Using a 12v car coil how can I produce a H.T. discharge at a rate of around 500 per min. not using a car distributer.?

That's a pulse rate of about 100mS. You could use a relay as a switch controlled by a microcontroller or 555 timer to give you the pulses. Relays have switch bounce so you might get several fast/short pulses at the pulse interval. If you don't want that, replace the relay with a power transistor or FET. If you use a transistor or FET you must include a flyback diode otherwise you'll destroy the semiconductor.

You could use a set of points driven by a motor/cam to make/break the coil primary. This is the way the system works in a car. Or....

You could use a capacitor discharge system triggered with a pulse generator feeding the coil primary.