THE TURBO RSR AND THE 935 FOR GROUP 5 Realizing that even 330 horsepower was not enough to win outright, Hans Mezger and Valentin Schäffer turned their attention to coaxing much more power from the 911 engines. This was especially important since 1975 FIA regulations dictated that only cars derived from production models could earn manufacturers championship points. The FIA set engine displacement limits at three liters, and it instituted a multiplier factor to take account for additional power that a fully developed turbocharging system produced. Using this 1.4 factor, Mezger and Schäffer developed a 2,142cc engine, establishing cylinder bore at 83 millimeters and stroke at 66 millimeters. They ran early experiments relying on the standard upright cooling fan. However, they concluded that mounting a fan flat on top of the cylinders cooled better. A single KKK turbocharger, similar to the unit driving Can-Am and InterSerie 917 models, forcefed the air/fuel mixture. With an induction air cooler, Schäffer cooled the mix by 100–150 degrees Fahrenheit. By the end of the 1974 season, they had 500 reliable horsepower at 7,600 rpm. To handle this substantial increase in a car that he had lightened to 1,764 pounds, Singer had his engineers widen the body from 63.4 inches to 78.7, and they fitted the largest wing anyone had yet seen on an automobile. Ernst Fuhrmann was so embarrassed by its size he ordered it painted flat black to make it less obvious. It was legal, just vast. Beneath the wing and inside the flared body, Porsche used 10.5-inch-wide front wheels and, depending on race circuit, rear wheels measuring between 15 and 17 inches deep. Standard 911 torsion bars no longer worked at front and Singer’s team instead fitted lower wishbones and Bilstein dampers with concentric steel coil springs. The rear suspension used newly fabricated aluminum-sheet triangles with similar Bilstein coil-over dampers. To minimize the effect of the weight change in the new 31.7-gallon fuel tank during a race, Singer’s group set it inside the car at the driver’s right shoulder. Porsche was ready for the 1975 Group 5 season. None of the other competitors were, and the FIA postponed those regulations until the 1976 season to encourage others to join the grid. With time on their hands, racing engineers lent assistance to the road car 930 launch and provided support to privateers who raced normally aspirated RSRs or to those who, having watched Singer’s success with their turbo car, asked for help in creating their own. No homologated Group 5 cars entered in the Daytona 24-hour race. The FIA canceled its international championship status and IMSA’s own rules simplified things to Grand Touring or Grand Touring-Under classes, that is, GT and GTU for cars with less than 2.5 liters displacement. Brumos, with Gregg and Haywood sharing driving duties, took the checkered flag again but had no such luck the next month at Sebring, quitting after 50 laps following a too-close encounter with a Corvette. A BMW “Batmobile,” one of the winged-and-widened 3.0 SCL models, won, while George Dyer, who had acquired the previous year’s Brumos car, settled for second with co-driver Jacques Bienvenue. Sports racers made a stand at Mugello, the first of the FIA qualifying rounds. No less than three of the sleek, low spiders on the grid were 908/3 Porsches with recently turbocharged engines. An Alpine-Renault won overall, chased by Alfa Romeos and a couple of the turbo 908s, leaving the GT win to a Georg Loos RSR in ninth overall. Loos’ RSR carried the day at Dijon, France, two weeks later taking GT honors and fifth overall, although neither of Georg’s RSR’s finished on the podium at Monza. Neither did any other Porsche product. But that was a one-race fluke and GT wins went to several RSR teams through the end of the season to give Porsche the three-liter GT championship for another year. Throughout the 1975 IMSA season, Peter Gregg and Brumos scored enough well-placed finishes to claim that season title once again. The SCCA experimented with new rules for 1975, and Corvette took season laurels, winning six of seven races. That changed for 1976 as the Trans-Am fell in line with FIA’s Group 5, and the SCCA invited the Turbo Porsches into their new Class 1. Normally aspirated production-based sport cars comprised Class 2.

