Hi!

How thirsty is a 650 hp 540 CUI 500 efi engine compared to the standard 500 hp version?

According to boattest.com the consumption with the 500 hp engine in a 29 fountain is about 2,9 - 2,8 MPG at cruise speed - the 496 Mag ho got the same result in a 27 fountain 🤔

If the 500 hp engine uses about 14 GPH @ 3000 rpm, what would a 650 hp engine use at same rpm? Sure, the speed would be different at same rpm if one isn’t stupid enough to use the same prop... Bravo one in both cases though.

Just trying to figure out if I could afford the gas with the 650 hp engine...

First, 650 hp using a 500efi injection system is near impossible and also, I highly doubt those boat test numbers are accurate.

For WOT fuel usage divide the HP by 12. It takes a certain amount of fuel to make a certain amount of HP when tuned correctly.
No way to estimate fuel usage without knowing how much HP is being made at that rpm.

It takes x # of hp to cruise st a given speed, usually around .5 to .55 lbs per hr of fuel per hp. So IF u cruise at 50 mph and it takes 350 hp to do it your boat doesn't care if your motor makes 650 or 500 hp PEAK hp as long as its tuned correctly, a 540 does have slightly higher friction losses fwiw but not much. So if you cruise at 50 mph at 350 hp its going to take roughly 175 lbs per hr of fuel with either motor. Now at wot, yes, it will use a extra 75 lbs per hr of fuel. I had a 1100 hp 272 baja, i could cruise at 55 and still get about 2.5 mpg fwiw, at wot, you could burn 90 gallons of gas pretty fast. And as far as a 540 w 500 efi intake, you run out of intake before motor really makes what it should, most bigger motors running that intake just peak earlier once that happens fwiw, Smitty

Ayuh,.... Fuel consumption in boats is directly related to the motions of the operator's right hand,....

If you have to ask about the gas, you can't afford it