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Old 08-09-2018, 01:52 AM   #1
Lt. JG
Join Date: Jul 2016
Posts: 21
Default Tachometer Calibration

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?

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Old 08-09-2018, 02:13 AM   #2
Join Date: Sep 2017
Location: Vancouver, Canada
Posts: 83

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.

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Old 08-09-2018, 07:29 AM   #3
mmwjr's Avatar
Join Date: Feb 2012
Location: Essex, Maryland
Posts: 7,771

Get a photo tach from an auto supply store to measure the rpms. Calibration is factory set and cannot be adjusted.
1997 2400 SCR
5.7 Vortec / Bravo 2
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