If gearing in outdrive is wrong will upping prop pitch still drop WOT RPM?

[LAC_STS]

Hi all,


I have a 25 ft Donzi with a new 350 and Cobra outdrve. Long story short the guy I bought it from had replaced the outdrive with a used one he bought from someone.

I looked up the info on this drive from the info on the tag on it.

Model 985271
Engine 5.0
Serial T0566515

In my manual it says that that model # was used in 88 on 5.0 engines and has a 1.50:1 gear ratio. It also says that the 5.7 had a gear ratio of 1.41:1

BUT it lists a couple other years that have the 1.50:1 gear ratio listed on 5.7's.

At WOT I only get about 4200 RPM and I already have a 15 pitch prop on. (came with boat). I found out that the original prop that came with the boat new was a 19 pitch prop.


So my question is, If I increase the pitch to say a 19 pitch or 21 pitch prop, will the RPM still go down as I have a gear ratio that (maybe) preventing it from reaching the correct RPM in the first place?

My reasoning in thinking this is that the engine can and is supposed to push a 19 pitch prop to 46-4800 RPM at WOT. But because I have a different gear ratio it is preventing it from pushing the 15 pitch one past 4200 RPM at WOT. So with a bigger pitch prop on it should still have the power to push to 4200 RPM???


Am I thinking right or am I wrong?


Thanks

 

[steelespike]

Re: If gearing in outdrive is wrong will upping prop pitch still drop WOT RPM?

The 1.50 ratio is easier to turn than the 1.41 So if your turning a 1.50 imo
you should probably be over reving with a 15. And 260 hp.A 15 is easier to turen than a 19.I think you have more going on than gear ratio and prop selection.
I would start with simple things on a new install.Throttle plates opening all the way,timing set right and advancing as it should,fireing on all cylinders?Bottom clean and smooth.

 

[LAC_STS]

Re: If gearing in outdrive is wrong will upping prop pitch still drop WOT RPM?

Yea I've checked most everything I could think of. There is a list in the FAQ on the I/O part of the forum on causes of low WOT.

I checked the fuel system made sure the secondaries were fully open at WOT checked compression and ignition. Bottom of boat is great.

I dunno. I'm at a loss at what to do next.

 

[Outsider]

Re: If gearing in outdrive is wrong will upping prop pitch still drop WOT RPM?

The result from changing pitch is irrespective of the gear ratio. Doesn't matter what it had new, it is what it is now. Don't know what changing diameter would do, but if you just go to a 19", your RPM will go down ... :redface: