I am totally with Gordon on this one!
I could go into a technical and windy explanation about BMEP, Adiabatic Efficiency, Stoichiometric Combustion and Ratios and etc, however this excellent work does it very well.
http://www.carbibles.com/fuel_engine_bible_pg3.html
Since the 1950s, there has been a plethora of magic liquids and pastes for cars which cure everything from worn bores and pistons (Krause Labs; anyone remember this?) to numerous snake oil additives guaranteeing instant massive power increases.
Together with various gizmos offering huge new fuel economies!
Perhaps one of the very best was one outfit years back flogging an economy kit for side valve Fords......... which were always notoriously thirsty.
This wonder kit was simply a much weaker main jet.
Which was sort of fine, if the driver never exceeded 25 MPH; never accelerated beyond sort of 0 to 25 time of half an hour.
Most normal drivers who bought the scam finished up burning out the valves and often the pistons too!
Wonderful Cost-Benefit result, then..................