These were two entirely different awards; that for the Index of Performance a relatively simple calculation; that for Index of Thermal Efficiency a far more complicated affair that surely only the French could have come up with. Much perusing has found brief explanatory references in Hodges 1963 book, and Clausinger's from 1982.
The Index of Performance was an annual award that grew out of the Biennial Cup (itself a replacement for the once only awarded Triennial Cup - 1923-25). The IoP was a simple calculation where the distance (D) actually covered by the car was divided by the distance set (presumably!) by the ACO based on engine capacity (D1). Of course, the distance set, I guess was changed every year as performance in cars in general increased.
For the Index of Thermal Efficiency, I am going to have to quote verbatim from Hodges, since it is beyond my ability to put into easier form! Initially, it 'equated speed, weight and fuel consumption as follows
P+200 + V + (V -100) squared + (V-140) cubed
200 16 500 12000
All the above divided by E, where:-
P (Poids) = weight in kilograms with oil, water and spares but no fuel
V = the race average of the car in kph
E = energy in litres of fuel per 100km.
Got that! The formula was then modified in 1961, and then allegedly simplified 1962 and expressed as
where Er = consumption in litres per 100km, and
Em = 1.5 + (P + 1) V cubed
4 x 105
Where P = weight (now including fuel, but not tools) and
V = average speed.
As you can see the format of posting here does not allow me to place the 'divided by' figures directly under the 'divided by' lines (at least, I don't know how to do it) but I'm not sure it would make it much easier to understand, if I could!
Last edited by John Turner; 24 Mar 2008 at 08:40.