Inaccurate Tach
 

Bought one of those little "prop tach" optical tachs from Sporty's. It just came in a couple of days ago.

Been out flying around some, and find that my factory tachs are off by about 100 RPM (reading too high).

Since they are overstating RPM it's no big deal. I've just been flying around at lower power settings than I thought.

Would be a different story if they were reading 100 too low. That's definitely enough to make a pretty big difference on the power charts. Could have been really bad for the engines.

Wonder if anyone else has run into this, and whether or not you can calibrate the factory tachs?

I had the same error in both of my tachs (i.e. reading 100 RPM higher than actual). I discovered it when I installed a JPI with the RPM feature.

I have just ordered new EI Tach's, Manifold, and Fuel flow/Presure gages. Found the same problems. But what can we expect, these gages are pushing 40 yrs. Also just installed the same on a friends 337A ....same issues. I also just finished cluster replacement with EI gages Oil pres/temp, Volt/amp, Gemini 1200 analyzer and Mitchel Fuel. Makes a nice looking panel and much better "accurate" info

Skying posted a response to this thread which was written a manner that was insulting, and would probably start yet another flame war, so I have elected not to post it. But his points were worth noting:

They are:

1.) That the device you bought to check your tach may not be accurate. That you might be buying a cheap, uncalibrated instrument to check an expensive calibrated one.

2.) That Cessna provided a quality instrument, no matter what the current age, and the an understanding of how the instrument works with the mags would reassure you of this.

Kevin
webmaster

Those are all good points.

I do know that this is the same instrument that a lot of shops use when they do annuals, etc. so I felt OK about it.

Also, the disparity between the published performance numbers and what I was actually achieving in cruise using the POH power settings led me to believe that either the MP or RPM gauges were innacurate.

Setting the RPM with the digital tach, my performance numbers are now within a few knots of the book numbers, which tends to confirm my suspicion.

It occurs to me that I should add that I had a similar experience with one of my tachometers on my '73 P. Both were tested by my A&P with a instrument that he trusted (and I trusted him). The front tach was right on, the rear was 50 RPM high. Instead of recalibrating the tach, we just placarded it, and I flew it indicating 50PM lower than the desired RPM.

Kevin

When I had a new rear prop installed, the shop checked the factory RPM gages on my plane, and they were always reading higher than actual - usually about 100 RPM. I can refer to their scaled list anytime I want to be more accurate, but I figure running about 100 low hasn't hurt me and the settings I use have worked well for some years now. Bottom line is I'm aware of it but not worrying about it.

Is there a way to calibrate, adjust, the tach, so that it reads correctly?