Z06vette1

Thanks for all the help I've received on this site! I'm about 15 hours into the break in on my 7.4 mpi rebuild. I'm finally able to run it at wot. The question I have is I'm seeing 4200-4300 rpms at 3/4 throttle and not gaining any more or any speed at wot the boat runs about 60 at those rpms I have a 23p mirage plus. Does the engine need to loosen up some and break in more before I look at changing props? Thanks for your help we are really enjoying this boat and like everyone we already wish we went bigger. I'm trying my best to keep my better half interested in performance boats as she already want to look at 40 foot sun dancers ! Yuck.

Bajapred

iTrader: (1)

For that boat and motor setup your speed is right on..60 - 62 mph...my guess is your tach is off a few rpm's..that is pretty normal though. You can hook it up to a merc handheld computer and see the real rpm's if you know someone you can borrow one from.

jokers wylde

I'm running the stock 454 mag in my h2x, not sure of the horsepower difference but I see 63 on GPS at 4300 and just under 70 at 5100 with a 23 pitch

MACDAD260

I think you are getting all the speed you can expect from a 7.4L. I would say you need a 21 pitch Mirage to get your rpms up to about 4800. If you are only getting 4200-4300 you are lugging the engine. That can lead to premature engine failure. Good luck.

Z06vette1

Thanks the 7.4 mpi is def less than exciting. I reach 4300 rpms at about 3/4 throttle and don't see any increase once I pin it . I will start looking for a 21 p mirage plus

Thanks

Larry

bats05

What is your HP,, is the mag engine?

Bajapred

iTrader: (1)

The 7.4 mpi is 310hp... The mag is 385hp... You dont need another prop, and your not going to hurt your engine.. My buddy has a '99 h2x, i drive it all the time... it came with a 23p 3 blade mirage (they all do with the 310 motor)..it ran about 4700-4800 rpm & low 60's, but he hated the way it handled. I told him to find a 4 blade bravo1... He found a good deal on a 24p, tried it and loves the handling, running about 60 now 4600 r's. He really needs a 22p or to have the 24p labbed... But he has a great midrange.... Also remember, the 310 has a lower opperating rpm range than the 385mag does.

Go have fun with your boat and your new engine!!! Sounds to me like your Setup is spot on.

jokers wylde

Not trying to steal your thread zo6 but quick question on props for bajapred I am running a 23 on 454 mag which I know will over rev not sure how much cause I won't go past 5100 what do think I should run also what was the handling difference from a 3 blade to a 4 blade? Also z06 you coming up for the buffalo poker run?

XT-Innovator

iTrader: (1)

I could always here the rev limiters hitting around 4650 rpms on my 2000 29 Outlaw the 310 MPI's. 7.4's always seemed to use the same prop size as the 454's and 502's because of the RPM range difference between them when we did the testing at Baja. Low 60's on a wishometer is about right.

XT

Bajapred

iTrader: (1)

I will respond to Jokers question on here instead of a PM because I think it is good info for Z06 too. There are many more guys on here with much better knowledge of props then me...but,...when using a 3 blade prop you only ever have 1 prop blade in the water at any given time, leaving the back of the boat feeling "loose"...with a four blade prop you have two blades in the water giving the boat an "on rails" feeling. The 3 blade props are typically a little faster, but you sacrifice handling. With the advancements of prop engineering you can get about the same speed out of the 4 blade props as you can the old 3 blade ones, aaaand have the handling!..my guess on your prop Joker would be a stock
24p bravo1. Then see what it needs from there...but again, much better minds for props then mine are on this forum. good luck!