Porsche made good use of the time it gained in 1975. Weissach developed its 935 for Group 5 and the 934 for Group 4 in relative leisure. Rules for Group 5, officially designated Special Production, left room for interpretation, a situation that favored Norbert Singer’s engineering style especially. Body engineer Eugen Kolb designed and fabricated for him a single piece of thin fiberglass incorporating the two front fenders and the front air dam. As one piece, its removal provided rapid access to the entire front end of the car during a race. FIA rules also said that, so long as the original rear of the body remained unaltered, it allowed longer tails for improved aerodynamics. In typical Weissach race-shops style, Singer’s staff pared weight of the car down to 1,984 pounds in anticipation of final regulations due January 1, 1976. When those appeared, linking minimum weight to engine displacement (with the turbo multiplier), Singer’s extreme diligence left them 154 pounds light! This allowed his engineers to position ballast where it established final weight balance at 53 percent rear, 47 percent front. The Turbo 930 production car rear torsion bar suspension went away to become titanium coil springs with an anti-sway bar adjustable from inside the car at speed. Titanium coils replaced the front torsion bars, but without the adjustable sway bar. Ventilated and cross-drilled rotors and calipers came from the 917, operating from twin master cylinders providing adjustable brake bias and front/rear brake pedal pressure. Standard wheels were 11x16 at the front and 14.5x19 at the rear. Hans Mezger and Valentin Schäffer, using the 1.4 multiplier and a four-liter maximum displacement, devised a 2,856cc unit that cranked out 590 horsepower at 7,900 rpm with turbo boost at 19 psi. Schäffer employed an air-to-air intercooler to drop temperatures and increase fuel density. According to Jürgen Barth in his The Porsche Book trilogy, their innovations returned 4.4 miles to the U.S. gallon, important performance for Le Mans that required 20 of its 8.476-mile laps between refuel stops. Singer’s staff had completed a single car for the 1976 season, and they did not expect miracles. As they did with the Turbo RSR, they competed to learn.

While Group 5 offered manufacturers leeway and creativity, Group 4 adhered to their production origins. There was no question this Typ 934 was a capable race car. A gutted interior, all black, with a rigidly mounted roll cage eliminated any doubts. However, Weissach fitted plastic wheel extensions instead of a one-piece plastic fender/air dam structure to accommodate the wider wheels and tires. Rules required stock window glass and the weight minimum was 2,470 pounds compared to 2,138 for the 935. Underneath the 934, Barth explained that “modifications were limited to adapting the suspension to the requirements of racing by a more-precise location of the moving parts, stiffer springing, different damping characteristics, the use of some reinforced parts, and racing brakes.” Knowing these were requirements for this racer enabled Weissach engineers to modify the production 930 to match during the year the FIA delayed the rules. The 934 ran on a turbocharged three-liter, and Mezger and Schäffer substantially modified the Bosch K-Jetronic injection system to accommodate the air-intake demands of racing compared to road touring. Because 934 bodies had to follow production lines, tricks that Singer and Kolb used improving aerodynamics and providing extra room for the 935’s massive intercooler were impossible. Engineers adopted a water-cooled system instead, requiring radiators in front, plumbing, and pumps to move the water around. Engineers calculated the weight addition at 44 pounds, not so great a problem with the high minimum requirement that even let the 934s retain the electric window lifts of the production 930s. The intercooler system reduced fuel temperature from roughly 300 degrees to 120 degrees Fahrenheit, dramatically improving charge density. The 2,993cc engine, with its 1.4 factor, displaced 4,200cc and developed 485 horsepower at 7,000 rpm. Privateers soon learned techniques to develop as much as 580 horsepower.

When the 1976 season started, only Porsche had a true 935 Group 5 car. And it had only one. The FIA had homologated it (and fortunately it required no minimum production in the Special Production category). The FIA also legalized the 934 in Group 4. In an effort to flesh out the starting fields, Singer’s competition group developed a kit to upgrade 934s into a 935. Georg Loos, Egon Evertz, and others competed through the racing season with these cars, known as the 934/5 or 934.5. In the United States, the shoving match between SCCA and IMSA challenged Porsche and car owners alike. SCCA’s Trans-Am series accepted the 934, as built, for the 1976 season. When IMSA opened its doors to the 934, it allowed owners to run the cars with the 935 conversion kits and Weissach responded with an IMSA package that former 934/935 racer Bruce Canepa called the 934 1/2. This meant that teams such as Brumos and others contesting both series needed two cars with two sets of spares. The year 1976 remains an odd one in racing history. In its regulatory zeal, the FIA eliminated the world’s three main endurance events from its points challenge. Daytona, Sebring, and Le Mans no longer counted. Whether it caused the FIA to shun the events or came as a result, the organizers of Daytona and Le Mans rallied round each other and created race weekends that celebrated not so much racing histories as political ones.

