the step to maximum advantage.

I guess the purpose for this thread is to document the experiment and look for any guidance you guys might have. Not many of these boats out there but maybe someone has attempted the same thing on a similar hull. Also curious to hear about any tuning advice from the collective audience. I have a TB V ignition with the knock sensor and a rev limiter; I need to check the module to see how much advance it has and what RPM the limiter will give me. I'd love to get this thing to turn 4800 on the pins.

More to follow once I get under the hatch. dh