I think the backfire remark was in reference to wet systems. The fuel & nitrous are mixed well before the valves, which is where the fuel on our trucks is injected. The danger is much like TBI injection, or carborated cars/trucks. The fuel & air are mixed aeveral inches from the engine, and travel down runners to the valves, hence there is more fuel/air to burn in a backfire. Possibly involving all 6 runners as all the air in the intake is mixed with fuel. The injection on our trucks limits the available fuel to small pockets around the valves. A backfire is not likely to involve more then one runner, as there is no fuel to propigate the burn from valve to valve.

What I want is someone to make new heads for the 3.3 that allows LPG direct injection. Can you say 110 octane!!