Innacurate Tach

[Razooli]

One of the things I noticed at the Dyno Day was that most of the dashboard tach readings were way off from the dyno tach outputs.
My tach registered 5000 RPM when the dyno tach registered 4000 RPM. Most of the other cars were off a similar amount. Interestingly, Pieros' Spider was just about dead on.
Anyone know what can be done to adjust the dash tach to the "correct" reading, assuming that the dyno tach was accurate?

[Zippy]

I believe my tach is in need of calibration as well. There should be an adjustable resistor (potentiometer) inside the tach for this purpose. The trick is you need a pulse generator to do it or a known calibrated tach to compare it to. You would probably have to take your tach out of the dashboard and open it up to find the potentiometer. This is on my growing list of things to do.

[So Cal Mark]

As I told razooli on dyno day, the mfg of our ignition systems tested a load of Spider tachs while developing our system. Every single tach was off; the closest one read 700 rpm low. Every one was low, most about 1000 rpm off at the top end. He figured Fiat had Veglia calibrate them that way to keep people from overrevving

[halg]

Here is a picture of the pot that is used to adjust the tach. Be careful that the tach you are using to set from is accurate. My SDS ECU has a direct link to the crank trigger so is very accurate. The 2 tach's I have checked were very close, you have approximately 2000RPM of adjustability. Both tach's I checked were linear when set correctly.
Hal

[baltobernie]

Great post! Thanks