Hi folks. I've installed an MSD StreetFire ignition in my 914. To get the tach to work, I had to buy their 8910 tach adapter, which solved the problem. Everything's working great now.

So today I decided to clean up the install by mounting the tach adapter somewhere, and dressing the wiring nicely. I'm sure some of you have done this, so where/how did you install the tach adapter? My first inclination was to bolt it to the firewall, which is where I put the coil and the StreetFire, but the tach adapter gets pretty hot, and the firewall is covered with that heavy vinyl (I think) sound insulation. Is that OK? Or is there a better place to put it?

I think the 8910 dissipates about 30 watts, so it gets pretty hot to the touch, and I wasn't sure how the vinyl would like that.

All suggestions appreciated, especially with pictures!

Thanks,
Jim

Get some aluminum L channel from home depot and make a bracket and mount it to the firewall. The aluminum will act as a heat sink.

A tach adapter that dissipates 30W??

That doesnt sound right. If it is, that's a really bad design and apparently stealing some juice from the ignition system.

I'd like to see the circuit diagram for that.

here's mine

Wow!! Nice work. Does your tach adapter get very hot?

I rethought the dissipation, and it's probably half that. It's 30W peak, with about a 50% duty cycle... So probably closer to 15W average.

Regarding the power, the "tach adapter" is just a big inductor. I brought it in to work, and we measured it at 5 mH and 5 ohms. Then we X-Rayed it. Just a big coil. It must provide the inductive voltage kick that the tach "counts" to determine RPM. It's a fake coil, basically, and a pretty brute force way to do the job, but it works. I'm running a Pertronix in my distributor, and at 5 ohms that's a lower load than the Pertronix flamethrower coil I was using, which was 3 ohms. (The flamethrower measures 8.8 mH by the way). Because the MSD has a separate power source- right off the battery- I don't think the "tach adapter" is draining anything away from spark.

Here it is, just a big old E core with a coil of wire around it, not a very pretty winding job on that bobbin, either:
Click to view attachment

Wow!! Nice work. Does your tach adapter get very hot?


I never checked it.