Okay, I don't know how familiar most of you will be with hot ridding, but here's the breakdown. A two step is a temporary rev limiter on a car engaged when you are going to launch the car typically. My plan is this.
There is a tachometer output from the ignition box, it supplies a ground when there is a signal from the trigger(increases with rpm) If I use the car's battery for voltage supply, I think I can simply hook this ground to a circuit with a resistor, then take a batt supply, run a 5v regulator and have that hooked to a digital input on the arduino. It should be high until it is grounded, which will read low, indicating a firing event. I've never used the freqcount library, but I've read about it. I plan on using it to find out the engine rpm, then based on an analog input from a pot(dial to adjust the activation rpm) when the arduino sees high on another digital pin(temporary contact switch in the cabin to hold to activate two step) it will then use a mosfet to ground the ignition coil until rpms drop below the pot controlled value.
Im writing this from my cell on the road to florida. Gotta have something to occupy my mind. And im still really new to electronics, but will this theory work?