DallasBAJA

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!

obnoxus

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.

DallasBAJA

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

Thanks for the help!

3pointstar

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

DallasBAJA

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.

VoodooRob

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

DallasBAJA

Yes that is the engine I have.

VoodooRob

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.

DallasBAJA

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.

Griff

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.