Quote:
Originally Posted by CharlestonSC
WOT is 5900 RPM at 43mph normaly I cruise about 4000 rpm at around 28mph so I thought the 21 pitch might bump my speed up a bit at the same rpm so that would improve milage.?
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There is a lot to be gained from prop testing and setup. You need a tach a reliable method to calculate speed (GPS)and take some notes. Time to plane, minimum planing speed, how does it react to throttle inputs, does it feel sluggish?
I agree with Ferm something doesn't sound right with the numbers. If the tach is right you may already be close to optimal for your setup. A 21 Pitch prop might bump your speed up at the same RPM but there will almost certainly be a trade off. You will be asking the motor to work harder to turn more prop so you may lose some holeshot and it will burn more fuel. I like my outboards propped to run around the maximum factory recommended RPM wide open with a light load around 5800 RPM in your case. Outboards do not like to be lugged around. The timing curve is manual based on your throttle input, meaning if you go to WOT, the timing does too. Maximum timing on a outboard without achieving the proper RPM can lead to pre-detonation. Pre-detonation is difficult to hear in a noisy 2-stroke and it eats away at the pistons and destroys the powerhead. I have a old 2.4L 200 Merc carb motor that I tried on the 20-V. I saw 48 on GPS turning a smaller diameter 19p to 6100 RPM. RPM was on the high side so I tried a 17P large diameter Mirage Plus and it worked better, getting the RPM's down to 5800 with nearly the same speed, holeshot was a tick better. My .02 - Let us know what you find out.