Keeping piston skirts happy in a small, heavily boosted engine
In Time Attack, a popular form of road racing in the US, the objective is not to compete wheel-to-wheel but rather to set down the fastest lap possible. According to Rob Cardona, manager of WORLD Racing, Toyota's Scion brand official team, "It has a bit of road racing with a bit of a drag-racing element to it, going for a time slip without racing each other."
While cars in any particular class will be on the track at the same time, the organisers try to pace each car so that the lead racecar never catches traffic and the second-placed car never hooks up with the first. "There's no traffic involved, and that way everyone gets a clear and clean shot at one lap," Cardona told me.
Engine builder Gary Kubo preps the engines for driver Chris Rado, who races a Scion tC that sports a 2AZFE 2.4 litre all-aluminium engine. "Right now we're running about 1000 horsepower and maybe a little bit more when we compete; we can run up to 50 pounds of boost, depending on the atmosphere and the track, of course," he said.
The team has been working closely with JE Pistons over the past 10 years to get the right package together for their drag and Time Attack engines. When they began racing in Time Attack, Kubo said, "Their off-the-shelf parts are so well developed we've actually, for the first two seasons, run their off-the-shelf pistons.
"We've made some changes because we try to upgrade the car every year, little by little. We try to get more power and more response, so we go back to the data to see what we can do to change that engine response," Kubo related.
He said the changes made annually to piston spec come in skirt design. "You don't want to run the skirt so loose that you obviously have a reliability or oiling problem, yet you want to run them tight enough so you have the ring seal. When you run as much boost as we do - and, mind you, this is a factory block - we're making 300 horsepower per cylinder and the pistons need to stay round. so the skirt design is critical."
WORLD Racing runs a quenched chamber, so it specifies a dome/dish configuration that matches the chamber. "We try to digitise the chamber and reverse-duplicate that same configuration on the piston side. I would say the biggest wear problems we're having is in the skirt because we're running methanol," Kubo said.
"We don't try to shoot for max horsepower on the dyno because it never reflects the real world. We're using 2500 cc injectors multiplied by eight so, for our engine to consume as much fuel as it does, it has to be well over 1200-1300 horsepower because the fuel consumption is so high."
Still, the service life of the JE pistons is about 30 hours, but it would depend on how many races the team runs. According to Kubo, "We've had engines that last all season, a minimum of 12 races plus testing and dynamometer work. We try to change the spec only once a year, coming back each year with a bigger bang.
"WORLD Racing does get a bit of wear on the top-ring group because we run our lateral gas port set-up hard, so there's obviously combustion pressure pushing the ring against the wall and there's some axial movement - not much, we still hold the tolerances. Even if the ring groove opened up maybe half a thou, with a boosted engine that's a minimal difference," Kubo said.
"It's not like there's a book on what we're doing - we're making our own data on this engine."
Fig. 1 - WORLD Racing pays great attention to skirt details on its JE pistons (Photos: Anne Proffit)
Fig. 2 - WORLD Racing uses a quenched dome
Fig. 3 - The team reports some wear on the upper ringland
Written by Anne Proffit