Re: RPMs?
Well, Jack: here's my take on this.
Recommended RPM range as set by the factory is not a range etched in stone. It is just that: recommended.
The 3 cylinder Force engines have a crankshaft stroke of 2.80 inches. Compare this to a Chevy 350 with a 4 inch stroke. Centrifugal force and piston speeds are way less for the same RPM. So the short stroke Force 85 with no valve train to float could probably turn 10,000 RPM without too much fuss. (in fact, during the 1970s when Detroit iron was usually red-lined at 4000-4500, a built-up Chev 283 could turn 10,000 RPM). Look at current automobiles: Many are red-lined at 6000 and above. My 2007 Sebring is red-lined at 6500.
In the old days, when Chrysler (from which Force is descended ) rated their engines, for example, the 75 would be rated at 75 HP at 4750 with an operating range of 4500-5500, the 85 would be rated 85HP at 5000 with an operating range of 4500-5500, and the 90 would be rated 90 HP at 5250 with the same operating range. So, you try to prop for WOT at the rated RPM.
The only real problem with operating a stock engine outside its recommended range at wide open throttle is that if you load down the engine (lugging) it tends to carbon-up. If you exceed the range, you are operating the engine outside its maximum torque range. Torque drops faster than RPM rises and so does horsepower (Horsepower = torque X RPM).
However, when an engine is modified for more power, it usually produces maximum torque and horsepower at a higher RPM. Thus, my 140 which has been ported, modified reeds, 2 to 1 gearing, and less restricted exhaust is only happy running at 6000 or better. When I am not concerned about cost of feeding the beast, I prop it to run that high.
But I digress. By running your 85 at 6000 RPM you are losing a little horsepower and actually going a little slower than you should. AND while your hole shot may be great, you are using more gas to go the same speed and the same distance as a higher pitched prop would do.
Thus: by going up 2 pitches in prop, you will drop 400-500 RPM, your top speed MAY increase by a couple of MPH, AND, you will get better gas mileage at cruising speeds and above. That is assuming that your load stays the same.
Changing a prop is no hassle--you simply determine what pitch you currently have, then buy a new or used one 2 inches higher. For example: current prop is 13 diameter X 17 pitch, find a 13 X 19. One nut changes it.
My personal opinion is that the stock black straight backed Force props (actually made by Michigan) are crap. The closed hub ones are OK though. Still, in either case, you would be much better with a branded Michigan stainless prop. But even a Michigan aluminum would be better than the stock prop. Bought new, the aluminum would probably pay for itself in saved gasoline in 1 or 2 seasons and the stainless in a few more. Less if bought used on auction sites.
So, there are my thoughts, now, you decide what you want.
