Can Am Series 1973


70's Racing

Can-Am 1973


Porsche dons McLaren mantle


PORSCHE ended the five-year McLaren domination of the Canadian American Challenge Cup Series of sports car races in America with, six victories in nine races during 1972. The vehicle was the Porsche 917/10, a turbocharged 5-litre derivative of the original endurance racing flat-12 917 coupe which so handsomely won the Championship of Makes for Porsche. The basic configuration of the 917/10 was similar to the original spaceframe 917 with slight dimension changes to accommodate larger tyres, fuel tanks and so forth. The long-tailed coupe body had been replaced by an open sports body while the original air-cooled flat-12 engine had increased its output from 600 to 900-plus horsepower.

However, there was far more to Porsche's Can-Am Championship than the racing car itself, for the winning potential of the 917/10 was only reached through the ideal partnership of Porsche KG and Roger Penske Racing. The Porsche contribution to the partnership is obvious but the Penske side consisted of driver and development engineer Mark Donohue and engineer Don Cox plus Penske's racing establishment in America.

Porsche had made two previous Can-Am sorties with the 917, both with Jo Siffert. In 1969, Siffert ran a cutdown Spyder version of the 917 coupe while in 1971 he ran the original 917/10 Spyder. Both cars were underpowered as their engines from 4.5- to SA-litres were just no match for the 7- and 8-litre Can-Am Chevrolets. Neverthe­less, Siffert was impressive by virtue of his personal style and the reliability of the basic 917 concept.

The Penske/Porsche project began in earnest 8 months before the first 1972 Can-Am. Donohue who became the focal point of the entire effort began track testing at Porsche's Weisach facility in Germany. In totalled days, he spent two of the next eight months in Germany. As weather and scheduling became a problem, a second 917/10 was shipped to America and Donohue did more testing at Road Atlanta, Georgia, one of the Can-Am tracks. All early testing was conducted using the basic SA-litre engine for at that time, the turbocharged engine was little more than a highly advanced dream.

Mark Donohue is his own severest critic. He sets his own personal standard of achievement at the very highest level for there is really only one acceptable result-victory. As the focal point of the Penske/ Porsche Can-Am effort, Donohue's personal standard became the standard for the joint effort. As engineer/driver he was the final judge and since he more than anyone else knew of what the McLarens were capable, when he was satisfied that the Porsche was a winner, they were ready to go racing.

At first there was a very critical period of adjustment while the Penske/Porsche people got to know each other, for in each other's eyes they were definitely unknown quantities other than by reputation. The crux of the problem was whether or not Donohue et al would be just another set of "preferred customers" who would race the factory equipment or whether they were important and valued equals who had a part to play in the development process.

I t was on the skid pad-an important first proving ground for the tough, analytical Porsche people as well as Donohue and ex-General Motors engineer Cox, that the marriage between Porsche and Penske Racing was consummated. They saw eye to eye on the importance of the skid pad. From then on, the Penske organisation was not just another bunch of "cut-and-try" racers but instead they were sane hard-nosed engineers worthy of respect.

With this gap bridged many things happened. Donohue suggested a larger rear wing than the Porsche staff felt necessary. But they tried it and found increased performance and from then on, ideas emerged, were tested, evaluated on their merits regardless of their origin and used or discarded accordingly.

An important point in time was the Riverside, California tests in January. Coming after successful record-breaking mid-winter tests at Road Atlanta, they were expected to support the indications that the 917/ I 0 could match the Can-Am dominating McLarens. However, in normally aspirated SA-litre form, the Porsche was a dismal disappoint­ment at Riverside. It was underpowered and at that point the success of the turbocharged engine programme became an absolute necessity on which the entire programme depended. Of the 9 Can-Am tracks, over half of them were pure and simple horsepower courses. The turbo motor had to work.

