Okay this is what i just bought http://www.jaycar.com.au/productView.asp?ID=KC5378

Im going to use it activate my water meth kit, instead of relying on boost reading from the manifold which appears to be VERY inconsistent, like sometimes car is revving like crazy the pump is not working, other times it works fine. So i have decided to use consistent device other then diaphragm based boost activated switch. So instead of boost i will use RPM reading, lets say water meth kit will kick in after 2500rpms

Okay the question, where is the best place to intercept RPM frequency reading?

Off one of the primary leads of the coils. Or inductive coupling to a spark plug lead. Lots of places carry an RPM related signal. But it would take some electronics to make it work.

You reckon i could use ground wire of the coil?

Yep, that should work. The kit should come with suggestions anyway. Tachometer input is the most obvious one though.

Thanks, yea i thought about that one too, perhaps someone can help with wiring diagram?

Im pretty sure there is a trigger wire on the firewall on the left hand side but i can check and get back to you. if u use rpm as the trigger wont you go through alot more water/meth why dont you use a micro switch on the throttle body so it only comes on at WOT.

There is a connection on the firewall near the passenger side tower with a white wire it says in the manual its for engine speed .This should give you the rpm trigger you are after.Someone correct me if i'm wrong.

In theory using the boost reading is a much better idea than just rpm, as boost is a more accurate indication as to how hard the engine is working. Maybe work on getting that setup more reliable?

Idea is not to get more performance but to keep "very reliable" sprintex charger cool given that im not running 7th injector, i bet their high failure rates is that they get too hot, so id rather spend money on methylated spirit instead of reconditioned charger (roughly $2700 from Sprintex), so i do want it running most of the time on purpose.

If you could please get back to with that wire that would be much appreciated.

Thanks for the input guys

Confirmed white wire on the firewall comes from the dizzy engine check speed according to the manual.

Thanks mate