[JohnAh]

Think I wrote it before, downsizing the engine is one of three ways to increase efficiency by increasing compression/combustion pressure and reducing ammount of engine friction per driven distance. The same thing can be done by changing gear ratio (modifying/replacing the gearbox or change to much bigger wheels) or switch to "P&G driving".

One thing that may lessen the improvement of fuel economy from the engine downsizing is fuel enrichment on a carburetted engine. Some (most?) carburettors give a richer mixture when operated near full throttle and since the engine will be weak the driver may be tempted to use a heavy foot. This will NOT increase power other than at very high revs. On my Fiat I noticed that if driving at full throttle I got no higher average speed but most of my modification´s 10-15% improvement were lost. The easiest solution to this problem is to put a wedge between the pedal and the floor.

I don´t know how an injection engine will behave but I suppose the airflow sensor will average out the pulsations from each intake stroke so the system will think you are driving with four cylinders on half the load, avoiding the fuel enrichment regardless of how much the pedal is pressed down.

I guess a carburettor will also average out the the pulsations at higher revs to a more continous spray but here the fuel enrichment may be mechanically activated by the throttle opening and not by actual airflow.

I also see a risk for imbalance in fuel mixture between the two cylinders if using a 180 degree config as I did on my Fiat. -If my theori is right the "second" cylinder will get a slightly richer mixture since it gets some of the fuel spray started by the "first" cylinder. Since it will take 540 crankcase degrees until next induction stroke the carburettor spray may have time ease down more than on the following 180 degrees. When I compared my sparkplugs I saw no sign of the cylinders getting different mixture but there may still be something to this theory...