In ideal conditions with a great tune for the motor, yes 12.5:1 is ideal, however that's typically not the case and most tuners wont tune for lean best/ideal, but rather run richer 11:8-12 for some added cooling and safety margin at the expense of efficiency and some power. So my afr suggestion was only for running in combination with water/meth, which does add fuel and lower the afr and is whats recommended among common school of thought. If your not confident in the system, then don't run it. Part of the benefit is being able to run a leaner more responsive/power friendly afr along with added timing. Over saturating the afr to help with cooling isn't required anymore with the addition of water/meth. Then your getting back to flooding your engine again by over fueling. Find a happy medium that works with the injector and meth fueling, and get a meth system that has some sort of flow metering/safety boost dump or timing pull built in. Snow performance, and Devils own makes them that I am aware of.

And based on your latest post. Losing 20whp after injecting water/meth means you are not tuning for it, you should be adding about that much power, not reducing it. Your just flooding the engine. You need to lean out the starting 11.5afr pre meth more and likely add some timing back in and possibly reduce nozzle size if you have to but not necessarily. More the the better up as mentioned, up to a point. A point to where your ignition system just cant cope with it and spark blow out cant be mitigated with reasonable plug gap, then back off on the injection. You should have no problem running 1000cc nozzle at your power level in my experience.

 

great information thank you, will take the car back to the tuner soon.

 

the knocking was there before i add the meth, now will try to re-adjust the afr for the meth.

 

yes there was knocking (3 knocks) in piston #2 at low RPM, after the retuning it's reduced to 1 knock. the tuner told me 1 knock is not big deal and it' consider as normal!