[serialk11r]

Re tuning:

Typically you'll find OEM spark timing to be close to optimal. They'll leave a little bit of margin for bad gas and hot weather and whatever. You can often take this margin out but the gains are pretty small, IMO. What I would personally do is either keep it stock, or use an off the shelf map + PCV catch can to give it more knock resistance.

(if the engine is radically modified then you want to redo the whole thing)

AFR at WOT is again set conservatively so the valves and cat don't cook on a hot day with bad gas. You can remove some safety margin without issues, or remove a lot of safety margin and accept reduced cat lifespan.

I believe all potential low-rpm fuel economy gain comes from EGR and VVT adjustments on 2000-2015 era engines. The oil-pressure actuated VVT is slow-responding, so the OEM doesn't use the full adjustment range to improve response. Hot EGR I believe is limited to reduce intake heat soak (generally speaking every paper I've read suggests 10-15% EGR dilution is a non-issue, but some cars only use a max of 4%, so leaving a lot of pumping losses on the table).

I was able to increase my Scion FR-S's mpg 1% by maxing out intake cam retard for as late valve closure as possible, then retarding the exhaust cam just a tiny bit to get some EGR back (IIRC any more than 5 degrees caused misfiring). Retarding the exhaust cam increases pumping losses though so again there's only so much that can be done without external EGR.

On your Honda K24, you only have intake side VTC available, so things are a little harder, since you increase volumetric efficiency when you try to get more EGR. Since the low cam has low duration to begin with, closing it later probably has only a small effect on VE reduction, so advancing it a tiny bit more is probably the best you can do.

Lean burn or adding external EGR would be your best bets IMO. You'd want to bump the ignition timing advance up a little when doing so. My gut feeling would be increase it by (lambda - 1)*(15 deg - current timing)/2 (pretending that combustion speed is linear in AFR, compensating by 1/2 since friction increases and knock and whatever, comes out to 3-4 degrees max at low load assuming 1.15 lambda).