What system do you have, and how big a shot are you wanting to give the car???? A couple different companies making some good nitrous timers, I used a Jacob's on a customer's car some time back and he is quite pleased with it.... Be sure you have a sufficient fuel enrichment circuit to go with the Nitrous!!!! Without an adequate increase in fuel flow when you hit the nitrous, it will lean out the engine and do all sorts of nasty and expen$ive damage to it!!!!! I would also think you might want to consider a timing retard as part of your control circuit.......