I made this deal that will allow me to monitor ignition timing on an engine. The device has two inputs, Tach and TDC. The Tach signal is a square wave that goes hi when the coil fires for each spark plug. The TDC signal from a variable reluctance sensor. The TDC signal goes to a component that turns the sine wave into a square wave.

The tach turns on the output of a flipflop and then the TDC turns off the output of the flipflop. So if the ignition timing is 45 degrees, the output will be high 50% and low 50%. I then take this PWM with varying duty cycle through a RC circuit to turn this square wave into a 0-5v signal. 45 degrees is not 2.5 volts.

It works pretty good on the test bench however I am picking up noise along the circuit on the race car. I have access to a dyno next week where I can really test the hell out of this thing. Any suggestions based on the attached schematic to help eliminate the noise? I am going to bring my scope and solder wire to pins along the circuit to figure out the problem area.