Le Mans Blog: Juggling Engine Performance

A broad range of engine options in LM P1

As is traditional in the prototype classes at the 24 Hours of Le Mans, the ACO (Automobile Club d’Ouest, the event organizer) allows a variety of different engine configurations even in the top prototype class, LM P1.

#16 Pescarolo Team Pescarolo Judd: Emmanuel Collard, Christophe Tinseau, Julien Jousse

Photo by: Rainier Ehrhardt

The all-powerful turbodiesel engines used by the Peugeot and Audi works teams are of 3.7L displacement with 3 bar of boost, compared to the 2010-spec semi-works Oreca Peugeot’s 5.5L and 2 bar specifications (down from 2.59 bar last year). Simple enough, certainly.

It’s with the petrol engines that things get complicated. In principle, there are only two options: normally aspirated 3.4L engine, and a 2.0L turbocharged (or supercharged) motor. Of those, the free-breathing engines get marginally larger intake restrictors, 43.3 mm vs the 42.9 mm of the turbo engines.

But 2010-spec cars are still legal this year, and that means not only the 5.0L Judd GV unit at the back of the No. 16 Pescarolo-Judd, but also the 6.0L GT1-homologated Aston Martin V12 powering the No. 22 Kronos Racing Lola-Aston Martin. The latter originally saw service in the Aston Martin DBR9 in LM GT1, but was then adapted for use in the Lola B09/60-Aston Martin (aka Aston Martin DBR1-2) in 2009.

The legacy engines are increasingly constrained by the restrictors, though: the purpose-built 5.0L Judd unit uses a 42.9 mm restrictor, smaller than the new 3.4L atmospheric engines. The production-base Aston Martin has an even-smaller 42.7 mm restrictor.

The end result of the restrictor rules is a reasonable parity in power output: the new 3.4L engines are rated by various manufacturers in the 475 hp range, and the restrictors have brought the larger engines into the same performance range as well.

The diesel-power cars, of course, are still well outside of the range. ACO regularly performs balancing between diesel and petrol engine performance, but the gap between the two groups is little changed since 2009.

“We have chosen to run with a six-cylinder turbocharged engine,” said George Howard-Chappell, Aston Martin team principal, at the time of the AMR-One launch in March 2011. “We believe this offers the best potential within the petrol engine regulations. With the ACO’s commitment to effectively balance the performance of petrol and diesel Le Mans entrants, our hopes are high that we’ll see the closest racing yet in the premier LMP1 category.”

However, in spite of Aston Martin’s best wishes, the hard reality is that the diesels effectively still do run in a class of their own. The slowest diesels are still some five seconds a lap faster than the top of the petrol-fueled cars, and their lower fuel consumption makes up for the reduction in fuel tank size. The only mitigating factor will be the smaller refueling nozzles for the diesels, and it remains to be seen how much those will slow down the Audi and Peugeot titans.

Within the unofficial “petrol division,” it has been the Pescarolo-Judd – with the 5.0L Judd GV engine – that has proved to be the most potent combination. In spite of the smaller restrictors, the team has clocked their proven combination in the 3:35 range, just five seconds slower than the times they set in qualifying in 2009 (the team did not run in 2010).

#12 Rebellion Racing Lola B 10/60 Coupe-Toyota: Nicolas Prost, Neel Jani, Jeroen Bleekemolen

Photo by: Rainier Ehrhardt

Rebellion Racing’s pair of Lola B10/60-Toyotas and Oak Racing’s Pescarolo-Judd DB were next on Wednesday’s qualifying timesheets, all with 2011-spec 3.4L engines. However, all three were well behind the Pescarolo-Judd GV, clocking times in the 3:37, 3:38 and 3:39 range, respectively.

The extra grunt of the larger Judd GV makes a big difference on a long circuit like Le Mans, and that’s reflected in the sector times. In the relatively tight first sector, the 3.4L-engined Lolas and Oak Pescarolo roughly kept pace with Team Pescarolo, losing only four to eight tenths, but it was a very different matter once the cars got onto the open track.

In the second sector, essentially consisting of the Mulsanne Straight, the gap was 1.2-2.0 seconds, and it was nearly as big in the final sector as well, with the cars returning through Arnage and Indianapolis to the finish line.

The key question for the race will be whether the 3.4L units will have sufficiently better fuel economy than the Judd GV of Team Pescarolo to allow them to stay with their rival throughout the night.