RACING CARS FROM THE SEVENTIES

PORSCHE 917 STEEL (1978) IN USA

GERMAN

Any builder of NASCAR stockers would probably be offended by the Porsche 935, mostly because a street car is still recognizable beneath the racer's swoopy bodywork. It's not a mistake on Porsche's part either. When the firm initiated its turbo 911 racing project in 1974, the tubeframed funny car that would have seemed logical to Banjo Mathews was deliberately rejected. Porsche has always sought to emphasize the link between its street cars and racing cars. So though the connection between the 935 and the Turbo is tenuous, there's no denying that the chassis of a genuine street car lies beneath this IMSA Camel GT competitor and FIA Group 5 champion.

For Porsche, the magic that transforms a production sports car into a flame-belching race car was clear. Porsche has a far more than usual interest in factory racing, and that has undoubtedly had much to do with its sports-car success. Besides, there was a fair amount of racing bits left over from the 917 cars that raced at Le Mans and in the Can-Am, so the racing engineers simply jacked up the Turbo chassis and slid a lot of 917 hardware underneath it. Jim Busby, IMSA Porsche driver and the man who prepared the cars seen here, calls the 935 a "steel 917."

There are a lot of details that distinguish the 935 from the street Turbo, but the one that confuses many Porsche zealots is the fact that there are no torsion bars in the race car. Instead, the rear suspension pickup points are changed and the use of a larger rear crossmember permits the use of coil-over shock absorbers. In front, tubular control arms are attached to coil-over shocks. And just as in any open-wheeled race car, rod ends take the place of ball joints and rubber bushings. This is one sports car that can offer a suspension engineer as much diversion as an Indy car.

But despite these changes in the 935's suspension, Busby reports that the rear trailing arms still deliver too much camber change and toe-steer. As a result, Busby specifies a standard 935 suspension setup with a 380 lbs/in spring rate in front coupled to a 14mm anti-sway bar, and 580 lbs/in spring rates with a 26mm anti-sway bar in the rear. "We've found that the stiffer we go short of wheel hop, the better the car is."                                 .

Weight is the enemy in any race car, so naturally everything possible has been done to shed nasty fat from the 935. The first racing Turbos that appeared were essentially highly-tuned street cars, complete with federal bumpers and electric windows. But thanks to Porsche's clever work with fiberglass, plexiglass and titanium, the 935 weighs 2250 pounds. For example, the first 934 doors weighed 80 pounds. The 935's doors weigh fifteen pounds. Careful detailing takes care of the rest of the fat, like the substitution of titanium springs for steel springs. Steel springs cost $110 apiece. The titanium items cost $780, but at least they don't rust.

There is an unpleasant consequence to this radical lightening project, however, because the logical place to trim weight is the already-light front of the car, away from the engine and its appliances. So despite the fact that fuel and oil tanks are located in the front bay, Busby describes the weight distribution as "about twenty percent in front and 80 percent in the rear." That's hardly the optimum for predictable handling, but the aerodynamic bodywork and huge 29.5xI4.0-19 rear racing slicks compensate somewhat. Even so, when the first 1974 racing turbo lost its front chin spoiler, the car would literally do wheelies while accelerating away from the corners.

The bottom line of this madness is of course the flat-six engine. Jim Busby's specially-modified 3.0-liter, 730-hp engine has about as much resemblence to the stock motor as you might expect. At least the engine cases are the same. The compression ratio is 6.8-to-one and boost level is controllable from the cockpit. The boost is usually set to 22.7 psi for qualifying and 21.3 psi for sprint races. The 8000 rpm revlimit must be strictly observed. The driver keeps track of the boost level with the aid of a series of electronically-controlled indicator lights-no- gauge can keep up with the KKK turbocharger. The lights go red when 22 psi is reached. Much of the horse­power increase over a stock motor can be traced to modifications like cylinder head design, twin spark plug ignition and the substitution of Bosch plunger-type fuel injection for the K-Jetronic. Operating temperatures are critical, so the 935's engine has a horizontal fan over the engine like a 917, the oil cooler is 40 percent larger in capacity and the turbocharger intercooler radiators located in the ducts forward of the rear wheel wells are charged with water. In fact, that 934 turbo was the first Porsche to ever carry water. All the horsepower is transmitted to the ground through a standard transmission with stronger synchro rings and a titanium spool developed by Mark Donohue for the Can-Am 917s that replaces the expected limited-slip differential.

If you were to slide into the driver's seat of the Porsche modified by Jim Busby Racing to 935 specifications as pictured here, it would not be as disconcerting as you might expect. The dashboard resembles a street car, but there are orange, red, yellow and green warning lights as well,

simply because a driver hasn't much time to peer at gauges at 160 mph. The lights are for low fuel pressure, fan failure, oil pressure failure and generator failure, just like some sort of funny Chevrolet.

Though the gearbox internals are sur­prisingly similar to the street machine, the linkage is taken direct from the 917. The four-speed box has about two inches of travel at the shifter. As Busby says, "It's very important to disengage the clutch fully each time you shift because the synchros are so sensitive. Double-clutching is of no value. It's complicated and you can actually do the engine more harm with a missed shift. And yet it's important to shift quickly or you'll loose turbocharger boost and your lap times will fall off."

If Busby were orienting you for your first solo in the 935, he'd begin, by telling you that the racer has an extreme rear weight bias, just like a 911, and it will oversteer, especially on trailing throttle. "If it begins to go out on you," Busby says, "just let it scrub, stay on the gas and add more steering lock. Drive it out of a drift, because unlike a Monza it won't just snap itself straight if you lift off the gas, it'll back into the fence."

It's crucial to get the tires warmed up to drive this car fast, Busby explains. "This car has a rear weight bias, a locked differential and a turbocharger all working against a good driver. So it will understeer to beat tlie devil when the tires are cold. As the front tires get warm, it oversteers.

When the rear tires finally get warm, the, car gets more neutral. But if you were to complain about bad understeer after twenty or 30 laps, all things being equal, I would say that you weren't getting on the throttle hard enough and early enough in the corners. Porsches just like to go around corners with their tails out."

Ironically, the steel 917 doesn't win its races because it can out-corner other race cars. Busby admits that even a 935 can't stay with Al Holbert's IMSA Monza in the corners. But because of its rear weight bias, the Porsche can brake harder and more effectively. And when the car is pointing straight at the corner's exit, Busby can let the turbocharger do its job because the rear weight bias aids traction. Apparently, when you drive a Porsche at racing speeds it's important to get the boost properly coordinated in the turns. But the car will outdrag anything on the straights and it can brake deeper than other cars in the turns.

Strangely enough, making a race car out of the Porsche street Turbo isn't a case of simply smoothing out its idiosynchracies. The car has too much power for that. Instead racers like Busby live with the car's faults and rely on its strengths to win races-speed and braking. So in that respect the 935 is just like the street car, only more so. 

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Author: ArchitectPage