Tachometer Wars

[Bob Ashlock]

Engine swaps, tachometer swaps, or ignition system modifications often result in erratic or non-functional tachometer behavior. Porsche tachometers look like relatively simple instruments with just a few connections so how can they be much problem? But, even though most Porsche tachs look similar, closer examination reveals that Porsche made periodic changes that kept them compatible as engine and ignition systems evolved.

A few years back I ran into a problem when swapping a 911 engine into my 912. The 4-cyl tach now read 1.5X faster than actual rpms. I wanted to retain my stock 912 tach as it matched my other custom-faced instruments. I designed a signal adapter that avoided removal of the stock tach and the cost of sending it out for modification. Members picked up on this and since then I have been providing these little adapters to Porsche enthusiasts all over the world. The applications have
been varied... some users just wanted to swap a different tach into their car, some installed larger 911 engines with motronic and several Chevy V8's found their way into 911's and 914's. Ignition systems ranged from simple points, Pertronix, CDI's, MSD's, Porsche Motronic and GM HEI systems.

There are many postings about TACH-ADAPT on other forums. Since Steve just started-up this new forum, I asked to post here to "re-introduce" TACH-ADAPT to the new-comers. TACH-ADAPT has a long track record of solving compatibility problems. Please take a look at my website to get the details on the product.

[Steve]

This is really a neat product and a genuine problem solver for anyone doing an ignition upgrade and/or engine swap/conversion. We are planning on carrying them for sale on our website very soon.

[304065]

Bob, what is the voltage level, waveform and duty cycle of the tach output?

[Bob Ashlock]

To answer the question generally, the output is ~12Vp-p, 50% duty cycle, current limited. And, for older tachs (pre-'74) there is a selectable high-voltage, low-energy pulse to simulate the Kettering points/coil/condenser output.

For more details, please e-mail me and I'd be happy to go into a deeper explanation: Bob@AshlockTech.com