Originally Posted by
chaffe
will do mate, the injectors are 4 ohms each (LPG gas injectors) as said ill buy six 8 ohm wire wound 25 watt resistors and put one in each earth, then join all the earths from the resistors together as i only have/want one wire going to the ecu/changeover reylay (from petrol to gas)
So you are trying to make low impedance injectors to work with a high impedance ecu? So, if you have a 4 Ohm injector and want it to look like 12Ohm for the ecu, then just add 8 Ohm in series to each injector. The current will be 12V/12Ohm=1A. So you will get P=4Ohm *1A*1A= 4W of power and U=4Ohm*1A=4V of voltage to the injector. So just make sure the injector is opening in the first place with the 4V (around 3V when cranking in the cold). If the injectors are in a batch configutation (2+2), you will need half the resistance not double, so 6 Ohm resistor. The power converter to heat in the 6Ohm rsistor will be P=R*I^2=6Ohm*2A*2A=24W. If you don't have a batch install, the power will be P=12OHM*1A*1A=12W, so just use a single resistor 25W is ok if you can actually mount it to something in the engine bay that dissipates heat. Remember also that the injectors are driven by PWM which almost never will reach 100% duty, so in an actual case, much less heat is generated.
An example of a good resistor is:
http://uk.rs-online.com/web/search/s...duct&R=1073891
Notice how the 25W rating drops to 12W only when it is not mounted to any metal for heat dissipation (convection only).