12 inches is generally recommended, however there are situations where you wont have any problems running them close together. I always set mine up with as much space as is practical for my set-up. I try to keep all my ignition components forward of the firewall, if possible. I think the newer ignitions are less of a problem than some earlier set-ups. Its really a crap-shoot, and either way, you need to have a good range check with engine running.