To begin with, engine development specialist Vallentine Schaeffer knew nothing about turbocharging but having personally developed every Porsche racing engine since 1964 he knew how to go about discovering its secrets and black arts. Starting from first principles he began a development programme for which few people will ever know the complexity of problems and frustrations. Jo Siffert had driven the first turbocharged flat-12 and found several seemingly insurmountable problems: throttle lag (of as much as 7 seconds) followed by enormous and unmanageable horsepower (at one point 1,088). In addition, the first efforts revealed that the engine would not idle and then once running when the driver lifted off for a turn, the engine ran on.

But Schaeffer was the man who carried on regardless. The delay in throttle response was reduced to the point that driver anticipation and correct gearing made it usable, the power was tamed, the run-on eliminated and the idle problem cured although the engine is still smoky while idling, a problem which will be cured next year. The feedback from Donohue to Schaffer tuned the engine to a usable form just as the feedback from Donohue to Cox and Porsche designer Helmut Flegel had tuned the suspension. In one instance, throttle re­sponse characteristics were improved by fitting another type of fuel injection pump which became known as the "happy pump" because once fitted, Donohue was happy.

The final turbocharger set-up featured two radial turbochargers feeding two long log manifolds which in turn fed the 12 cylinders. The secret of the development was a series of one-way valves which cut down the throttle lag and effected the responsiveness of the engine through its working range. Horsepower had been reduced to around 900-under for practice, tyre and suspension testing, over for qualify­ing and somewhere in betWeen for the race. Horsepower output was simply controlled by turning the turbocharger wastegate screw and for racing the first consideration was fuel consumption.

The turbocharged engine became the key to the Porsche Can-Am success for once it was running, everything else fell into place. With as much as 200 horsepower more than the Chevrolet powered 8.1-litre McLarens, Lolas and Shadows, engineer Flegel could design a high downforce body which would literally glue the car to the ground. Thus the big rear wing and snowplough front, with horsepower to burn, transformed the original sleek 917 into a Can-Am winner.

It was not all as easy as it sounds for to achieve the correct aero­dynamic and suspension balances still required days and days of skid pad and track testing but Schaeffer's turbocharged engine made it possible for Flegel, Donohue and Cox to tune the rest of the car to its highest level. Through the slow boring days of testing Flegel was often the one with the most patience to stick it out to find the ultimate combination.

Generally speaking, the Porsche was faster through slow and medium speed turns than its main competition the M20 McLaren as a result of its shorter wheelbase and the high downforce body. These benefits came at the expense of high speed handling where the McLaren excelled. With its abundance of power, the Porsche excelled in acceleration and top speed and in certain circumstances on braking as compared with the McLaren. Because of the greater thirst of the turbocharged engine, the Porsche went to the start line with more petrol and a resulting weight penalty but as the fuel load lightened this penalty disappeared and in fact it was only an apparent serious advantage on one occasion.

During the season the original turbo set-up was altered to use smaller turbochargers, giving better throttle response at the cost of less horsepower on two of the tight twisty tracks (Mid-Ohio and Laguna Seca). Late in the season, a medium size turboblower was adopted which replaced the original large one and gave almost as much horsepower but with greater throttle response.

Once the racing starting in June, it seemed only a matter of course that the Porsche would win. Donohue led the first race at Mosport from the pole position until a turbocharger valve stuck, forcing a pitstop to free it which dropped him to second at the finish. A minor fault had given Hulme in the latest Gulf McLaren the victory after team-mate Revson's engine broke its crankshaft with three laps to run.

Disaster struck the Porsche effort in private testing for the second race at Road Atlanta when Donohue crashed heavily, breaking his kneecap and putting him out of racing for some time. A spare chassis was hastily prepared and five days later George Follmer stood in for Donohue. Follmer became the Cinderella of the Can-Am for in the remaining eight events, he won five, started from the pole three times, set fastest lap five times-four of them lap records. He was unquestionably the best.

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Can-Am 73

Can-Am 74

Can-Am cars 67

Can-Am 67

Can-Am 68

Author: ArchitectPage