Twin vs single outboard setup
Hi!

I know we have discussed this topic before, but I hope you all can look at again.

I have a Phantom 25 with a Promax 300 1999 model running 78 mph in spring (75 mph in the summer). 7,5 inches setback, blueprinted Tempest 26 spinning 6.000 rpms. Torquemaster lowerunit.

I am looking for more speed. I have 4 options:

@ Buy another Promax 300 second hand, and add this to my 1999 model Promax 300. Since the boat is only 5,5 feet wide, I am afraid it will be a little tail heavy.

@ Sell my Promax 300, and buy a new Promax 300X. Speed will increase somewhat - approx. 2-3 mph according to my calculations (30 hp extra over the old 300) making in excess of 80 mph.

@ Sell my Promax 300, and buy two 2.5 EFI engines. Since these engines are lighter, the balance of the boat will approx. the same as one Promax 300 (7,5 inches setback) if 2-3 inches setback.

If I am correct the 2.5 EFI should be as strong as the old Promax 300. Is this correct? Do anyone have any dynoresults? What about longvity for the 2.5 EFI compared to the Promax 300?

Could the less weight give a better speed for the 2.5 EFI's compared to the twin Promax 300's?

@ Sell my old Promax 300 and buy twin Promax 300X. Expensive solution, and still uncertain if the boat will be a handful to drive in high seas despite trimtank in front.

I am a little confused here! Phantom 23 (new smaller model) is raced with twin installations (2.5 EFI) in the race circuit in Europe with great results. Even twin Yamaha/OMC 225 work fine ...

The big question is ofcourse: is the extra cost of a twin installation worth the money over a single Promax 300X....?

Cheers and thanks,

Toffen

Ha!

Put a blower on that ProMax!
You'll need a MagnaCharger like the ones they put on dragbikes. Will also need a Rapair EFI module to tune the fuel curve, a boost-indexed fuel pressure regulator, and an Aeromotive or Weldon electric fuel pump.

I've got a complete 14-petal Merc front half with a 4-53 (or similar) blower grafted on it. It is set up for dragracing on a 2.4. Ran alcohol and 12 pounds of boost. Scared the living h$ll out of me but ran reliably (200 # of cranking compression, 12 # of boost, 38 degrees of ignition timing, 25:1 with Golden Spectro 2-cycle oil, mechanical fuel injection (belt driven pump and a simple spool valve for tied to the throttle plate on a big throat rectangular bore air horn). Ported standard 2.4 (tried the Bridgeport block, but it would melt that thin bridge with less than desireable results). Short exhaust tuner. 7000 rpm out of the hole and 8800 a the end of a quarter. I never raced it that way (after all, I was married), but a buddy of mine did - 132 in a quarter mile on a 19' STV hull (pleasureboat upper deck).

Not figuring you want that wild of a combo, but the blower did so well on the race motor, that I cannot figure out why nobody is peddling kits for the big outboards... L&S used to market a turbo which I have driven on a boat. Nice package, too but looks funky. The blower stuck out the front of a one piece housing not as far as you would think and looked too cool.

I'd think you could run 4# of boost on a promax 300 with good results. I am not familiar with the promax injection, and you may need to upsize the nozzles (they are more like car injectors aren't they?).

If you just want occasional wild speeds, Nitrox makes the most trouble-free nitrous kits in the world for outboards. They inject nitrous AND alcohol to suppress knock and cool the internals. Really work great in those aluminum motors...

That being said, a single 300X would be a bit faster, but not where you want to be. Twin 2.5's will aggravate you for the $$ you will spend on them versus the end result..

$6k should get you a good blower setup - just find a good blower guy in your area and meet with him and a good outboard guy in the area. Let them hash it out.

After all, in a coupla months you got the whole winter to get it ready...