In 1973, for the second year in a row, Porsche dominated the Canadian-American Challenge Cup Series for sports cars with a sweep reminiscent of the great days of Team McLaren. Porsches won eight out of eight races while the works Roger Penske/Sunoco entry of Mark Donohue won six of them and was fastest qualifier by a considerable margin for all eight races. In the two races which Donohue didn't win, he was placed seventh in one after a shunt with a slower car and second after a leaking fuel filler cap forced a pit stop. Never before had any driver started and been running at the finish of every race of the season, a credit to the preparation of the Penske machine.
In fact, there were two Penske machines, matching 5.4-litre turbocharged 917/30 Porsches for Donohue, the latest long wheelbase, long body works model, developed by Porsche and Penske Racing from the previous all-conquering 917/10. Donohue's regular race car was subtly tweaked throughout the year while the spare remained in reserve and rarely left the transporter. The only time the spare car actually raced was at Watkins Glen when a hemispherical suspension joint failure caused Donohue to crash in practice and the spare car rolled off the transporter, secured the pole position within minutes and saved the weekend by winning the race in Penske/Donohue style, from start to finish.
Penske with his usual iron-clad deal had exclusivity for the 917/30 so that the best that Porsche customers had to race with was the 1972 model 917/10 but with the latest 5.4-litre engines which Donohue was using. In truth, the only opposition Donohue had throughout the year came from Porsche customers but at best they could slip ahead briefly or tag along behind as Donohue won race after race. Southerner Bobby Rinzler had purchased both 1972 Penske cars for his own Royal Crown Cola team to be driven by defending Can-Am Champion George Follmer and Charlie Kemp. For Rinzler it was for the most part a year of disappointment as his mechanics were not capable of handling the enormously complicated turbocharged cars and the drivers rarely got them to handle as well as they wanted them to. When everyone else fell out in the season opening race at Mosport in June, Kemp scored a lucky win after Donohue shunted with a slower car. In the second race at Road Atlanta in July, Donohue had his fuel filler cap problem and Follmer was an equally lucky winner. From then on it was Donohue the winner, although on two occasions, at Mid-Ohio and Laguna Seca, Follmer led Donohue for much of the race. At Mid-Ohio Donohue was sandbagging and took the lead in the closing laps while at Laguna Seca Donohue was starting from midfield due to a blown engine in the qualifying heat and when Follmer's engine blew Donohue led without opposition to the finish line. In addition to his Road Atlanta win, Follmer scored two seconds and a third but failed to finish the remaining four races while Mosport winner Kemp's next best finish was third.
California Porsche specialist Vasek Polak mounted an impressive private team. Polak is himself a master Porsche mechanic and he had a good crew of German mechanics so that the 917/10 for Jody Scheckter was always at or near the peak of its performance. The young South African was on his way to winning Mosport when tyre failure put him out, and in fact on several occasions later in the season he suffered from tyre problems. In the second race at Road Atlanta, Scheckter's 5-litre was no match for the 5.4-litres of Donohue and Follmer but he finished third.
He scored another third plus a second to Donohue at Road America but it was an unlucky season for, even with a 5.4-litre, things such as being rammed from behind, which damaged his clutch mechanism, spoiled every race after Road America.
Polak had a second Porsche, the old original 917 Spyder which Jo Siffert drove in the 1969 Can-Am. Actually a cutdown coupé (917-028), the second Polak car had been updated to 917/10 specification by some neat refabrication of the rear chassis. Steve Durst drove the car naturally aspirated in several early season races but Polak's piéce de résistance was to fit a turbocharged engine for Brian Redman in the last two races of the season. Redman was a strong qualifier and should have finished second at Laguna Seca but engine failure forced him out after being second in the qualifying heat. In the season-ender at Riverside, a recurring chassis failure in the remodelled car forced him out of the race, again losing a certain second place.
In his first season driving anything bigger than a Porsche 911S, Hurley Haywood revealed himself as a promising newcomer in his Bnimos Porsche 917/10. In fact a protege and partner of Porsche specialist Peter Gregg, Haywood was driving Gregg's old Can-Am car which with a 5-litre turbocharged engine he placed fifth once, third twice and second once to give him third in the Can-Am Championship standings with 47 points, behind Follmer with 62 and Donohue with 139.
Against the Porsches, the Chevrolet-powered opposition was very thin indeed. The only works team other than Penske's was Don Nichols' Universal Oil Products sponsored Shadow effort. With a new Tony Southgate design, Nichols had high hopes, starting the season with a naturally aspirated car while a second turbocharged Chevy powered car was being readied. In fact, the Shadows were too little too late for the naturally aspirated car first turned a wheel in the Mosport paddock the day before the first race while the turbocharged car did not appear until midseason in equally unsorted condition. Nevertheless, the naturally aspirated car became a strong runner and Oliver was placed third at Edmonton and second at Laguna Seca. Vic Elford drove the turbo-Shadow in the last two races of the season and reported that its 1,240 horsepower made it the strongest car he had ever driven and that in fact he thought the power characteristics better than the turbocharged Porsche which he had driven in the Interseries. Regrettably, Elford retired from both races, one with lack of brakes and the other with body damage due to hitting course markers. Elford's brief run in the Shadow however revealed what was potentially a Can-Am winner with the necessary pre-race development, a factor missing due to the requirements of Nichols' Formula One debut.
While Nichols' UOP Shadow crew have been turbocharging a big 8.1-litre version of the Chevrolet V8, Mike Slater's Commander Motor Homes team had been working on a smaller 7.1-litre higher-revving version for Mario Andretti to drive fitted to Commander's ex-works M20 McLaren. Like the Turbo Shadow, the Commander Turbo suffered from lack of sufficient development and only ran the last two races of the season. Surprisingly, Andretti ran third in the qualifying heat at Riverside but decided to bypass the final for he had another handicap, a lack of competitive tyres because he is a contracted Firestone driver and Firestone has left Can-Am to Goodyear for many years.
While the turbocharged Chevrolets had as much as 1,200 horsepower and the turbocharged Porsches as much as 1,100, the balance of the Can-Am entry with naturally aspirated Chevrolet V8s were hopelessly out of the picture with less than 750 horsepower. In this category was David Hobbs in Roy Woods' ex-works M20 McLaren-Chevrolet, called the Carling Black Label Special. Woods' plan was to run a steady race, hoping to finish well when the more powerful cars failed to finish. With Donohue finishing every race, needless to say, this plan didn't work although Hobbs did manage one second place at Watkins Glen. In fact, mechanical problems (mostly engine) spoiled Woods' "run to finish" plan or Hobbs might have scooped up more high finishes when the lesser Porsches failed.
One driver with a similar plan to Hobbs who made it work was veteran Bob Nagel with the ex-Jackie Stewart works T-260 Lola-Chevrolet. Running on a low budget, Nagel's 8.1-litre Lola scored points in six of the eight races with third at Mosport his highest finish. As a result he was fifth in the Championship points, having won over $30,000 in prize money which was more than the purchase price of his car.