I plan to use inturrupts to count the time between each pulse, do the math and know the RPM. Not only do I want to capture the RPM but I also want to smooth it a bit as some ignition systems are a bit "ugly", so I plan to capture the last 5 pulse times average them, and then come up with my RPM. I then want to output a clean RPM signal at the spcified rate.
Know if my calcs suit me right... at 8000 RPM, I will have 4,000 pulses per minute (each cylinder fires once per 2 RPM), which works out to 66 pulses per second, .06 pulses per ms, which is roughly a pulse every 15ms...
Will this give the processor enough time to do the RPM calcs, output it to an LCD, and even reoutput a "clean" RPM pulse? Any suggestions on other ways you can think to do it?
As to timing, I have done something like this for a motorcycle which easily handled 8000 rpm. What I did in the interrupt routine was to process only every 10 tach pulses. Even at idle I'd be getting an update evry second and I never had to worry about overruns. My code was handling wheel interrupts as well as tach and just loading them into a queue or table. The "Loop" routine would pull them out of the table and do the calculations. That way I could do anything I liked for debugging and, worst case, I would find the table full and just flush data until I caught up. This did happen occasionally when I had very complex debugging or crappy signalling so it's worth checking.
As to data sources, I was pulling from a formal tach signal. You might find that most cars at least have that interface. I have also heard of catching a tach signal by wrapping a turn or two of wire around a spark plug lead but I don't know what signal level you'd get.
I would recommend getting it working for some particular car that you've got easy access to and go from there. Search for my stuff here and you may find the code or other people's comments of use.
Keep posting - I'm interested and others will be.
Be careful of your circuit interfaces to the car, the spikes can be wild. consider using an ipod/usb type power plug for your supply voltage to make life simpler.
I saw a basic tach for sale in an auto discount store for $20, again, might be a source of ideas.