Re: How do I attain higher RPM's on my 105hp?
There are a couple of things you can do to wake-up that 105 and they are not too expensive.
Certainly, stepping down 2 pitches in prop will raise RPM by 400-500, BUT it will usually result in either the same top speed or most likely a few MPH less.
You can also experiment at raising the engine on the transom about 1 inch. Running it higher results in less backpressure, and less drag from the lower unit--net result a bit more top speed. Cost--0. However, raising it too high will result in cavitation--that's why you need to experiment. Sometimes a Cupped prop will help--but that costs money. If your 105 has the two piece lower unit (earlier than 1979), that lower unit and midleg was 1 inch longer than the one piece lower unit. If that is the case, your cavitation plate is most likely one inch below the hull bottom and you certainly can raise the engine that one inch to put the cavitation plate even with the bottom or at the apex of the VEE if the hull is a Deep Vee style
Now, that 105 almost certainly had Tillotsen WB carburetors. They had a venturi of 1 5/16 diameter. The intake manifolds were never changed, so if you can find a junk Force 125 or even a Chrysler 125 or 140, you can remove the manifolds, reeds, and carbs. They will bolt right up to your 105 and since the carbs are Tillotsen TC carbs with 1 5/8 venturi diameter, they will give you a bit more horsepower. ---Not 125 since your 105 is ported differently and has about 4 cubic inces displacement less. But it will make your 105 which is only delivering about 90 at the prop give you closer to 105-110 at the prop.
The other thing you can do is to find a used lower unit with 2 to 1 gears. (If you have a two piece lower unit, look for one off a 90 horsepower engine)Your 105 has 1.78 to 1 gearing. When you go to 2 to 1 gears, the prop spins slower through the water. There is less drag and slightly less slip. The net result is that with a prop 2 pitches higher, although the THEORETICAL top speed is exactly the same, less horsepower is wasted in turning the prop, more HP goes to moving the boat, and the actual top speed with a 2 to 1 lower unit is about 5 MPH faster.
So if you perform both bolt on modifications and keep the prop you now are using, you should see MAYBE 5-8 MPH more. Not too bad for an investment of say 300 bucks.
Oh, BTW: If you are using an aluminum prop, the exact same prop in stainless will give 200 rpm more and about 2-3 MPH. But new stainless is expensive and even used are pricey--if you have a two piece lower unit--before 1979-- then it will be difficult if not impossible to FIND stainless.