The Automobile Club de l’Ouest (ACO), which organized Le Mans, reached out to fellow 24- hours organizers IMSA and Daytona host Bill France. They extended invitations to pairs of cars from NASCAR and from SCCA to race in honor of the U.S. bicentennial. In a similar spirit of inclusiveness, IMSA and Daytona invited eight NASCAR veterans to run their cars among the Ferraris and Porsches for 24 hours. Daytona also hosted the inauguration of this AAGT category, All-American Grand Touring cars. This was a class meant to encourage Ford, Chevrolet, and others to put their cars on the track, and rules made generous allowances for modification. In hopes of enticing BMW to expand its competition efforts in the States, IMSA banned the Carrera Turbo RSR and allowed only two of the new Turbo 935s. These constitued the entire Group 5 field. Daytona proved to be another endurance race ripped from the pages of a novel. Peter Gregg, longtime Porsche loyalist, started from the pole. However, he had deserted Porsche for a BMWentered 3.5 CSL, sharing driving responsibilites with Brian Redman of 917 fame and John Fitzpatrick, who earned dozens of wins in Loos Carrera RSRs. The best starting Porsche, in fifth on the grid, belonged to Al Holbert and it was not one of the Group 5 cars, but his own normally apsirated RSR. The fastest Group 5 Porsche was in fact Egon Evertz and Sepp Gregor in Gregor’s 934. It started 11th, though that was two spots ahead of the best NASCAR entry, a Chevy Nova that father and son Bobbie and Donnie Allison ran. Ford race car builders Holman & Moody entered a Ford Torino. NASCAR legend David Pearson qualified it 17th on the grid. There were Mazda Cosmos and AMC Gremlins and a Honda Civic among the starters. Twenty-four hours is a long time. Ask any endurance racer and most will say they hate the long races. No sleep. Driving in the dark. It’s what they get paid for. It still is hard. It also seems that in the dark things break. Or human bodies fail. Peter Gregg, normally invincible, suffered gastric distress and he handed off the CSL to Redman, who just had finished a stint. Fitzpatrick stepped in, normally a co-driver on the secondplace CSL. At that moment his co-driver, Tom Walkinshaw, was off the track, needing to perform repairs on their car by himself in order to stay legal. Then at dawn, with Redman back at the wheel, their BMW nearly stopped on the track. A moment later Al Holbert’s RSR lost power as well. They pitted. Another half dozen cars, front markers and back fielders alike, slowed. Gregg’s longtime chief mechanic Jack Atkinson caught it first. He saw water in the fuel lines. “The gas truck had been through the pits,” Atkinson explained in an interview in 2012. “It started at the end near where we were set up, resupplying the feed tanks in every pit as it went till it ran out. It didn’t take long to realize what had happened. The track supplier had picked up a tank load of contaminated fuel. We all let race management know.” At 10:10 a.m., IMSA president John Bishop halted the race.

