Le Mans Blog: Prototypes Not All Quite Equal

Story Highlights

  • ACO continues to equalize engine performance
  • Petrol engines still outclassed by diesel


Restrictors, boost pressure and fuel capacity balance performance


There may be 17 prototype cars in the LMP1 class, but some of them were definitely created more equal than others: the defending works Audi team with three cars, the three works Peugeot challengers, and the semi-works Team Oreca Peugeot all run turbo diesel engines, while the other 10 cars are powered by petrol (gasoline) engines.

#33 Level 5 Motorsports Lola Coupe-Honda Performance Development
#33 Level 5 Motorsports Lola Coupe-Honda Performance Development

Photo by: Eric Gilbert

The front-running diesel cars all have 3.7L turbocharged engines, in a V6 configuration for the Audis, V8 for the works Peugeots and V12 for the old-spec Oreca Peugeot. All produce somewhere in the neighbourhood of 550 hp, though well down from the near-700 hp their engines produced a few years ago before ACO (Automobile Club de l'Ouest, the event organizer) began to reduce engine output in earnest.

On the petrol side of the fence, though, the engine situation is much more varied. The current ACO regulations permit either a normally-aspirated 3.4L engine, or a turbocharged 2.0L unit, but a number of entries have been grandfathered in under previous years' regulations.

Hope Racing and the works Aston Martin team are both using the small turbocharged engines: Hope's is a hybrid-equipped four-cylinder unit producing 500 hp, while Aston Martin's AMR-One cars are equipped with the company's bespoke inline-six engines capable of 540 hp.

Meanwhile OAK Racing (Judd), Rebellion Racing (Toyota) and Quifel-ASM (Zytek) all have taken the normally-aspirated route, both having 3.4L units producing in the neighbourhood of 475 hp in race trim. Additionally, Kronos Racing (6.0L Aston Martin V12) and Pescarolo (5.0L Judd V8) have larger engines under the grandfathering rules.

However, it's not all about cubic inches as as the ACO continues to fine-tune the performance parameters and balance engine capabilities. ACO's primary weapons are the intake restrictor and the maximum boost pressure for turbocharged engines.

#12 Rebellion Racing Lola B 10/60 Coupe-Toyota
#12 Rebellion Racing Lola B 10/60 Coupe-Toyota

Photo by: Eric Gilbert

The number of restrictor options boggles the mind, but suffice it to say that the larger the engine displacement, the smaller the permitted restrictor will be. For example, for normally-aspirated engines with a single restrictor, the permitted size drops from 45.5 mm at a 2.4L engine size to 43.3 mm at 3.4L. The 2010-spec engines will use even smaller restrictors, a 42.9 mm for the Judd and 42.4 mm for the massive six-litre Aston Martin.

The end result is that the power outputs are remarkably well balanced, with even the older units within 20 percent of the leading cars' engine outputs, regardless of the displacement, turbocharging or the fuel type.

But that's not the entire story. The diesel engines, like their road-going cousins, produce enormous amounts of torque, giving the cars massive grunt coming out of low-speed corners, where the drivers of the normally-aspirated, petrol-engined cars are trying to keep the engines' revving near their relatively narrow torque peaks.

And then there is fuel consumption. While there is no limit on the total amount of fuel, every pit stop takes time and loses ground to a team's rivals. The fewer stops, the less time spent time in the pits, the more time spent on the track -- that's what one needs to win the 24H.

#5 Hope Racing Oreca Swiss Hy Tech-Hybrid
#5 Hope Racing Oreca Swiss Hy Tech-Hybrid

Photo by: Eric Gilbert

The petrol-engined cars can carry 65L of fuel on board, while the fuel tank capacity for the diesels has been brought down to 55L. Still, with the diesels' greater efficiency, at best this is a tie for the petrol brigade.

The end result is still a substantial gap in performance: in this year's first practice session, the seven diesel-powered cars were all covered by just over two seconds -- and then a 4.6-second gap to the best of the petrol-engined cars, the Pescarolo-Judd.

The petrol division, then, becomes a bit of a championship fight of its own, even if no trophies will be handed out to the winners. Tomorrow, we'll take a look at the title contenders in this fight.