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"         The aim of current racing regulations for the 24 Hours of Le Mans is to give equal chances to all cars of the same class, irrespective of their engines&amp;#039; type and size. This is done by restricting the flow of air into the engine&amp;#039;s intake system by one or two cylindrical flow restrictors of appropriate diameter. The system is based on the knowledge that when air is aspirated through such a restrictor, the maximum flow possible is reached when its speed equals the speed of sound. This automatically limits the weight of the fuel that can be burnt, irrespective of the engine&amp;#039;s size. Any increase in power can be achieved only by improving the combustion process or reducing friction. It must be noted that 2-valve engines are granted slightly larger restrictors than 4-valve engines of the same capacity.           However, restrictors do not limit maximum torque, which is essentially related to the engine&amp;#039;s capacity and is always obtained at lower rpm than the power peak. This is an obvious advantage for a big engine, and to compensate for it, the regulations allow restrictors of a slightly larger cross section for small engines than for big ones. This compromise is more related to guessing than science, but has proved to be rather fair. In the case of turbocharged engines of both the prototype and GT classes, the size of the restrictor(s) \u2014 which must be installed upstream of the turbocharger(s) \u2014 is the same for all cars of the same class, their maximum power being equalized by allowing smaller-capacity engines a higher boost than the big ones.   There are two prototype classes, LMP1 and LMP2, differing only by their weight, tire sizes and engine specifications. The LMP1 prototypes must have a minimum weight of 925 kg (2040 lb.) and their restrictors are programmed to produce 620\u2013680 bhp. The LMP2 prototypes have a minimum weight of 775 kg (1709 lb.) and narrower tires, and their restrictors have been revised to keep power less than 480 bhp. (This followed the performance of the new Porsche RS Spyder that strongly challenged the LMP1 cars in last year&amp;#039;s American Le Mans series.)   As from last year, provision is made for turbocharged diesel engines up to 5.5 liters to run in the LMP1 class; these are granted larger restrictors than gasoline engines of the same capacity. But following the performance of the Audi R10 TDI 5.5-liter V-12 in 2006, the fuel tank capacity of diesel cars has been reduced from 90 to 80 liters. This should reduce the distance between refueling by one lap and, by my reckoning, make little difference to the overall results.   The current restrictor-based formula has attracted fewer major manufacturers to Le Mans than the previous fuel-consumption formula. This in turn has increased the interest for the two GT classes for which only homologated production models (suitably modified for racing) are eligible. In both the LM GT1 and LM GT2 classes, the maximum allowed engine capacity is 8.0 liters, and more running-gear modifications are allowed in GT1 than in GT2.           The air restrictors follow the same basic rule as in the prototype classes and their cross section has been reduced by 10 percent compared with 2006&amp;#039;s, reducing the maximum power by an equal amount. They are smaller in the GT2 class than in the GT1 class with target power outputs of around 430\u2013450 bhp and 540\u2013560 bhp, respectively. The size of the restrictors depends not only on the engine capacity and the number of valves, but also takes into account the weight of the car; this, in an attempt to equalize the power/weight ratio of the vehicles running in the same class, adding another &amp;quot;guess factor&amp;quot; to the formula.   The leveling of performance within the various classes has certainly produced close racing, but it does not necessarily reflect the real potential of the contenders. We will also have to wait until a potent and experienced manufacturer comes up with a state-of-the-art gasoline-engine LMP1 prototype to see if the regulations applying to turbodiesel-engine cars are too generous, as has been suggested by many. But the sanctioning ACO organization must be credited with having opened the access of its world-famous race to diesel-engine cars, which, up until now, have never had a chance to demonstrate their performance potential in a major event.   Whatever the situation, in the 2007 Vingt-quatre Heures du Mans, the focus will be on the battle between the Audi R10 TDI and the Peugeot 908 HDi, both powered by 5.5-liter V-12 turbodiesels, and that will be worth watching. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.roadandtrack.com/motorsports/news/a13112/le-mans-how-it-is-today/>