[wandering the forums looking for a thread to comment in...]
I'd point to the HotVWs Mileage Motor magazine series. Based on work by John Karcey, an AA Fuel engine builder, for his Karmann Ghia as documented in VW [Trends or Greats](, I forget) and available at
CB Performance; and developed over 8 installments by Bruce Simurda.
The plan involves a stroker crank and high reciprocating mass. Stock pistons and cylinders means better cooling and the 1/4" stroke pulls the torque curve downward. A Freeway Flyer transaxle, electronic ignition, low friction lubricant, &etc.
Karcey used a 50lb Jaguar pulley (prolly a little slow off the line
), I forget what HVWs used; but the motor sitting on my bench has a Gene Berg 8lb crank pulley (and his Type IV cooler upgrade and a spin on oil filter).
HVW's test vehicle started at 28mpg and their goal was to crack 40mpg. They got to 39.99999ohIgive up. And this was with no aerodynamic mods to speak of (IOW
Herrod Helper). Karcey claimed 50mpg in his Ghia in the 1970s.
I think a lot of it is in the build quality. Did you know VW racers drill holes in the tops of their pistons to go faster? True story.
Edit: Fixing a typo, but what about this: Entire engine assemblies can be dipped in liquid nitrogen. Apparently it doesn't affect rubber seals and such. Individual part machine better after treatment so maybe it's best to re-crystallize the metal grains and then machine to tighter tolerances before assembly.
All I know is that my brake rotors are holding up great. Some claim 3—4 times the part life. But it doubled the cost of the stock part. So maybe you could assemble an engine too loose or too tight, freeze it to -273°F and have the various alloys settle in together before you fire it up.