The craziest ecomod I have tried was when I lobotomised the engine in my previous car, a little Fiat 127. A conrod big-end bearing seized and I could not find a replacement engine cheap and quick enough. Instead I ripped out the bad conrod with it's piston, and it's neighbor to get better mechanical balance. I welded the oil jets in the crank, removed the valve lifters (OHC) and cut som clearance in the cams with an angle grinder. I re-used the cylinder head gasket with some cheap liquid seal and drove the now only 525cc two cylinder engine for 4000 km in 1½ year with almost no problems before I sold the car as spareparts to another Fiat 127 owner.
I had to run the engine extremely hard with lots of down-shifting and high revs all the time. The fuel consumption went down almost 15% due to increased load and reduced engine friction. 20-25 hp max in a 750 kg non-aerodynamic car is a bad equation. It would be interesting to try something similar with a car that has better power margins.
Engine downsizing may definitely be one way to improve fuel economy. It can be done very cheap, it doesn't change the appearance of the car (except for accoustic appearance
) but I would personaly go for things like aeromods and Burn & Glide driving and keep the power margins in the engine.
Here's my old downsizing blog/report (mainly in swedish) with photos and som videos:
Projektblogg - Fiat 127 engine downsizing