Where is the best place to inject gas, before the turbo or after, at the inlet manifold?

at the inlet manifold.

are you talking about nitrous? or LPG?

I think what you are asking is whether to use a suck through system or a blow through system.

Blow through is better.

If your talking about LPG then blow through hands down in terms of both safety and performance.

You do not want your entire piping system to be full of LPG if your car ever backfires.

And in a performance sense the mixer will always flow a greater mass if the gas is forced through it rather than sucked through it. In some cases blow through LPG setups offer over twice the power potential of suck through setups.

GasResearch Australia has a good section on this and the differences between the two - you might be interested in finding it and having a read on there website...

just for the safety factor i would say blow thruogh, like cabbi said under boost compressed gas would cause one f**k of and explosion, i guy i know had a ute that was wired wrong and the lock off soliniod on the converter and tank were always on when the key was on, he had stalled it at the lights and tried to wind it over it backfired and blew the manifold clean of the head , you can still see the dents it the shock tower where the manifold hit. this happened a week after he got the thing and it was a "on road" deal form a car yard.

end rant