stevesxm

have long thread going over in general tech discussion but got refered here so here i am.

have 96, 38 top gun. 2 502 mag mpi about 100 hours. motors run perfectly in all respects. props are 4 blade 24 p.

boat gets to 53 mph at 4500 and just stops there. at first i thought is was rev limited in the ecus because of how consistent it is but general consensus is no...

so... everybody says this boat w/ these motors should go 60 mph. a 22 p prop is about 8 % so add 375 revs for smaller props to go the same speed and are these extra revs going to get me into an rpm range where the boat will then accelerate thru the drag limitation and go faster ...

so.... for all you guys here that know a hell of a lot more about these boats than i do :

1) is 60 mph an unreasonable expectation for this boat

2) what would the proper prop selection be for stock 502 mag mpi be ?

3) anything else you can think of.... and yes i have already checked full throttle on the controls

and when i say 4500 i mean the tach needles split the 4500 line on both motors... not 4425 or 4562 .... just like it was programmed to stop there.

motors were 100 hour takeouts from a donzi. absolutely factory original.

offshoredrillin

something is not right, you should be turning them at 5000-5200 and your speeds would be right. there is a limiting factor somewhere. how is the dist and wires? with the smaller props what was your rpm?

sounds to me like there is something in the comp that is maxing them at 4500, when you hit 4500 does it die down? if so your rev limiter is too low.

MrCIG

Are you trimming the drives up and are the tabs fully retracted?

stevesxm

tabs full up but running the drives full down. absolutely nothing suggests a mechanical problem in the motors like cap/wires etc... they run clean and sharp.... other forum said rev limter supposed to be 5700 or something .... so ...since BOTH stop at 4500 are you saying that the rev limters are user serviceable ? that they can reset somehow ?

comp

not sure if my last reply went thru,
I have a 95 top gun with the exact same set up.
in salt water at 5000 rpms 68mph on gps
how are you measuring speed? speedometer or gps?
we are repowering to bring our speed up
you could have a problem in one moter and that will hold the other motor back

Stormrider

What ratio are the drives?

comp

I should know that, seeing how i had to replace one this year but I have a brain block
over the winter I am upgrading all the uppers to handle the new hp
nothing crazy, 502 with 500hp plus headers. going back to carbs and closed cooling

DORaymond

Steve - to see top speed you must not operate with the drives full in. I would say you should be trimmed at least to between 5 and 8 on the indicators. Only experience with the boat will let you know how much trim is too much. Drive trim makes a huge difference in top speed.

stevesxm

no idea at all what drive ratio is and taking boat out tom and will start playing w/ drive angle... nothing to lose. as i said...new to this so can and will listen to all intelligent advice.

speed noted is on my gps dash board speedometer a couple mph slow at top speed but very close at 40 and below.

don't believe for an instant notion of " one engine holding other back" all engine wiring, plumbing, fuel and ancillaries completely independent of each other. they share no systems at all. one engine could fall directly out of the boat and the other wouldn't even notice until it snagged in the prop.

will advise about drive angle dif if any. and thx for all thoughts.

lucky strike

Trim it out----------You will go faster ! ------alot faster !

LS