THREE WAYS TO CALIBRATE A MECHANICAL TACH:

Most tachs are either 2310 RPM or 2566 RPM. That means that, if your tach is accurate, and if you run an hour at that RPM, your counter will read (you guessed it) one hour.

First Method:

The only problem with fire wall forward is over speeding the engine. Above 8,000 feet you don't have enough air pressure to get high mp's. You can however be running the engine faster that you think you are.

The mechanical tachs we use are notorious for reading low. The older they get the more slippage sets in. You can check you tach real easy. Follow these steps.

1. Most tachs are either 2310 rpm or 2566 rpm. Our Mooneys have the 2310 flavor. If you are not sure then time your tach. Set your rpm to 2310, if it takes 6 minutes and 0 seconds to roll over one tenth on the counter drum then you have the 2310 version.

2. Now set the tach to nice round number like 2400. Time how long it takes to turn over a one tenth digit.

3. Divide the indicated time by the actual elapsed time, multiply the result by 2310 and you'll arrive at the actual rpm of the engine.

I'll bet you are reading 50 rpm low. The other downside of overspeeding is using up your accessories. Lycoming engines run vacuum pumps at 1.3 shaft rpm. Brad Farrar bfarrar@flash.net
Fort Worth, TX (817) 927-2613

Second Method:

During cruise, adjust engine speed to an easy number, like 2400.

Make a chart of actual time vs. tach time.

On the ground, convert the actual time to 10ths (to match the tach time, which is also in 10ths.

since (for any given pair of numbers):

(elapsed clock time)          (base RPM of tach)

______________________  =  __________________________________

(change in tach time)       (indicated RPM during the trial)

(elapsed clock time)(indicated RPM)

____________________________________  =  (base RPM of tach)

(change in tach time)

Now, compare the calculated (base RPM of tach) to 2310 RPM and 2566 RPM. Unless your tach is way off, it should closely match one of those numbers. If it is, or you can read the base RPM from the case, the difference between your calculated base RPM and the base number on the case is your tach error.

By way of example, if your calculated base RPM is 2490 and you have a 2566 tach, you are reading approximately 76 RPM slow, or 3% slow.

Third Method:

Borrow a strobe tach and divide by the number of prop blades to get actual RPM.