Drive Ratio
 

Hello guys,

Having changed my engine lately, I have now troubles!
I bought a MAG MPI 350 with a bravo drive but dealer made a mistake and send an alpha Gen 2 (1.47). Since the drive was too small for my engine, my engineer told me that putting a Bravo I would be a good alternative because re ordering a package and receiving it takes 3 months.

Bravo I was installed but since my performances are terrible: 3800RPM max and 11kts!
I do not think that a bad ration with a bad propeller settings could cause such low perf! no?

Besides, I reached the 3800RPM with some angle left for the throttle to be pushed forward. The remaining angle rangle does not increase the RPM's. The engine is already giving all that he has at a lower throttle angle.

I checked everything on the engine ( cylinder compression, fuel pressure, filters...) everything is fine and the diagnostic made with the computer shows nothing except a probe temp fault. I tried to run the boat with the probe disconnected but still teh same ****ty perf! I am getting desperate

Thank you for your help

A 350 mag is not too large for a alpha drive,
what was the power/ drive before this install,
what’s the specifics on the boat?

made a typo: too small for my 240 sundance's originally sold with a 260hp but a bravo III 2.2 and a 20P prop.
My B1 has a ration of 1.5...
I am wondering if the ratio is too small for my boat....but well I have an engine of 300hp so having only 11kts makes me doubt of that hypothesis.

yesterday, I found 2 purple wires ( do not know what one of them was, and the other one was I guess for the engine to see if I have engaged the forward or rear gear engaged) , I disconnected them and I tried to rev up the engine at the peer, the engine went at 5500RPM whereas with these wires connected I cannot go over 3800RPM.
I thought that I solved the problem but when I tried at sea, same results: 3800RPM max and 11kts.

Those wires were probably for the shift sensor ( so the ecm knows it in gear, rev limit when in neutral, like you’re experiencing)

what size prop are you running now?

I need to check my prop size...do you think a bad pitch or diameter would cause such a loss of perf?
For the wires I do not get why I only reach 3800RPM without load at the peer when these wires are connected and 5500 when they are not..
.

The 40hp difference in engines is next to nothing.
If you only spun 20 pitch props with a 2.2 ratio Bravo 3, then you would need about 15 pitch prop with a 1.5 ratio Bravo 1.

Why not just put the Bravo 3 and 20 pitch prop back on ???

because unfortunately boat fell of the trailer during a hurricane and drive was broken...

So from you are writing, you are not surprised by my perf 11kts at 3800RPM. A change of prop size and pitch should do the trick?

I would try a prop 4” lower in pitch than whatever you have on it now for starts.

How fast was the boat before? and at what rpm?

with the B3 and 260hp (20p prop) I had around 35mph at 4500RPM

If the prop is too big it will never let the engine spin up to where it makes it’s real power in the upper band of revs. Rough example of you may have 3/4 of throttle but only half of the power. Engines need to be free to eat!

Prop pitch is 21...I will try a 17 or 18 tomorrow and see if I can get my 4500RPM

I know, oldish thread, but it's interesting.
It does need the smaller pitch propeller due to the different drive ratio.
But as said, I'm interested. It might probably still need that wiring issue addressed to get wide open throttle to run past 3800 rpm.