85mph seems pretty optimistic with an older v-hul like that.But keep us informed as to how it turns out. I know a guy with an old Wellcraft nova and he put 800hp in the engine room and it never got over 70mph the hull just didnt get out of the water. GOOD LUCK.

You might want to check into using your old injection with the guru's. Those computers can be a nightmare with any other motor than what it came on.

If it were my boat, I would be using a good intake and Holley carb.

Anyway, doing a slip calculation, you were at about 20% slip!!! That's a LOT!! But you mention a Mirage and not a plus. The plus may be a much better prop for your boat.

Given the same situation, you are going from 330 to hopefully 600 hp, if the 540 ends up built right, so that is a lot!

If the 540 ends up needing to turn 5200 or so, maybe a 27 Mirage Plus would be a good place to start. That's probably where I would start. If the 27 hits the rev limiter, then the 29 would be next. Of course, maybe your boat might like a Bravo prop. If you can try one, just use 1" bigger than when using the Mirage Plus. Bravo's only come in even sizes, but are actually 1" less than marked.

My best guess is that you might be in the mid to upper 70's with the new power, once it's all dialed in. Maybe better! You never know!

I would recommend calling Throttle Up propellors for better info!
I'm no guru, just been through the repower myself a couple of times!

Also remember that many small boats (yours and mine) run out of hull at about 75. They get really funny. You will need external hydraulic steering at those speeds, and probably some large tabs to calm it down.
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Here's a recent example. My old power (22ZX Donzi) was a 7.4 MPI. I hit a best of 63 (on gps), usually around 61-62. I put an HP500 in mine a while back and now I have hit a best of a little over 78 (on GPS), with 76-77 every day. The difference in speed is mind boggling!! Just a measly 15mph, but it's HUGE!! I went from a 24p Bravo to a 28p Bravo with this. I have not started playing with props, yet.

Here's why:
I installed full hydraulic steering and it is still a handful over 75. The tail end starts to move around, and it's generally scrary! I'm still working on the handling!

Best of luck with your new power! It should be really FUN!!
Please be careful!
Kent

Hi all,

thanks for answering so fast. :-) It's always very interesting to hear about your experience with almost simular sice boats. I guess we wont procharge it first but sure build full hydraulic and long trim tabs in it. I sure will have to change the 4 injectors from the original throttle body (Holley) to 85 pph to get enough fuel. Sounds like fun already... ;-)