Setting Loose the Horses on Dyno Day

by Edwin Fotheringham Edwin Fotheringham

I need to find out, once and for all, how big it is. I need to see if I measure up to the other guys, if I have what it takes. And to do that, I’ll have to hang it out in public, let them size it up, in a very open display of the one quantity most important to the American car guy. Horsepower.

DeMan MotorSport, one of New York’s best Porsche racing shops, is a tiny facility behind a high chain-link fence on a side street in Nyack, a Hudson River town best known as the home of Rosie O’Donnell and lots of cutesy antiques stores. Not much bigger than a tennis court, DeMan MotorSport on a hot Saturday morning is jammed with 30 Porsches waiting for maintenance or modifications, race prep or resale—Porsches of every sort, from pleasant little 2-liter narrow-body early 911s to battle-scarred, decaled, waddle-ass track cars.

And today is Dyno Day. The deal is that 10 of us, all electronic acquaintances from an online Porsche bulletin board (pelicanparts.com), have put up $100 apiece to hire shop owner Rick DeMan’s dynamometer, a machine that will plumb the depths of our various Porsches’ horsepressures.

A dynamometer is not something you’ll come across down at the Toyota dealership or at the local we-fix-flats garage. It’s an expensive device that applies a load to a car’s drivetrain (in the case of a chassis dynamometer) or to a naked engine in a test cell (in the case of, yes, an engine dynamometer) in a way that allows the load to be measured and converted to a horsepower rating.

The classic chassis dyno is a big rotating steel drum, like a huge barrel lying on its side, located in a deep pit. The car is strapped down atop the drum so that the tires can turn it; the amount of time it takes the car to overcome the drum’s inertia and accelerate it to a variety of speeds can be converted to a horsepower curve. It works reasonably well but isn’t particularly good at sensing small variations in engine performance (the inertia of the big drum smoothes out the power curve), and it offers too many opportunities for variables, such as how tightly the car is tied down to the dyno.

Not to mention that if the car isn’t snugged down tightly enough, it can slip off the dyno, usually with the tires spinning at a simulated 150. This can launch a ’Vette or a Viper through mechanics, bystanders, shop walls and perhaps the Wal-Mart next door.

DeMan’s dynamometer, however, is entirely different–a compact, portable, made-in-New Zealand, computerized rig called a Dynapack. It sells for $56,000, and even more for the 4-wheel-drive version. "For $100 each, you guys are getting a bargain," DeMan says with a laugh.

Two boxes, each the size of a dorm-room mini-fridge, straddle the car, which has had its drive wheels removed so that splined extensions can be bolted to the hub studs. The splines mate smoothly with receptacles in each of the boxes, called power-absorption units, and the car sits with its rear end suspended in midair, held aloft by the two PAUs.

When the car is accelerated—and we’re talking foot feed to the floorboard, not some wussy EPA city-driving cycle—the extension shafts spin hydraulic pumps. The engine applies a maximal load to the pair of pumps, and by measuring the hydraulic pressure versus rpm, the computer graphs the amount of work being performed per unit time. Which is another way of saying horsepower.