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can I increase outboard hp?
Just thinking out loud here, but I'd "guess" this is an urban myth. It
seems easy enough to check, though. Changing just the jets in the carburetor is not rocket science but it is tricky - and you can end up with a useless carburetor if you make a mistake. Instead, why not try installing a used carb from a 15hp motor (larger jets and as you claim identical parts otherwise), seeing what happens, and if you don't like the results, switching back to the appropriate, original size. I wouldn't be too optimistic though. The reason I don't think it would work is that horsepower is a function of the amount of gasoline converted "efficiently" into smoke. It is the pressure from the smoke that drives the pistons during the power stroke. There is a physical limit to how much fuel a cylinder of a given size can burn properly. Adding more than that would result in incomplete combustion in the cylinder, fouled plugs, back firing and higher exhaust manifold temperatures. While it may be possible to increase the performance of your engine slightly by using slightly larger jets then what are currently installed (but perhaps not ones spec'd for twice the rated horsepower) my guess is that you would also need to somehow adjust the timing as well as the air entering the carob manifold, to maintain the proper air/gas ratio. You could possibly achieve this by adding a blower (turbo) type arrangement and adjusting the dwell, but I think we've now strayed in way more effort than you had originally hoped for. Oh, BTW, one side effect of "peaking" your engine performance, generally, is reduced engine life. So what you gain in additional hp tends to add stress and strain not originally intended for that hardware. Hope this helps. Cheers, Robb wrote: O.K.: Urban myth or not? I've heard over and over again that everything is the same on a 7.5 h.p. Johnson as on the 15 horse, except the carb. I also heard it's not even the carb, it's just the jets. I have a 1980-ish 7.5 seahorse that just does not have the guts to pop my inflatable up on plane with two people and a full tank aboard. I added those wings, which do help, but still just need a little more. Anybody actually done this carb or jets upgade, or at least seen it work? Every dock rat seems to have heard of it, but nobody's done it. Failing that, any ideas on how to squeeze a bit more out of a two-stroke (short of going 100:1 on the mix)? Thanks! |
#2
posted to rec.boats,rec.boats.cruising,rec.boats.building
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can I increase outboard hp?
AMPowers wrote:
Just thinking out loud here, but I'd "guess" this is an urban myth. It seems easy enough to check, though. Changing just the jets in the carburetor is not rocket science but it is tricky - and you can end up with a useless carburetor if you make a mistake. Instead, why not try installing a used carb from a 15hp motor (larger jets and as you claim identical parts otherwise), seeing what happens, and if you don't like the results, switching back to the appropriate, original size. I wouldn't be too optimistic though. The reason I don't think it would work is that horsepower is a function of the amount of gasoline converted "efficiently" into smoke. It is the pressure from the smoke that drives the pistons during the power stroke. There is a physical limit to how much fuel a cylinder of a given size can burn properly. Adding more than that would result in incomplete combustion in the cylinder, fouled plugs, back firing and higher exhaust manifold temperatures. While it may be possible to increase the performance of your engine slightly by using slightly larger jets then what are currently installed (but perhaps not ones spec'd for twice the rated horsepower) my guess is that you would also need to somehow adjust the timing as well as the air entering the carob manifold, to maintain the proper air/gas ratio. You could possibly achieve this by adding a blower (turbo) type arrangement and adjusting the dwell, but I think we've now strayed in way more effort than you had originally hoped for. Oh, BTW, one side effect of "peaking" your engine performance, generally, is reduced engine life. So what you gain in additional hp tends to add stress and strain not originally intended for that hardware. Hope this helps. Cheers, Robb All of this is true. Now imagine that an engine manufacturer wants to offer a range of engine sizes, but wants to minimize the cost of doing so. Make the 15 HP motor, and install a smaller carb on it, and voila! you have a 9.9 hp motor. This was the tactic followed by Johnson/Evinrude for a long time ( and is a common practice in the automotive industry as well... how many V8 engines were sold with small 2 BBL carbs?). Changing the jets will only screw up the mixture. The ratio of gas to air is critical, and is carefully established by the jet size. The AMOUNT of fuel/air mix is controlled by the size of the carb. But Steve's advice about just getting a 15 HP motor is well worth considering... bob s/v Eolian Seattle |
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