Classic cars forum & vehicle restoration.

[traction39]

Experts,

Is it possible to hook up a BTH magneto with a tachometer? (Riley 9)
If so, how?
I know how to connect a tacho to a distributor/coil engine but unsure whether this is possible with a magneto.
The usual pick up off the inlet cam is unavailable without major changes.

Anyone have the knowledge?

Alistair

[Old Wrench]

An electronic tachometer is simply a pulse counter, plus diode to rectify the AC current and uses a large scale deflection meter.

You would have to detect "pulses" from the low tension side of the magneto points: rectify and then "count".

Alternatively, you could detect pulses from a plug lead.

Most cars of that era used cable-driven chronometric mechanical tachometers, via a drive gear take off normally from the camshaft.

Later versions (e.g. Jaguar sports and saloons) used a small electrical generator driven from one camshaft end and again a standard moving coil meter to indicate approximate RPM.

A case of experiment, trial and terror ( ) and work on first electronics principles.

Many light aircraft still use magnetos, so this could be a point of research.

Some references here may assist:
http://www.donzig.com/tachometers.html

http://www.bowersflybaby.com/tech/tach.html

http://www.eng-tips.com/viewthread.cfm?qid=320604

[Old Wrench]

Here is a simple circuit based around a 555 timer chip.

http://homemadecircuitsandschematics.blogspot.co.uk/2012/05/make-this-simple-tachometer-circuit.html

And here is an analogue meter.

http://www.ebay.co.uk/itm/10-Volt-Moving-Coil-Panel-Meter-10V-FSD-Class-2-5-Voltmeter-fp-/111430483601?pt=UK_Sound_Vision_Amplifier_Parts&hash=item19f1c63e91

Or you could elect to go completely digital.

http://circuitdiagram.net/digital-tachometer-rpm-meter.html