75hp Chrysler Switching from a 1 to 1 ratio lower Unit to a 2 to 1 off a 90hp????

[fcross69]

If I switch from a 1 to 1 ratio which my 1975 75HP ha to a 2 to 1 ratio lower unit from a 90hp how much of a pitch change will I have to make I was a 13 inch by 15 pitch prop I have a 13 inch bye 19 pitch prop should I run it to start. Hope the Fleet Admiral can help me on this one...

 

[Frank Acampora]

Re: 75hp Chrysler Switching from a 1 to 1 ratio lower Unit to a 2 to 1 off a 90hp????

The 75 is a 1.76 ratio and the 90 is a 2 to 1. If the engine is currently running within the recommended RPM range, start out by going up 2 pitches. That is, from a 15 to a 17.

Since you already have the 19 pitch prop and since the 18 spline props are getting more difficult to find, try it. However, if you don't have a tachometer, buy one. As with any voyage: "You cant get there unless you know where you are starting" and unless you know what each prop is turning, you simply can not evaluate the changes.

The bright spot is that the Motorla ignition uses the standard 12 pole Teleflex tach which is still available

 

[fcross69]

Re: 75hp Chrysler Switching from a 1 to 1 ratio lower Unit to a 2 to 1 off a 90hp????

I need to get me a tach for sure that is my next purchase I did mark both shafts and turned them the one I just got was a 2 to 1 and my original was like 1 to 1 1/4. I originally had a 16 pitch on there with old lower unit and I was getting 31 mph on gps with 4 people and 18 gallons of fuel, 80lbs of tools, and a cooler of soda and beer. and with 6 people I was running 23mph. I will try the 19 first with 2 people and GPS this weekend then beech it up and do a prop swap and see what the 15 pitch does. But yes a tach is very important all I have is the cheapo that wraps around a plug wire and I find it not very accurate. But in any case I got the lower unit and another prop with prop nut for 140 bucks and I am finding running a CHrysler I better buy spare parts why I can find them and now I can atleast play around and get my best setup for tubing and have a top speed setup when its just 2 of us out on the water so I am excited and when my gear pack gets here I will be setup with 2 killer lower units and a 6 bolt switch with one cotter key is a fast easy swap. Thanks Frank you are the master!!!!