Spark Plug Heat Range

[parmas]

Discussing Spark plug Heatrange on boost.

Engine in the example is a:

- Toyota Yaris 1.5L
- Normally aspirated engine
- Stock 10.5 : 1 Compression / Turbo 8.5 : 1 Compression
- Stock Plugs BKR5EYA 0.8mm gap
- Street Use

Normally when going from N/A to F/I many use 2 steps colder plugs or more according turbo and engine compression. Is it the same case when using water injection ? Water Only for now.

Let's say I did go 2 steps colder on the 8.5 compression. Would this result in poorer engine efficiency while idle and cruising than using stock plugs?

I think answer is obviously yes!

Would water injection reduce spark plug heat range in a way to make it work cooler during combustion?

I think the answer is yes if using same fuel and ignition settings.

So why would a person switch from N/A to F/I go to colder plugs if he is not running lots of boost ? (above 20psi ? )

I tried it and eventually didn't blow the engine but instead got the engine efficient by leaving stock plugs and inject water immediately on boost.

What are your thoughts about this ?