Way back I bought a Tach Adapter off eBay that was programmed to take the 8 pulses of the V8 and turn it into 3 pulses for the stock tach. It was sort of successful, but not something you would rely on. It kinda fidgeted and floated around. I contacted them, and they said to put a 100K resistor inline with the input as that would affect the Time Constant of the circuit and smooth it out. Now, I'm not an electrical engineer, but I figured the tach adapter wasn't getting a significant signal, and adding a resistor inline would just make it less significant a signal. And it did. The tach was lazier and worse.
So, because I didn't want to have to troubleshoot this, I decided to put the Firebird factory tach guts in the Firefly cluster. Something I should have done back then. It was actually easier than I feared.
First, pull the tachs out:
Here are the tach guts side by side. Firebird on left, Firefly on right:
Backside, Firebird on left, Firefly on right. No, I didn't notice any similarities yet:
I sanded down the perimeter of the Firebird tach face (I smoothed it up more later), and drilled two holes so it could line up with the existing holes in the Firefly face. The sweep is different, and the scale is different between the two, so I can't just plunk the 'Bird guts and use the Firefly face:
Then I noticed they were very similar. The Firebird cluster is made by Denso. The guts are super super similar. I bet if I just changed these mounting pins.....
By unsoldering them....
And attaching the Firefly pins to the Firebird tach guts....
It will all fit inside the cluster, and use the existing wiring and everything:
And it did, even using the Firefly needle (though I may change it back to the longer 'Bird needle)!