GIVING IT BOOST
Karl Ludvigsen continues our 917 theme telling of the turbo Can-am motors
“THE PRINCIPLE IS SIMPLE, PUTTING IT TO WORK IS NOT…”
The problem was simple enough: Porsche wanted to enter the prestigious Can-am series in 1972 with its Type 917 but it didn’t have enough power. With their modified Chevrolet V8 engines as big as 8.2 litres the dominant Mclaren team had access to 730 horsepower and 600lb ft of torque. Even when others had similar power they couldn’t match the Mclarens. What was the answer?
The 917’s flat-12 engine was no slouch in power production. Porsche enlarged it to 5.4 litres, from which it generated 660bhp at 8300 rpm and 470lb ft of torque at 6400 rpm. Good, but only for also-rans. For Ferdinand Piëch, Hans Mezger built a flat-16 version of the 917’s Type 912 engine that produced 755bhp and 542lb ft of torque. In the car, however, it was a disaster. Engineering director Helmuth Bott decided: Porsche would build a turbo-supercharged engine for the Can-am series.
Porsche’s engineers understood supercharging. What was not certain, however, was whether it would produce controllable power without excessive fuel consumption and without turning the air-cooled Type 912 engine into a bomb with a short fuse.
In most classes of auto racing supercharging in any form was either prohibited or penalised. Can-am racing had no such restriction. Apart from a ban on gas turbines, Can-am cars could have any sort or size of engine that could be packed aboard. Such rules were an engraved invitation to those who knew how to use supercharging – or thought they did.
Supercharged entries came to the starting line in the first Canam season, 1966. Among the Lolas fielded by the John Mecom team were some with Ford power and belt-driven Paxton centrifugal compressors. Jackie Stewart drove such a car in several of the West Coast races with no special success, after which the idea dropped from sight.
Boosting returned to the Can-am in 1969 in the form of special
Oldsmobile V8 engines with twin turbo-superchargers installed in two chassis built by Bob Mckee of Chicago. One had a tubular frame and the other had a monocoque structure. Funds were insufficient even to race the Mckee creations, let alone develop them properly.
In Europe, in the meantime, the art of exhaust-driven turbosupercharging was being advanced by Swiss engineer Michael May. He liked the way a turbocharger used otherwise wasted exhaust gas energy to increase power. In such devices the engine’s exhaust gas is passed through a small turbine wheel which it sets spinning. A shaft from the turbine wheel turns an adjacent impeller – a small centrifugal blower that can run at very high speed, to 100,000 rpm and more – to pump more air into the engine. When mixed with the proper amount of fuel, this extra air boosts an engine’s output.
The principle is simple, but putting it to work is not. The two turbine wheels on their common shaft are in delicate balance between the flowing columns of gas entering and leaving the engine. If the boost is high enough at low speeds to give good throttle response, it can soar to excessive pressures that will destroy the engine at high speeds. Temperatures on the exhaust side can rise perilously. With the right boost pressure for peak power, mid-range performance can be slack. Because the turbocharger is not mechanically connected to the engine, its response to the throttle tends to be delayed. Adjusting fuel delivery to the engine’s appetite is harder.
Starting with May’s initiative, BMW fitted a turbo to its 2002 model to compete in the European Touring Car Championship. In both 1968 and ’69 the boxy BMW beat the Porsche 911T and others to win overall. It was the first time that a turbocharged car had won an important road-racing championship.
Porsche could not and did not overlook this example. By 1970 experimentation with turbocharged engines was under way at Zuffenhausen. The first tests were carried out on a 2.0-litre Type 901 engine with the Bosch fuel injection system. Soon thereafter, as interest in the Can-am series grew during 1970, turbocharging was applied to the twelve-cylinder Type 912 at the express instruction of Ferdinand Piëch.
These early trials were mounted to see whether the engines’ air-cooling systems could handle the higher heat loadings imposed by turbocharging. Water cooling was weighed as an alternative if heat loadings exceeded the ability of Porsche’s traditional fins and blowers to dissipate excess heat. No insuperable problems arose.
