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Bob Ashlock 09-03-2011 09:34 AM

Tachometer Adapter
I’m going to do a little self-promoting here and hope I don’t get my hand slapped for trying to “go commercial” here on the forum. From time to time the topic of adapting tachometers to work with different engines (without sending them out to be torn open and “re-calibrated”) has come up. In my case where I installed a 911 engine in my 912, I wanted to use my existing 4-cylinder tachometer because it was in good shape and had a custom white face that matched my other gauges. Problem is that it was reading 1.5 x faster due to the two extra pulses/cylinders. Since this was a “one-off” situation, I kluged-together a little external analog circuit to change the pulse rate to the tach and went on with other adventures. Over time, I've engaged in discussions with various forum members about how I did this and that got me inspired to further improve the design to be more "universal". My new design is easy to install and can be used with any combination of engine size (4, 6 or 8 cylinders) and tach design (4, 6 or 8 cylinder). It is intended to work with Kettering (points) ignitions, or with electronic (Pertronix) and CDI units. It may also be useful for solving "tach-bounce" and related mis-match problems as it provides a selectable "regenerated" low-voltage or high-voltage drive pulse to the tach.

All that to say that I now have a prototype of this new adapter running in my car and am trying to determine if there is enough interest out there to move forward with producing and offering some units.

Porboynz 09-03-2011 10:33 PM

Good idea, this is not just a Porsche problem. I had the same issue when I swapped a 4 cylinder instrument set into a V6 Audi to fix a sticking odometer. It will probably iron out the issues with tach bouncing many 911 owners have reported in this forum. I ended up binning the Porsche movement and swapping in a GM movement to solve the bouncing tach issue. (not quite true, the original mecahnism is safely stored away) Good luck with it, how much do you need to sell it for to make it worthwhile?

Bob Ashlock 09-04-2011 08:22 AM

Technically, this device can solve a lot of problems and is definitely less costly and a lot less hassle than sending out your tach to be torn-open and re-calibrated or converted to work in the new application. To make this a viable product, I estimate it will need to sell in the $50 - $60 price range.

Bob Ashlock 10-11-2011 02:53 PM

Update: This started off as a fix for my own car, then I made some for other Porsche enthusiasts. I can now produce these units on demand. The design has been refined and the unit is now in a smaller size housing at 2” x 1.5” x 1” .

To summarize, Tach-Adapt locks to the points (or electronic ignition) signal, and then regenerates a new pulse-train at the rate needed to make your tachometer read correctly, even if it was intended for use on an engine with fewer or more cylinders. Tach-Adapt is useful for…

1) Engine swaps: For example, swapping a 911 engine into a 912 body, or vice-versa. Use Tach-Adapt to make the original tachometer indicate correctly with the new engine.

2) Tachometer changes/upgrades: Tach-Adapt allows use of (for example) a 6-cylinder tach on a 4 or 8 cylinder engine.

3) CD ignition upgrades. When adding CDI to a car that had basic Kettering ignition, the original tachometer usually won't respond because the 'new' points signal lacks a high voltage pulse. Tach-Adapt regenerates this pulse to your tach, making it indicate correctly.

Tach-Adapt is a quick, non-invasive, low-cost, solution that can be easily reversed. It’s simple to install and avoids the hassle and higher expense of tachometer removal and re-calibration by an outside service.

gumba 10-17-2011 04:49 PM

I'm trying to install a 911 '75 tach in a '79 930. The tach won't read. According to the Clewett site the older tachs are high voltage. Will your device do the same thing as the Clewett adapter. Tach adapter [10000] - $99.00 : Clewett Engineering, The complete solution for ignition & fuel injection.
The tach works fine on my '86 3.2 w/eletromotive hpv1.