“The gas truck had been through the pits. It started at the end near where we were set up, resupplying the feed tanks in every pit as it went till it ran out. It didn’t take long to realize what had happened. The track supplier had picked up a tank load of contaminated fuel. We all let race management know.” — Jack Atkinson

Bishop allowed teams that had gotten fuel to drain the tanks in their cars and their supply reservoirs. It took four hours until fresh fuel reached the pits, during which time every team checked their supplies, cleaned tanks, and blew compressed air through injectors and fuel lines. Historian Janos Wimpffen, in his calm, understated way, summarized what started as frustration, became chaos, and finally evolved into an unexpected respite. “The whole situation was most unprecedented,” Wimpffen wrote, “and charges and countercharges about bribery, sabotage, and other crimes against racing were bandied about. Most distressed was Hurley Haywood, who had the only leading car not affected, as his section of the pits had not yet been replenished by that particular delivery truck. His anguish only became worse when Bishop ruled that the race would be restarted based on the positions at 9 a.m., when the problem seemed to first occur.” Haywood remembered it: “You asked about my best day in racing? That was the win at Daytona, with Peter in 1973. I didn’t even understand how important it was, but I knew what it meant to me. The worst day? Daytona 1976.” Bishop’s restart meant that all that Haywood in the Brumos Carrera and Holbert in his RSR had accomplished during those 70 minutes of racing against Gregg and Redman in their BMW before Bishop stopped the clock counted for nothing. That’s racing, as many will say, and it was exactly that on that February 1, 1976, when the 24 Hours of Daytona finished on time. But it ended four hours short, racing only 20 hours, 6 minutes, 54 seconds. Brian Redman had driven 14 of those 20 hours to win; Al Holbert and Claude Ballot-Lena came second in an RSR, 14 laps down and one lap behind the winners. Haywood and friend and co-driver Jim Busby finished third in the Brumos Carrera. Sebring saw Porsche earn a sweet revenge as Al Holbert and Michael Keyser started from third on the grid and finished first in a Holbert-owned and -entered RSR. The racing world had to wait until Mugello later in March to see the real debut of the Group 5 Porsche, however. There, Jochen Mass and Jacky Ickx qualified on the pole just ahead of a Group 5 BMW CSL. Six hours later, Ickx and Mass finished five laps ahead of the ailing BMW. But it falls to Wimpffen again to best describe what Group 5 really represented. “The most memorable aspect of the 935 era,” he wrote in his essential two-volume reference, Time and Two Seats, “indeed that of the whole turbocharging epoch, was the sight of the flame-out on the overrun coming into the corners. This was the first time a ‘Silhouette’ car produced such an awe-inspiring act. It was another illustration of how the Group 5 era was to road going sports cars as NASCAR was to the American family sedan. Somewhere in the observers’ imagination is that fantasy that their own car might be capable of the same thing.” Group 4/Grand Touring went to Leo Kinnunen and Egon Evertz in his 934, in third overall. The 1976 season was off to a good start. Ickx and Mass and the company 935 repeated at Vallelunga. Manfred Schurti and Rolf Stommelen drove a factory 935 to fourth overall and Group 5 victory at Le Mans, behind Ickx and van Lennep in the hastily executed 936 Spyder for Group 6. The wholesome welcome of NASCAR to the Daytona 24 hours extended to Le Mans, and French racing fans howled as John Greenwood’s monstrous Corvettes chased Michael Keyser’s Chevrolet Monza, bedecked with wings and spoilers larger than the car body itself. NASCAR legends Hershell and Doug McGriff and their Dodge Charger suffered from fuel-octane shortcomings, while Richard Brooks and Dick Hutcherson’s Ford Torino, equipped with long-distance driving lights for its first adventure in the dark, lasted until well after dark when its transmission failed. Not one of the American cars finished, but it was not for lack of support from a rapturous and vocal French crowd. Porsche’s next big win came at Watkins Glen, when Schurti and Stommelen were first to the checkered flag. Ickx and Jochen Mass were first across the finish line at Dijon in late August. What race organizers concluded at the end of 1976 was that Group 5 was about to be a single-marque series. It was not what organizers had hoped, but spectators seemed okay with what appeared to be a match race between differently painted Porsches. “That’s racing,” many spectators, officials, and entrants repeated then, and they repeat it to this day. Competition was close; the cars bumped, bashed, and even crashed each other out. And for about five years, Porsche owned racing. In 1977, Porsche introduced twin-turbo versions of the 935 and its support of private team racing grew. Weissach prepared three of these 935/77 models. It proved essential as worksentry 935s won four of the nine championship points races while five privateers, Dave Helmick at Daytona, Georg Loos at Nurburgring, Ludwig Heimrath at Mosport in Canada, the Kremer brothers at Hockenheim, and Scuderia Vesuvia in Italy at Vallelunga added the other necessary points to the totals. It read like a movie writer’s plot plan for the next several years. As Wimpffen wrote, “the terms ‘silhouette’ and 935 are nearly synonymous.” This was truer than the words suggest; regulations allowed aerodynamic enhancements so long as the original silhouette remained unchanged. Singer’s solution shrouded the original body with an aerodynamic shell that mounted a second rear window outside the first. Throughout 1977 and 1978, Weissach sold 935/77 models to a variety of customers.