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the online instructions I found arean;t real good for my purpose
I'd like to know whether these commercial units break the normal IGN circuit, and have its relay switch the ignition circuit, and so the IGN wire from
the key switch is now used as a sensor line for IGN ON?
Or do they do something more clever?
Eg detect IGN circuit voltage 12V->0V (ie key off) and switch relay on ? would the relay be fast enough to keep the engine running?

Wiring is significantly different, the 2nd approach would be easier, as the relay is a simply bypass, without need to disturb the current wires, which are a b** because there heavy gauge wires.......
and the relay is only required to work during the timer interval....

I ask cause I want to add turbo timer mode to my H2O Inj computer..hopefully got enough free code space to add the function....
I'm going to use RPM as the safety feature, ie if the RPM changes whilst in turbo timer mode => engine off!!

thanks if someone can help with info...

90% of them use the second method. dectect the key off and switch power into the circuit. they also use the handbrake as a safety, no brake no timer.

big thanks Chris
just what I wanted to hear,tho I don't think i'll do the hand brake sense...
already got the rpm signal (maybe use TPS signal too)
but looks like gonna have to make computer #3, not enough code space and I'm NOT gonna do it in assembler.........

this is PIC based I gather. use a bigger PIC. 16F84 would be my choice.

Chris_Rogers wrote:this is PIC based I gather. use a bigger PIC. 16F84 would be my choice.

must be kidding...its using 16F876 atm...8K code space...looking at PIC18LF2550 maybe.....tho I do use atmel chips, i'll keep this project pic based.
Its taps into signals for AirTemp,WaterTemp,TPS,boost,rpm pulse from the ECU box, controls PWM of water pump, plus solenoids etc.......

I did a similiar thing years ago with one - drove a 2 x 8 LCD screen and looked at similar things. runs a 16f870 with room to spare.