Electronic rev-counter and wasted sparks

[Boxprod]

Dear wise folk,

I'm off down a rabbit hole again, this time about having an electronic rev-counter on my '74 widecase 350. This started simply because I was trying to get more accuracy than the original small needle-waver offered, so in about 1979 I fitted an old Norton Smiths mechanical version, which was the 80mm size and considerably better - but it vanished at some point over the years of storage.

So I recently acquired a Chinese marine tacho quite cheaply, and worked out how to set it for a single cylinder (despite the 'instructions'), and that seemed fine, but after a couple of months it started reading inconsistently and variably, so that one was removed.

I then found on eBay an old Elliot Design rev-counter, which was a very reasonable price, and seems to have the right spec. As you can see from the photos, it's probably a late one in the company's life judging by the graphics.

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It is expecting a single pulse every two revolutions - it clearly says so on the back: "1p/2R". Should work a treat. But of course, I've still got the Motoplat system, which I've had refurbished and checked, and it works very well indeed. The trigger rotor, running at half crankshaft speeed, is not 'sided', but has a two trigger magnets at 180 degrees - so you can't get the rotor on the wrong way round.

Which means, I've realised, there are two pulses per revolution rather than one. So the smart new rev-counter reads double the speed it should.

I have, therefore, a number of options in the absence of any further information - the company vanished in about 2005, I think, and there's no sign of any details about their products anywhere I looked on the internet.

Option 1 is to revert to a mechanical rev-counter by buying a 4:1 from a reputable supplier rather than an eBay Indian business.

Option 2 is to contact a firm like Speedograph Richfield to see if they feel able to re-calibrate (which may not be possible, and will certainly be expensive).

Option 3 is to investigate the Elliot's insides and see if I can make sense of them without causing any damage.....

Finally, does anyone have any idea what that yellow wire might be for? It's showing a resistance of 750 ohms when put to an earth (-ve). No photo of any old Elliot instruments has a yellow wire, and it's not mentioned on the back of the case, so what on earth does it do?

All suggestions accepted with gratitude!

Pete.

[blethermaskite]

Pete, been there with this on my own 350 mk3 when I had the Motoplat ignition. in the end I was lucky enough to acquire a mechanical Smiths race rev counter (which is the same size as the stupid std rev counter) and was/is a perfectly functioning instrument for the job.
Cheers George

[Jordan]

I had an analog 3 wire type aftermarket tacho that had a switch on the back.
It was to select the number of cylinders in a car application.
I looked inside, and there were different value resistors for each selection.
So calibrating yours may be as easy as installing a correct resistor value, perhaps with a temporarily fitted variable resistor.
Your fourth wire may be for illumination.

[Boxprod]

Thanks for the suggestion, Jordan, about the yellow wire, but I'm afraid that this instrument is pre-LED and there's no hole for a bulb-holder, so it's definitely not for illumination. It's also connected somehow to the -ve (earth) side. Research needed.

On the matter of reconfiguring the signal to match the expected one pulse per two revolutions, I've dug around and finally found the following:
https://www.ebay.co.uk/itm/284440826701?mkevt=1&mkpid=0&emsid=e11400.m144671.l197928&mkcid=7&ch=osgood&euid=ff7507bdf55e467fa5b3b13fbd52b13c&bu=43093538218&exe=0&ext=0&osub=-1%7E1&crd=20251109033847&segname=11400
Looking at it carefully, it seems that even though it's originally intended for multi-cylinder applications, the chart of its switches shows that if you had an 8-cyl engine and swapped it for a 4-cyl, or a 12-cyl swapped for a 6-cyl, then you need to halve the number of pulses - and in both cases the switch pattern is the same.

s-l140.png

I reckon that should work for halving the number of pulses from 2 to 1. After all, the little black box doesn't know how many cylinders it's dealing with, just the number of pulses. Fingers crossed.

I also found a US version here: https://www.ebay.co.uk/itm/141979625888?chn=ps&_ul=GB&mkevt=1&mkcid=28&google_free_listing_action=view_item, which would do the job according to the maker's detailed description, but it's a bit bigger and buying from the USA these days can be a bit fraught.

But then I find that https://www.iconic4x4parts.co.uk/product/uiversal-rev-tachometer-rpm-converter-adapter-4-6-8-10-12-cylinders/144 may well have some stock of the first one here in the UK. I await their reply.

If it works, when it gets here, then all should be well. I'll let you know.

Pete.