ditchdigger Reader
6/4/08 11:11 a.m.

Long story short I am dropping a VR6 into a 80 rabbit pickup. So the VDO Tach in the nice late 70's gauge panel I really want to use isnt gonna work. I cracked it open and the circuit seems pretty simple. Is there a "grassroots" method to modify cyl count?

Nashco Dork
6/4/08 2:26 p.m.

I know there are typically circuits that can be modified in most tachs, but each one is different. For a reference point, go over to Pennock's Fiero Forum (www.fiero.nl/forum) and do some searches. Lots of guys swap between 4-6-8 cylinders and changing the tach to read correctly is well documented on them. Like I said, I know it's different for every tach, but the principles might be the same on yours and you could probably test the same theory out by snipping some traces/leads on yours and running some test parts to try it out.


thatsnowinnebago New Reader
6/4/08 2:52 p.m.

I have no clue how to modify tachs but kudos to your project, it sounds like it should be pretty awesome.

Dr. Hess
Dr. Hess SuperDork
6/4/08 3:27 p.m.

I'd look into a divide by 3 multiply by 2 logic circuit if I couldn't find a simple "change this thingie" reference.

Nashco Dork
6/4/08 3:29 p.m.

On a related note, I saw some pretty wacky VW swaps at the local VW show last weekend. The diesel Scirocco was probably the biggest double-take. Engine swaps are fun. :)


ditchdigger Reader
6/4/08 3:48 p.m.

I spent so much time and money collecting and then restoring the parts for the dash setup I have, I would hate to switch it all out now.

Besides I really like what I have put together

I might be able to modify a late 70's Porsche tach to fit, The fonts would match up, but I would lose the fuel and temp gauges that are built in.

I know that companies like Palo Alto speedometer provide this service for a "reasonable" price but I like to do things myself.

The idea of a digital converter hadnt crossed my mind. I will look into that.

jamscal HalfDork
6/4/08 4:00 p.m.

Granny's speed shop (Rx-7 swap people) had a site listing how to convert the rotary tach to read correctly for a 4-6 or 8 cyl.

I did it to my Rx-7 Tach to V6. It involved taking the tach apart and a 12v power source. I had to turn a couple screws (pots) on the tach until a certain value was reached under power....

I'll try to find the link later.


Edit: easy find. Hope this helps:


confuZion3 Reader
6/4/08 4:02 p.m.

I think Dr. Hess is onto something. If I am understanding him correctly (there's a good chance I'm not) you really wouldn't have to modify the tach. A simple circut could be built (or bought?) that divides the number of pulses on the input side (from the engine bay) by 3, and then multiplies that number of pulses by 2 on its output side. The tach would only see 2/3 of the number of pulses that the engine is putting out - which is 3/2 of the ratio that the tach is set up to see - it should work.

Dr. Hess
Dr. Hess SuperDork
6/4/08 4:35 p.m.

Yeah, that's what I'm thinking. I don't see why it wouldn't work. The logic should be fairly easy to work out with a digital logic reference book and some gates, if off the shelf divide by chips aren't available. I'm not a gEEk. Just wire it to the coil negative with some proper insulating circuit, like an optical coupler thingie. Feed the output of that to the divide by/multiply by logic, feed the output of that to a transistor with a relay coil in collector lead to +12v to generate the voltage spike for the OEM tach. I think it'd work. Probably less than ten bucks worth of parts at crap shack prices. Ten cents bulk/wholesale.

Evan_R New Reader
6/4/08 6:27 p.m.

Not quite cheap, but Dakota Digital sells this...


...for $79, that will do the trick. I used a similar Dakota product to install a gasser tacho into my diesel.

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