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8 cyl tach in 4 cyl


finkbuilt

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I am converting my NK 2000A to a 5 speed, and I would like to use this cool period correct VDO tach that I bought on ebay. The only problem is that the tach is for an 8 cyl engine.

I want to make a little 2 into 1 diode lead to drive the tach so that it doesn't read at half RPM. My question is, what can I tap into in the ignition system to get double pulses? hooking it to the negative side of the coil, it reads half of the actual RPM.

Has anyone done this, or know how I should do it? There must be a clever way to use this cool old tach.

73 Riviera Daily driver 2002 - MS, 9.5:1, cam, LSD

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Unless you have 2 sets of points like an some old datsuns do, I don't know how you would get 2 pulses. If you want to build a circuit, do a search for "frequency doubler" maybe you can find something. I think diode frequency doublers are only good at RF freqencies, but 2 other ways for 0 to 10K range come to mind, you can use V to F IC like used in megastim 2nd version, put 2 back to back for freqency multiplier. Or use a pic chip or other small micro, probably what is inside the device ray pointed out. there is also a way to do it with a simple XOR gate & a delay, but that is even more trouble. Digital IC's divide by 2,4 etc.. are easy, but to multiply is not as simple. This circuit looks easy, but you have a chip

that no one sells in small quantity, unless you can get an engineering sample.

PLL & XOR circuit

Here is a link to the V/F idea, the TC9400 chip is about 6 bucks at digikey in single quantity.

Figure 24 in the datasheet shows how.

AN795 Link

2002 owner since 1980

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http://www.technoversions.com/

Howzzat?

;-)

Cheers,

Ray

Well, I was thinking more of a DIY solution like "yeah, tap into 2 of the contacts on the distributor" or some such advice, only better. :)

If I was looking to spend $80 bucks, I would just troll eBay for the right tach (which it may come to) .

but thanks.

73 Riviera Daily driver 2002 - MS, 9.5:1, cam, LSD

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Unless you have 2 sets of points like an some old datsuns do, I don't know how you would get 2 pulses. If you want to build a circuit, do a search for "frequency doubler" maybe you can find something. I think diode frequency doublers are only good at RF freqencies, but 2 other ways for 0 to 10K range come to mind, you can use V to F IC like used in megastim 2nd version, put 2 back to back for freqency multiplier. Or use a pic chip or other small micro, probably what is inside the device ray pointed out. there is also a way to do it with a simple XOR gate & a delay, but that is even more trouble. Digital IC's divide by 2,4 etc.. are easy, but to multiply is not as simple. Here is a link to the V/F idea, the TC9400 chip is about 6 bucks at digikey in single quantity.

Figure 24 in the datasheet shows how.

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en011959/a'>

AN795 Link

perfect! thanks.

73 Riviera Daily driver 2002 - MS, 9.5:1, cam, LSD

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If I was looking to spend $80 bucks, I would just troll eBay for the right tach (which it may come to) .

That certainly would be the easy, cheap way.

There isn't any simple way to get 4 coil- type pulses

per rev, and the tach is going to want a coil- type pulse

(high voltage spikey - type waveform, not a polite 12v square wave...)

t

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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