View Full Version : Tacho?

Mart366 - n/a
12-Apr-07, 08:22 PM
At some stage in the (hopefully) not to distant future, i will need to run in the beast.

Now being that i,m loath to spend money unless i have to, http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif i was looking at tachco options...


do i have to have a matched tacho to my engine (rotax), or will any tacho do?

if its the latter, would it be feasable to use one from a motor cycle? and if so, are there any minimum requirements? must it be from a two stroke? twin cylinder? or will any tacho do (car even?)

also what wiring requirements are needed, does it need specific wiring, or as long as it has xx wires it would be ok

any help would be appreciated


jon_curtis - n/a
13-Apr-07, 11:28 PM
the rotax coil puts out approx 35v if i remember correctly and the frequency of the sinusodial output changes acording to engine revs. so no not any old tacho will do,you might find something to work but you will need to calibrate it.

why not just spend the 25 quid and buy one from a light aircraft supplier?

LAS (light aero spares) are a little cheaper than skydrive for instance.

or you could get one of those ones which take an impulse of the ht lead, im not convinced how acurate they are.

Cpl. Bee - n/a
14-Apr-07, 04:07 PM
answer: Very - Every cylinder fires once every 2 revolutions, the digital tachs pickup the EM pulse when the spark comes down the lead calculates the rate and then doubles it, there's your digital readout of your revs per min

kevinf - n/a
14-Apr-07, 06:56 PM
Slight correction - on 2 strokes each cylinder fires on every revolution. It's a 180 degree engine so there are potentially 2 firing pulses for every revolution of the engine which can signal the tacho. With a Rotax, it will depend on whether you have points ignition or CDI. The points ignition models can drive a tacho directly from the wiring (I'd need to check to make sure which wires to use), whereas the CDI ignition systems tend to use the type with a pick-up wire wrapped around a plug lead (sometimes you need to wrap the pick-up around two plug leads).

What type of engine do you have Martin, points or CDI? - I might have an old spare tacho you can use. Send me a PM - I'll be at the Branch meeting Thursday, unless you can wait 'til the 29th.

Kevin F

Cpl. Bee - n/a
15-Apr-07, 01:12 AM
Ah whoops http://hovercraft.org.uk/images/icons/smiley_icons/icon_redface.gif

Sorry - i'm used to Renault Clios where we have a digital rev counter and timing light combination so it does have to multiply by 2.

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sometimes you need to wrap the pick-up around two plug leads

although i wouldn't advise it as you could get messed up readings - simple solution - just wrap it round 1 lead twice as many times. and make sure it's a lead from the dizzy to the spark plug rather than the coil to the dizzy if your engine has them separated.

Iceman - n/a
15-Apr-07, 07:30 AM
More corrections!

Some 2 strokes fire once per cylinder per rev, others fire twice per cylinder per rev.

Mart's engine is points ignition so will only have one spark per cylinder per rev, other Rotax engines with electronic ignition have two sparks per cylinder per rev.

The signals can be quite weak and using two leads for a trigger is fine, the sparks all occur at the same time so will not upset the reading.

All rotax engines (that are generally used on hovercraft) come with engine wiring to power tacho's, its usually the grey wire.

Some of the better Tacho's are settable to the number of cylinders and 2/4 stroke engines.

P.S. Rotax engines do not use distributors! Any HT lead will do for a signal!