XKR Jaguar pistons

Tony Gentilozzi is the sole engine builder for RSR Jaguar, running the combined GT class in American Le Mans Series (ALMS) competition. He's racing against other factory teams that have far more resources than he does and, in many instances, far more cars from which to learn.

The Jaguar XKR engine is a 5 litre of about 500 hp, using a pair of air restrictors (29.92 mm) that keep his useable revs to 7000 rpm.

Gentilozzi uses two different manufacturers' pistons as he zeroes in on the engine set-up that will work best in his two racecars. "We like the features of both manufacturers, and we're working on getting it all together with one piston maker," he says.

"We're pretty equal on wear for all eight pistons. We see the most wear in the top groove and work very hard on that. We run a hard anodised coating on, and special ring in, the top groove. There is no coating elsewhere on the engine."

Because the XKR Jaguar coupe and convertible use direct injection on the street, the race engine is using the same induction system. "With direct injection we run very lean, and that's why we're heating up the top ring on the groove; it's relatively substantial."

All pistons are of Gentilozzi's design. "We designed the piston and did some CFD work with Jaguar in the process, and dome development was done in CFD and using high-speed photography on a single-cylinder development engine.

"The most important part of the direct injection system is the dome shape of the piston, allowing the combustion event to work in conjunction with the injection event and allowing them to get the most efficient and most effective burn," he says.

"The spray pattern of the injectors and the dome shape of the piston - or lack thereof - is critical to direct injection. We run both concave and convex dome shapes. For the endurance races we go with a dish piston because it's necessary when running stratified injection, as it allows a very lean burn below 3500 rpm. For sprint races we run a dome piston."

He says rod length is unaffected by the different types of piston, and that the sprint piston dome does not change from port fuel injection to direct injection. ALMS contests last for as little as 2 hours 45 minutes to as long as 24 hours, for those invited to the 24 Heures du Mans.

For the Petit Le Mans 10-hour or 1000 km contest, Gentilozzi fits a dish piston and runs about 12.5:1 compression. "The point of the dish is to focus the charge as near as you can to the spark event; you want an efficient and effective burn of the charge."

The Jaguar XKR pistons go through two rebuilds or roughly 80 hours, as Gentilozzi stipulates rebuilds at 40 hours. "We do a fair amount of testing because we're still in development of both the engine and the car," he says.

The team switched to direct injection at the Mobil 1 12 Hours of Sebring held last March. Gentilozzi says, "We changed our piston spec to accept direct injection. We did our research and development in the off-season, and it's constantly evolving as we learn more. Skirt shape and design hasn't changed all that much; we run a relatively short skirt without coatings and only hard anodise the top ring groove."

Because the Jaguar engine is continuing to evolve, Gentilozzi has different piston combinations on his mind in this off-season. "We're thinking we may try a two-ring piston for endurance and friction reduction, as well as some different bore coatings. We've been using regular nickel silicon carbide coatings, and have stayed away from moly-based coatings because we're not certain about their longevity. We're not about to sacrifice on that end. With our air restrictors you're only going to use a certain amount of air no matter what you do - you're going to hit a wall there, and all you can do from there on is work on friction advantages."

Fig. 1 - Gentilozzi piston (Photo: Tony Gentilozzi)

Written by Anne Proffit