I need to order a 1 or 2 step colder spark plugs for an Evo 9, 2.3L GTX35 turbo. NGK site lists the LFR7AIX as the number for the Evo 9 but it seems that numbers LFR8AIX or LFR9AIX don't exist! What's the part numbers for 8 or 9 heat range for an Evo 9?

 

denso IKH24 are the coldest ive found. they are supposedly equivalent to ngk heat range 8.
ive had great luck a bit beyond 30 psi on 93 with them

 

Hi,

How wuld you exactly know when you will need a colder plug? Any general rule of thumb to follow?

 

About 1/2 to all of the ground strap should be clean. This would indicate the plug is the correct temperature. If the entire ground strap is dirty, the plug is too cold. If the entire strap, plus any of the threads are clean, the plug is too hot.

 

Oh thanks. How many Kms of driving before we look at the plugs for this?

Is there a guide on what mods warrants us to step up our plugs? Searched. Not found.

 

A few WOT pulls will tell you. generally colder plugs aren't needed if the car is tuned properly, and your below the 600-700whp mark.

 

it all depends on the fuel and timing and compression and boost youre running. I run the ngk8 range densos i posted earlier in a stock evo 9 motor on 93 at ~30 psi. the colder plugs let you be more aggressive with boost and timing without getting into knock as easy. The downside is they can foul easier. I have about 5k on my plugs and ive been getting some light misfires at cruise but its a beast under any kind of real load.

the plug strap rule mentioned earlier is good, but you need to make a couple wot pulls and check them. cruising and normal driving generally will make any of the normal visual cues on the plug invalid.