hello! i want to ask why 4k-u engines use long spark plugs and not the normal type! if you use in this type of engine normal spark plugs how will affect it? do long spark plugs raise the compression? thank you very much!

The 4K-U had a dished pistons and a slightly different design to the head/combustion chamber... hence the longer plugs. No other reason.

 

I have used the normal length plugs in a 4K-U (when I thought it was just a normal 4k :) ) and it still ran well enough.... but definitely better with the proper plugs.