Bob Ashlock 10-17-2011 05:36 PM

Hi Harold, YES, that is the intent of my design. Besides altering the pulse "rate" to the tach, it is also capable of regenerating the high-voltage pulse. For example, I have a couple of the early 70's Porsche tachs (911 and a 914) here on my workbench that ONLY will operate if they see a leading-edge pulse that exceeds about 150V. My Tach-Adapt unit can be set to delivering this high-voltage pulse without modifying the actual rate. Please private message me and we can exchange info.

gumba 10-17-2011 06:21 PM

Hi Bob,
I sent you an email.

elflamo 10-25-2011 01:45 PM

Hi Bob,

I have an early tach (stamped januari 1968) that I would like to work with my 1972S engine, it has a CDI box (is that the HKZ unit?).

Will your adaptor make this work? The back of the tach says "12V 24000 Imp."

fastfredracing 10-25-2011 02:06 PM

Great Idea, I assume this will also work with MSD installations? If so, you can count me in for one, I would love to try it.

Bob Ashlock 10-25-2011 03:00 PM

The TACH-ADAPT unit I developed will work with both the 3-pin and 6-pin Porsche CDI boxes (HKZ units) and the MSD systems. In the case of the 3-pin CDI, it accepts the low-level signal from the points (or electronic ignitor) and in the case of the 6-pin CDI, it accepts the low-level TD (Tach Drive) signal that is generated within the 6-pin CDI. It will convert this low-level signal to a more robust signal that includes the high-voltage pulse the older tachometers are triggered by. Please contact me by email for details and any questions: Thanks! --Bob

tdskip 10-25-2011 03:02 PM

Guys - just a note that Bob has been great to deal with and the instructions were very clear and he knew the details of all my components. Got called out of town so I haven't installed it yet (will do Sat), but he is a good guy to deal with.

gumba 10-25-2011 03:20 PM

The tach adapt worked on both '75 tachs, one stock and one re-calibrated to 10,000 rpm when installed in my '79 930. It's small enough to stuff anywhere and the instructions were straight forward. Saved me a bunch of grief.
thanks Bob.

tdskip 11-09-2011 06:51 PM

OK - quick update here. I finally got this installed and it turns out that my tach problem was a bit different that what I thought. A couple calls with Bob later and he helped me sorted it out by changing how the unit was configured. Neat product, and Bob was a huge help.

Flying Toaster 11-21-2011 08:14 PM

I agree with what others have stated. Excellent communication and very quick shipping.

I recently purchased the Tach-adapt device for my conversion:

Car: '74 911
New Motor: 5.7L GM small block
Problem: Tachometer was reading significantly higher than it should, as Bob put, 1.33 times what it should be. The needle also bounced erratically as it was unable to handle the HEI signal.
Resolution: Bob preset my tach-adapt device in advance based on my setup & I had it installed in about 5 minutes. Tach reads right on now and the erratic bounce has gone away.

Thanks Bob, greatly appreciated. - John

sandiego 12-12-2011 08:21 AM

Great service
I had an '89 911 engine with a 6 pin CDI and an unknown tach stamped "4 cylinder". Bob took the time to listen to my symptoms, shipped me his unit without paying up front with a no questions return policy. It worked great. Cost less than sending a questionable tach to a speedo shop and paying $75, plus shipping, just to have it tested!!

amallagh 01-24-2012 03:18 PM

Hi Bob, just seen you thread here on Tach-Adapt.
Is there anything it doesn't do !!!!!

I'd be interested in buying one or more of these if you still have some. What is the price ?

Can I just check out my application with you. I have 1966 912 converted to a 6 cylinder 911 engine. Car also has fitted a MSD 6AL ignition.

Will your tach adaptor allow me to use my original 912 4cylinder green face tacho with this new set up ?


Bob Ashlock 01-24-2012 03:55 PM

Thanks for asking about my little product. Your conversion is similar to mine and YES, it will make your 912 tach read correctly with your 911 engine.

Price is $60. I'll send you a private email to work out details.

tdskip 01-24-2012 04:03 PM

(Bob is a great guy and has been very helpful to a number of forum members...just say'n)

amallagh 01-24-2012 11:50 PM

Thanks Bob, I'll email using the address you sent me.

Bob Ashlock 03-23-2012 08:44 AM

Check out the latest issue of 9-Magazine (March/April 2012) to see a feature article on my Tachometer Adapter project. To view the magazine article and to see more info, go to my new website Home Page

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