Hello,

I have the boat and engine above and I took it out to get a good GPS speed before the end of the year so I can either lab or order a new prop for next year. It was me, the wife, 1/4 tank of fuel, and pretty good water. Trimmed all the way up I was able to tap 56 but it was only for a couple of seconds. With the throttle in the dash I was only spinning 3900 RPM. I have read that these boats should spin 4200 so I am curious what y'all think I should do with my prop. It is a 23P Mirage Plus. Also once I get to around 3/4 throttle there is really no change in rpm going further.. Not sure if that is prop related or cable related, but I am sure someone on here would know. Let me know if there is any other information you might need.


Thanks!

To much prop for that boat. Thats what I ran with a 502 MPI in our old 272.

Try a 21 M/P or get that one turned down a bit and labbed.

Quick cable check, go WOT on the dash and verify at the plenum that you cannot pull the throttle arm anymore.

Anyone in the DFW area have a 21P MP I could borrow for a day??

Thanks for the help!

We have a 7.4 mpi (310 hp) Bravo 3 (1:2 ratio) pushing a 226 Cobalt - we can easily hit 60 MPH on GPS and WOT is 4600 rpm's with 2 people in the boat at about 25 gallons of fuel. We are spinning 3 blade 28P props.
I know it's late in the season - and we really can't compare Bravo 3 - dual prop to a Bravo 1 but it seems to me that I would start with the basics - were all the connections tight on the electronics - Ours is a 2000 year boat also and every once and a while gauges will get a little squirrely only to see a little corrosion on a terminal- once removed all is well. Was the air intake clean - a 7.4 goes thru a lot of air each revolution and anything that restricts this could cause a decrease in performance.

The second I would do I check the throttle linkage - are you really opening it up - sometimes these can get out of adjustment.

Please let us know what happens

All the best

3pointstar

Do you have any head work done? I was under the impression these should hit between 4200-4400 RPM max stock. I could be wrong, Ill have to check my engine sticker when I get out to the boat next.

I keep the air cleaner as clean as possible. It gets dirty pretty quickly. Just swapped plugs,wires,distributor cap, and rotor at the beginning of this season. ( the boat was only spinning these RPMs last year also from what I remember) It sounds strong and runs great. I do believe i need to replace my IAC but other than that no issues I can see.

We had the same boat but with the carb 454 310hp. Exactly which 454 engine do you have? 7.4 MPI?

Yes that is the engine I have.

In the future refer to it as 7.4 MPI, it eliminates confusion with other engine models. Your RPM range is 4200-4600. A 21 Mirage + should get you right at 4200-4300. We had a 23+ and it lugged when the engine was stock, like what your experiencing. I had BBlades work the prop to get it to spin but for the $ you can just buy a new 21. Dont expect much more speed, youll be right at 58-59. Thats all we could get in stock form. Your engine will be under less stress also.

Thanks for the tip on the engine. I have read posts where people call them by all kinds of different names so I was always unsure. I think the prop change is just what we need to an all around better performing boat. I’m assuming this 21 will help with getting up to speed also.. I have a buddy with a 272 back home in KS that is going to lend me his prop for a month to test. It’s a 21pitch mirage plus so im gonna pick that up a couple of weeks from now and test.

You want to prop it to spin close to 4600 rpms for best performance. The WOT range of the engine is 4200-4600 rpms.

I also highly doubt your speed is an accurate GPS speed.