I always thought the FD had 4 ignition coils for the 4 spark plugs, but after removing the UIM, I was confused to only see 3 ignition coils. I looked at the manual and saw the following:



This is mostly a curiosity question, but why did they decide to put both leading plugs on one coil? But not the trailing plugs? From my knowledge, the leading plugs generate 95% of the energy during combustion, so I would assume that running them both on separate ignition coils would give them a bit more energy, since now 2 plugs are firing even though the other spark is wasted. Is there a reason for this?

 

The most likely answer is cost savings. It's cheaper to build a car with one two-post coil than two single-post coils, and cheaper to have a 3-channel igniter than a 4-channel igniter. This sort of 'wasted spark' setup was common on other cars, including the EVO9. Each of the leading plugs are firing twice as often as needed, but they happen to be located in a position on the rotor that firing them 180 degrees too early or too late isn't a problem. The trailing plugs use separate ignition because it would not be safe to fire them 180 degrees too early.