Arduino based two step ignition

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?

Lol, cellphone :P haven't been in prison yet.

The signal wire is normally open, it grounds when there is a trigger pulse(I believe its hall effect) So that's why I need to have a seperate power supply(I think) these are typically used at a stand still, so no mph needed. The momentary push button could be hooked to the clutch though(versions like this enable flat shifting or no lift shifting). One thing I messed up, the mosfet would need to be normally closed, would that be a problem(would always be closed, during all driving, would open only during two step limiting) instead of grounding during two step action. If this works well you can also induce a delay in the firing circuit to retard the engine timing. This builds boost on the line in turbo cars by causing the unburnt fuel to ignite in the exhaust(google or youtube antilag) which is pretty violent, but cool. Boxes that do this typically cost $200+ so im just wondering if I can do it with the arduino and some misc components. If I get it working ill upload the code somewhere and make it open source. I have found other people doing similiar things but I haven't found a completed one yet.

The knob should be sufficient, you can change it and the rpms should change real time, allowing you to just read the tachometer in the car and adjust the knob to the desired rpm

Lol. No offense taken. I understand and appreciate the help. http://www.summitracing.com/parts/SUM-850610/ and click instructions, then scroll down to" ford tfi ignition with harness" The coil is our final output. The red wire going into the ignition box is switched power(always on with key on) im pretty sure the orange is the same. The black wire from the factory grounds the coil primary, when ground is removed it triggers the coil to fire. This is done 2 times per engine revolution, since I have an 8 cylinder with a single coil and a 4 cycle engine. The box just turns it into a capacitive discharge output and supplies multiple spark at lower rpm. Now the gray wire would be our arduino input. If you look at this diagram http://www.summitracing.com/parts/SUM-G2945/ and click instructions, you'll see that the shift light is always grounded, supplied keyed power, and gets the signal wire from the box, which is negative, and determines engine rpm from that. Then lights up at a certain rpm. Im doing the same thing, but the arduino receives the signal, and when it hits the target rpm and we have it activated, the mosfet will interrupt the coil operation.

I ordered a pro mini for this project. Im going to be mounting it in a vehicle, will it NEED to be vibration isolated or away from engine heat(possibly mounted on an inner fender??) I can't find any specs on operating temps on these, but the ignition box this reads from has been on the fender for over a year, so it seems like this might be capable too? Mounting inside is an option, though more complicated, but that's why I went with a pro mini. Its small enough that it shouldn't be hard to find a place for it

Okay, I've never used a mosfet before, but I've done a lot of reading and I believe I have two candidates. They are both logic level so they should have no problem being driven from the arduino, since I am driving a coil, I believe I need to add a diode to protect the mosfet from back emf, is this correct? The coil specs say a max current of 220mA, but I went to digikey, found logic level mosfets, and these were some of the cheapest in stock, and they handle way more than that, so should I pick the one with the lowest capacitance??

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=RFD3055LE-ND

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IRLU024NPBF-ND