Tachometer Calibration

[Jduncan99217]

I have a 2800 SCR with a 7.4 and Bravo 3 out drive and a 22 pitch props.

My tach is reading 5900 rpm WOT at 36 mph. Doing the calculations for the pitch of the prop and the gearing of the outdrive, the speed is correct, but the RPMs are reading about 1100 rpms too high.

Is there a way to calibrate the tach?

[rwc]

Speed is not a good measure as resistance curve becomes exponential past plane. Speed won’t change that much with RPM, but engine load and fuel consumption will.

You might just need a higher pitch on your propeller.

Your propeller would be designed to cavitate 5 to 8% at full power, design RPM (4800), so at 5900 you would also be cavitating heavily, which could also cause you not to increase speed.

[mmwjr]

Get a photo tach from an auto supply store to measure the rpms. Calibration is factory set and cannot be adjusted.