4 cyl tachometer/rev counter and a V8...

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Caprimaniac
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4 cyl tachometer/rev counter and a V8...

Post by Caprimaniac »

Well, a bit embarrased since I asked the question earlier, but am not able to find the thread.

Have a 1970 car, originally a 4 cylinder. Have put a V8 in it, and been running fine for 10 years now.

However; redoing things here and ther and now I'm getting rid of the Autometer tach. Instaloling original dash, with original GT gauges.
So, the tach is for a 4- cyl. What changes do I need to get it to display the V8 revs correctly?

Let's see- a potentiometer.... or a resistor...., maybe?
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Re: 4 cyl tachometer/rev counter and a V8...

Post by Caprimaniac »

Seem to remember answer I got last time around was some kind of an component- 2000 ohm resistor?- in paralell with wire from coil to fet the pulses halved.

It cannot be that simple?

Dakota digital is selling a box; it's like 100$. However- I'm not sure it will work for my purpose. Found a youtube vid with a part no not longer available: SGI 8D would do exaclty what I want.
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Re: 4 cyl tachometer/rev counter and a V8...

Post by In-Tech »

I seem to remember it was a simple matter of cutting a wire for 6cyl and cutting two for 4 cyl, if so, you can just solder the cut wires back together. I have a project that WAS a 6 cyl and is now an 8 so I will be looking in the next couple weeks if you can wait. Probably just peruse Autometer site for old diagrams and your answer will probably be there.
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Re: 4 cyl tachometer/rev counter and a V8...

Post by BillK »

Not sure what would be the best way to do it. The V-8 makes twice as many ignition pulses per revolution so you need something to actually change the number of pulses. A resistor will not do that. I will look at the circuit tonight and see what I can figure out :)

Exactly what car is it ? A 70 Capri ?
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Re: 4 cyl tachometer/rev counter and a V8...

Post by BillK »

Actually this looks like it would do the job:
https://dr911.ca/shop/tach-adapt-digita ... ll-models/
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Re: 4 cyl tachometer/rev counter and a V8...

Post by Geoff2 »

Bill,
I am not sure if that will do the job. Description is a bit vague. It may just adjust the tach accuracy, not change the number of cyls that the tach works with.
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Re: 4 cyl tachometer/rev counter and a V8...

Post by BillK »

Geoff2 wrote: Wed Dec 22, 2021 1:12 am Bill,
I am not sure if that will do the job. Description is a bit vague. It may just adjust the tach accuracy, not change the number of cyls that the tach works with.
Geoff,
If you look at the pictures it shows the back of it and there are dip switches that you set depending on which combination of engine and tach you have. In his case it looks like switches 1 & 6 would be on for a V-8 engine with a 4 cyl tach.
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Re: 4 cyl tachometer/rev counter and a V8...

Post by Baprace »

Caprimaniac , Looking at the picture of the RPM correction box it appears to do exactly what you wanted.
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Re: 4 cyl tachometer/rev counter and a V8...

Post by Caprimaniac »

In-Tech wrote: Thu Dec 16, 2021 12:41 pm I seem to remember it was a simple matter of cutting a wire for 6cyl and cutting two for 4 cyl, if so, you can just solder the cut wires back together. I have a project that WAS a 6 cyl and is now an 8 so I will be looking in the next couple weeks if you can wait. Probably just peruse Autometer site for old diagrams and your answer will probably be there.
Hi, yes- I think you mix this with an Autometer tach; I remember looking at this when installing (have done a couple).

Yes,Bill and Baprace- I do Agree. That box looks correct for the job. I'll try to find it on ebay or in Europe....

And you ARE correct about one more thing, Bill- it's a 1970 Ford (Merc) Capri, and I got hold of GT instruments from a German car.
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