Originally Posted by JKMorahan

Just a sidenote, I did find earlier, with a stock M103 engine and just a cam upgrade I had no trouble with resistor type plugs and heard from a couple of mechanics who said it was only an issue with M104 engines due to their spark recovery (?), but with a more heavily modified M103 I now find it seems to be more sensitive to plugs and runs better with non-resistor, especially sorting teething and troubleshooting in the fresh setup.

The CIS3 ignition is still somewhat of a mystery to me but it appears to adjust stored ignition maps based upon a reference call using a resistor value and whilst it may not be apparent in a stock engine, I now believe a modified motor maybe more sensitive to the original factory fitment of non-resistor plugs in order to give best ignition, ie. extensively modified engines tend to operate more frequently at performance curves and take advantage of maximum flow rates where subtle tuning makes quite a difference, where stock engines tend to operate more flexibly where you don't notice minor alterations and they give little or no WOT changes anyway. I also noticed outside temperature made a huge impact on performance which it didn't when the motor was stock, so it is generally far more sensitive to minor things when modified extensively, eg. +70hp from stock...it actually fluctuates roughly +50 to +75hp at best guess depending on the day's conditions and minor things, where stock the 2.6 was reliably the same output every day, any day, within 2-3 unnoticeable hp.

I'm using NGK non-resistor at the moment, which work excellently. I did have resistor type Bosch performance plugs with multiple prongs in the near-stock motor without trouble, even believed it pulled better up hills slightly. But in the heavily modded motor I had some extra troubles with the resistor type Bosch plugs when troubleshooting other issues, that disappeared when I put non-resistor plugs in. It was making troubleshooting difficult since I was having multiple problems and some of those disappeared with non-resistor plugs. But again, no problems at all with a near-stock motor.