Avoiding piston skirt wear

One series where engines see tumultuous life is the World of Outlaws winged sprint car campaign in the US, with 60 contests coast to coast that keep competitors far away from their engine builders - sometimes for a month or more.

So to help their customers in the field, Shaver Specialties Racing Engines (SSRE) of Torrance, California, chooses a robust Mahle Motorsports piston of 2618 alloy that's coated with phosphate; skirts are coated with Mahle Grafal.

Shaver's operation, supplying engines for four-time champion Donnie Schatz and his True Speed team owned by NASCAR Sprint Cup star Tony Stewart, has been using this particular spec - an off-the-shelf part - for about two years, with only a few minor changes. Company owner Ron Shaver had some design input on the piston, according to Dennis Hardesty, his third in command at the workshops.

The engines used in World of Outlaws run between 408 and 410 cu in and, from Shaver's shop, are Chevrolet-based. Horsepower is 850 and Shaver's engines produce more than 700 lb-ft of torque at 6300 rpm, with redline at about 8500 rpm.

"We dyno them to 8000 here," Hardesty says. "We see some wear around the skirt but not around the ring land area unless they've detonated them a lot. Every once in a while we will break a ring, but that is because of detonation."

Pistons used in Shaver's World of Outlaws engines have to be heavier than, say, a NASCAR Sprint Cup pot. "They have to handle a lot more torque, which means they also have to be heavier because of the alcohol (methanol) and because of detonation," Hardesty says. The piston has a small dome on it that helps with strength at the top.

The best drivers ride the rim of the racetrack - the cushion - but when they come across slower drivers, an accident or suffer a mishap of their own, they have to lift the throttle from this single-geared engine. "The engine detonates until it gets back on the cam curve," Hardesty explains. "So our piston has to take a lot more abuse than a NASCAR piston.

"They tend to be a bit heavier so that they can take it. Our Mahle piston weighs 475 grams."

Detonation is the demon for World of Outlaws sprint car pistons. To stop detonation and the resulting wear to the piston skirts, Shaver specifies a piston life of then to 12 races, or about 400 miles for the hard runners.

"We see some wear but it's at a minimum; what we see sometimes is wear to the piston skirt, so that is why we don't run these pistons longer than ten to 12 races," says Hardesty. "We use a box-style piston and skirt length, cam shape and ring land area specified by Mahle.

"These pistons rock at the top and bottom of the bore more than the pistons we used five to ten years ago. This helps the mass load, as the piston is changing direction at the top and bottom of the bore. This has given us excellent reliability with the 16:1 compression ratio the hard runners like to use."

To help customers keep track of piston wear while on the road, Shaver specifies Total Seal rings in the triple-ring land piston it supplies. "That way they can tell if there is anything wrong with the motor," says Hardesty. "We have customers out in the field and you've got to have a way to let them know if they've messed up with their tune-up, so they leak them down and if they're not leaking past the rings or past the valves they can run them again."

Never wanting to stand still, Shaver is experimenting with other manufacturers' pistons, and has used their test pistons in practice sessions as well as races, always trying to find ways to avoid the detonation problems that can ruin an engine. Still, it stays with Mahle for its primary choice because, as Shaver says, it's consistent.

"A motor that sees a lot of rpm needs to get its piston weight down; a motor that doesn't see as much RPM doesn't need to go to that edge," says Shaver. "We need to get as close to that edge as we can."

Fig. 1 - The only type of wear Shaver Specialties Racing Engines sees on its World of Outlaws is the occasional wear in the pin boss area (Photo: Anne Proffit)

Written by Anne Proffit