Calibrating tachometers?

[67Dart273]

Steve, 4spdragtop ask me about calibrating a tach

I'm trying to come up with ideas that the average guy could employ to do so.

There's always "find a guy" with a good tach. How do you know? (It's a good tach)

Or you could "find a guy" with a tuneup machine, but that cost's money, most of the time.

My Fluke won't trigger off the ignition, and most guys don't have access to an o'scope or signal generator

What I'm thinking is

Most of us have laptops or at least computers with soundcards

There are numerous places to download various sound tones on the www

A Radio Shack audio transformer to step up the audio voltage, maybe a diode, and hook to the tach trigger wire

The old Knight Kit/ Heathkit tachs calibrated right off line voltage with this:

The problem is, 60 hz AC generates a very low RPM, 900? I believe for a V8

(60HZ X 60 seconds in a minute / 4 because a V8 fires 4x per crank revolution)

 

[KitCarlson]

Full wave rectify it to get 120Hz for 1800 RPM. Danger !!! That circuit is line powered, isolation transformer and fuse advised.

To get most tachometers to trigger you need 60+ volts, so the sound card idea will not work. Unless you run an amplifier and transformer. Beware most amps will not like the high inductance of the transformer and might be damaged.

I program a micro controller and use a MOSFET and inductor to simulate ignition from a 12V supply. The inductor boosts the voltage, like is done on the primary of a standard ignition. The micro has a crystal time base so it is easy to get accuracy to 50 parts per million. This feature is built in to my engine management systems for waste spark systems, since the spark is distributed from multiple coils. While my system displays RPM via USB on a PC, the tach drive fires for every ignition event, enabling the tach in the car to work.

 

[needsaresto]

Interesting,so Im subscribed! Look forward to a good,useable and understandable method for testing a tachs accuracy. Sorry,I've got nothing to contribute..

 

[67Dart273]

Let's continue this thing over here:

http://www.forabodiesonly.com/mopar/showthread.php?p=1969578221#post1969578221

 

[67Dart273]

Full wave rectify it to get 120Hz for 1800 RPM.
.

I thought of that, but 1800 still doesn't give one a very comprehensive idea of how linear the meter is

. Unless you run an amplifier and transformer. Beware most amps will not like the high inductance of the transformer and might be damaged.

I'm thinking a 1K or so resistor across the transformer output to the tach should tone it down, no pun intended.

I was only getting about 20V AC (RMS) so peak to peak is a mite higher