Want to build my own tachometer recording device.....

I know these devices are available for drag and circle track race cars, but it would seem that the technology can't be too very complex. What type of recording media would work, and what type of electronic signal (voltages) are being fed from the ignition system to the tachometer? The recorded information needs to be able to be played back through the analog display on the tachometer (signal in and signal out need to be the same) and should be able to record for at least 40 - 60 seconds at a time.

Any feedback would be helpful.

Thanks,