Big Loss of RPM

[John3]

I replaced my original Mercruiser Outdrive (1967 160 Hp in line 6), Gear Ratio 1.65:1, with a used, pre Alpha OD 1.5:1. With my old prop 15.8 Dia X 15 pitch Wide Open Throttle, RPM = less than 3000, 0 vacuum on the gauge. I got a new prop 15 Dia X 11 pitch, tested it on the water today. WOT = 3100 RPM, again 0 Vacuum. Tests run less than a week apart, only me on board.
Seems to me there must be a problem in the OD, going from 15 pitch to 11 should produce a much better gain in RPM. The Engine seems OK, Temp & Oil Pressure OK, Vacuum Ok at idle. Today coming back the engine died in the no wake zone @ 1000 RPM, seemed to be laboring, started right up again. Could it be a LOT of friction in the OD? Shifts fine, no loud noises. I replaced the oil, there were fine metal chips on the magnet.

 

[Bondo]

Re: Big Loss of RPM

I replaced my original Mercruiser Outdrive (1967 160 Hp in line 6), Gear Ratio 1.65:1, with a used, pre Alpha OD 1.5:1. With my old prop 15.8 Dia X 15 pitch Wide Open Throttle, RPM = less than 3000, 0 vacuum on the gauge. I got a new prop 15 Dia X 11 pitch, tested it on the water today. WOT = 3100 RPM, again 0 Vacuum. Tests run less than a week apart, only me on board.
Seems to me there must be a problem in the OD, going from 15 pitch to 11 should produce a much better gain in RPM. The Engine seems OK, Temp & Oil Pressure OK, Vacuum Ok at idle. Today coming back the engine died in the no wake zone @ 1000 RPM, seemed to be laboring, started right up again. Could it be a LOT of friction in the OD? Shifts fine, no loud noises. I replaced the oil, there were fine metal chips on the magnet.

Ayuh, of course it's laborin',... it's the Wrong drive Ratio...

 

[tpenfield]

Re: Big Loss of RPM

+ 1 on Bond-o's comment.

You went the wrong way on the drive ratio. Not sure why you made the OD change, but it should have been no less than the previous ratio. I would recommend . . .

Check engine alignment
Check gimbal bearing
Put the 1.65 ratio OD back on, if that is a possibility.
Get a smaller prop, a 15 - 16" pitch prop is a lot to be spinning with a "6 banger"; usually those are for the big V8's

 

[achris]

Re: Big Loss of RPM

...Get a smaller prop, a 15 - 16" pitch prop is a lot to be spinning with a "6 banger"; usually those are for the big V8's

??? What??? The size of the prop is dependant more on the size of the boat. My original 165 turned a 15" and pulled 4300. The replacement engine, a 4.3LX turned a 21" and still managed 4900. My current engine, a 4.3MPI turns a 19" and bangs on the rev limiter....!

 

[tpenfield]

Re: Big Loss of RPM

??? What??? The size of the prop is dependant more on the size of the boat. My original 165 turned a 15" and pulled 4300. The replacement engine, a 4.3LX turned a 21" and still managed 4900. My current engine, a 4.3MPI turns a 19" and bangs on the rev limiter....!

Diameter, not pitch.