Hi all.

I modified my tach (4cyl) to 6 cyl operation and I thought I'd share.

I have a second tach which I modifed for the 350 in my boat, which also has a version of hei installed.

I found this:

http://www.fieros.de/en/articles/tach.html - http://www.fieros.de/en/articles/tach.html

And it works great! Perform the 'Modifying the tach' section. I used a 500kohm pot instead of a 1 Mohm pot, but it worked fine and gave me lots of adjustability.

I set up a square wave frequency source at 50% duty cycle, 5v rms, and injected the signal directly and then through the tach filter. Both were successful. I set the tachs up for full scale deflection (6krpm) with the following formula:

freq = (rpm)(/60)(/2)(*6)

Explained: rpm--duh

                /60--minutes to seconds

                /2--1 spark for each 2 revolution (4 stroke)

                *6--Number of cylinders

So, 300hz = 6k for 6 cyl, 400hz for 8cyl

The tach filter, with it's resistor in, resistor out, and cap to ground will short out high frequency transients, like the one generated when the ignition module turns the primary current off to generate a spark. It doesn't modify the frequency of the signal itself, it just protects the input of the tach. Therefore, changing out a 6cyl tach filter for a 4 cyl tach filter will have no effect.

This clears up a buch of misinformation out there on tach mods. Some places refer to changin the cap in the tach filter, but what they really mean is changing the cap on the tach circuit board. The variable resistor method is way better for accuracy.

Yep we did this mod to my tach to convert it to the V8 and did it a little diffrently we mounted the pot on the end of 2 long wires soldered to the board, so I have easy access if I need to adjust it at a later date, I just pop the top of the cluster housing off and turn the wheel a bit, its a 10 turn high precision pot, so I can tweak it to get it perfect. 

I think I have a few pics BRB,   this is robbed from my old pff thread when I built the car the first time around back in I think 2005

I went to SMI today in Langley and got what I needed today. I actually get something much better than what I expected. It is called a High Precision Pot. Instead of a simple 1 turn full to min, it is 10 turns to go full to max. It is a 200K High Precision Potentiometer. I wired the bad boy in today with my $1 soldering iron I got at the dollar store and my 1.95 solder. Tweaked it up and it is prefect. You can see on the Scan Tool it is dead on. I love this pot thingy, you can tweak it almost 1 rpm at a time.

My soldering skills are a bit on the lacking side, in the first pic it almost looks like there is so much solder that it bridged the 2 points, you can see in the 2nd pic that is not that bad. It is just a simple matter of wrapping some tape on it and pushing it down under the panel and then torquing the panel down. It will be nice too, if I ever have to recalibrate the tach it will be a simple mater of popping the panel off and twisting the pot.

Oh it is so nice to have a proper working tach again.

Well here are the pics.

My Mad Soldering Skills


My 1 dollar soldering Iron

Happiness is knowing what rpm your motor is at

Nice.

Chay