[Daox]

Originally Posted by Floordford

My thoughts go to polishing the intake and exhaust ports on the head so that the engine strains less.

Nope.

Possibly a slightly bigger exhaust system.

Nope, unless its turbocharged.

Or more powerful coils for better combustion.

Nope (ok, maybe marginally, we're talking a fraction of an mpg)

A custom grind cam, but it would be very much blind guessing.

Yes, but only if the custom grind increases low end power.

Higher compression for more power with less throttle.

Yes, but only to the point at which it starts effecting timing and you have to retard it which eliminates any benefit. So, this is a very hard thing to balance.

Lighter rotating assembly like a lightweight flywheel, crankshaft, valves, cam gears, rockers, spring retainers.

Yes. Included in this are piston rings that aren't as tight, OEMs have used these on recent engines to improve efficiency. They've also used offset crankshafts to reduce friction losses.

I also wonder what makes for a more fuel efficient engine as means to more horsepower or more torque. By that I mean, some cars have long intake runners to make more torque. Other cars have short intake runners to make more horsepower. No doubt this would need to get matched with things like cylinder volume, bore, and stroke. But one or the other might fetch better efficiency.

Most of my ideas come from making an engine more powerful to race but it stands to reason that if an engine doesnt have to strain as hard to make power then it should get better MPGs. So long as those mods dont cause it to use more fuel.

The easiest and IMO best thing to do to increase engine efficiency is to use a smaller engine. Increasing the load DRASTICALLY increases the efficiency of the engine. If the power output isnt enough, add a turbocharger.