Re: WOT RPM's
vran, that's what williamettejeff is saying. You are getting the same RPM from a 17" uncupped as you are from a 15" cupped. You can't rely on just the pitch number to base your expected RPM on, you have to take into account the rest of the measurement as well as if it's cupped or non-cupped, like Dave F. is saying above. The engine doesn't know what number is on it's shaft, it just turns whatever is there and the prop itself performs to the way it was designed, you are basically trying the same performing props on this motor.<br /><br />I'm actually working on the same problem with my '76 115 right now, Teleflex tach and 5400 RPM with a 13 1/4 X 15 uncupped aluminum prop no matter if I'm alone with a 1/4 full tank or with 4 guys and a full tank, kind of strang after reading this.<br /><br />It seems like you already understand this since you are trying to get your WOT higher, but maybe some people don't quite get it. The best way to understand WOT is by equating it to your car's transmission. If you had a tranny on a boat you could adjust your pitch and gearing, but you don't so you can't. <br />If you are driving a 5 speed car, do you go 20mph in 5th gear? No, you'd be lugging the thing to death. You'd be in 2nd gear. So if you drove around all day at 20mph you would be fine in 2nd gear, but if you wanted go faster you would have to change the gearing to adjust the load on the engine, same as if you wanted to go slower. If you wanted to go 2mph, you would have to go down to 1st, but you can control that in a car. You can't in a boat, so the reason you prop for WOT is not so you can go WOT the whole day every where you go, it's just so that you have the engine geared properly as to let it perform properly at any RPM you feel like going.<br /><br />With a properly propped and setup engine you could go 2000 RPM all day if you felt like it and the engine would be geared properly and not lugged, at the same time if you wanted to go 6000 or 4500 RPM all day you could because the engine is propped and setup right to handle any RPM.<br /><br />Just the way I understand it at least. I'm starting to lean towards my tach also on this problem, but am gonna work with the setup a little more before blaming that. It never occured to me that there would be a rev limiter set at 5400, I thought they were all in the 6300 range. But I gotta tell you, it's gonna bother me if I can't get this WOT up to at least 5800, and like you not for speed but for engine health. Let us know what happens, Mike.
In any event and despite every thing else, vran still doesn't know if his tach is reading correctly. I cross checked my new Faria one with my older Teleflex and the Faria reads at least 200 - 300 rpms low. The Teleflex tach had been previously verified against my boat's original OMC tach. Verified this also with a slip calculator, which gave me impossibly low and even negative slip numbers using the readings from the Faria tach at GPS verified speeds.