If you're tach is troubled by a bouncy needle, or the needle "floats when you get on the gas, this little filter will solve the problem.
Why the tach bounces: A tachometer "counts pulses" If the pulses from your ignition are not smooth, the tach may see more or less than there really are, and this can cause erratic needle movement. With most aftermarket high performance tach's, this may not be a problem, but if you have a factory tach, and add an HEI or other high powered electronic ignition, you may get a bouncy needle. This device will smooth out the tach needle on any engine.
What the filter does: This is a simple "low pass" filter. It contains a resister and a capacitor. Low frequencies are passed through to the tachometer, but high frequencies are sent to ground. In this filter, the cutoff is about 10,000 rpm with a V8 engine.
Will it affect my ignition: No, it can't. It is a completely passive device. The resister isolates the ignition system from any effect.
Can it hurt my tach: Again, no it cannot. In fact, it's ability to free your tach from unwanted hi frequency, high voltage spikes may help extend the life of your tach!
What if I want to build my own: If you can solder, and you have a Radio Shack nearby, no problem. You'll need a 270 ohm, 1/2 watt resister, a 1 ufd electrolytic capacitor, and some wire, terminals, and shrink tubing.