In 1971, when Porsche confirmed internally its plans to go Can-am racing, the development of a turbocharged 912 engine was shifted into high gear in parallel with work being done on a new Type 917/10 chassis and body. Each of the cylinder banks of the Porsche flat-twelve was given a separate turbocharging system. This duplication was not required because a single large turbocharger was unavailable; suitably large units existed. It was implemented because the rotating inertia of each of the two small blowers was much less than that of a single large one. Their inertia needed to be as low as possible so the turbochargers would spin quickly up to speed to increase boost as soon as exhaust gas flow started increasing – or sooner.
The turbochargers were supplied by Eberspächer, which began its work on such devices in 1947. Based in Esslingen, it had design links with the American Airesearch company, which made most of the turbochargers being used by USAC racing cars. Under Hans Mezger’s supervision their units, adapted from their designs for trucks, were applied to the 912 engine by Valentin Schäffer, a stocky, broad-nosed fireplug of a race mechanic who, said Mezger, wanted ‘to work more like an engineer. He became involved in all the experimenting that we did on the turbo.’
Schäffer’s first system was as simple as he could make it. Two
turbos pumped air into straightforward log manifolds feeding the inlet ports of a fuel-injected 4.5-litre twelve. Maximum boost pressure was limited the same way it was in the USAC cars—by a single Airesearch pressure-relief valve or ‘waste gate’ attached to the exhaust pipes of both banks at points upstream from the supercharger turbines.
The valve of the Airresearch unit was controlled by a diaphragm that was fed pressure, on one side, from one of the inlet manifolds. When that pressure rose to the desired level of maximum boost, the force on the diaphragm was enough to overpower an adjacent spring and thus open the valve in the exhaust system, dropping the exhaust-gas pressure and preventing the turbocharger speed and pressure from climbing further.
Even with boost control, durability of the first blown 912 engines was not the best. By early summer of 1971 the first such engine was installed in one of the new chassis and taken to Weissach to see how it behaved. Tester and racer Willi Kauhsen recalled what happened. ‘When I settled into the car and started to warm up the engine, the windows shot up in the nearby buildings and the Porsche people waited eagerly for the ‘turbo’ to appear. But the first times out I rarely covered more than 500 metres!’
Valentin Schäffer’s battle to build reliability into the blown engine was not made easier by a fire in the test cells that caused serious damage to the area where he was working. This put a three-month kink in the progress chart of the turbocharged twelve. Nevertheless, that summer of 1971 the engine was running reliably enough for Jo Siffert to try it in the first 917/10 both at Weissach and at Hockenheim.
These tests were traumatic for Siffert and Porsche. The engine’s response to the throttle was far too slow. When the throttle was pressed coming out of a turn, long seconds elapsed before a swelling sense of propulsion against Siffert’s back told him the turbochargers were revving up. When he backed off for a turn those seconds seemed even longer, the engine roaring away at high boost even although he’d long passed the last braking point. More than once the white Porsche test mule bounced off the track into the boondocks with an all-but-helpless Siffert behind the wheel. Power was impressive but its controllability was not.
On 2 August 1971 Porsche broke the news that it would bow out of Manufacturers’ Championship racing in 1972 and race instead in the Can-am and Interserie with cars based on the 917. On 16 November Porsche and Porsche+audi announced jointly that Roger Penske’s team was their chosen ally in the Can-am and that their agreement covered the 1972 and 1973 seasons.
While the negotiators dotted Is and crossed Ts and while Jo Siffert campaigned the 917/10 in North America, the technicians were hard at work developing the car over which all the fuss was being made. Since the Siffert car represented the state of the Porsche Can-am art at that time, and since it was not capable of winning in the 1971 Can-am, let alone facing against the even tougher competition the next season would bring, it was obvious that a lot of work remained to be done.
After the first alarming track tests turbocharged engine development went back to the design office and the dynamometer. The test 917/10 was wearing higher-downforce bodywork in late October 1971 when driver Mark Donohue and Penske’s racing engineer, Don Cox, visited Weissach for the first time. In his book, The Unfair Advantage, Donohue told how a first encounter with that car for picture taking and ceremony on 26 October turned into a do-or-die challenge to improve on the 50.5second lap record for the Can-am track set by Willi Kauhsen after exhaustive testing.
‘Germans are very tough people,’ Mark learned that day.
‘They expect the maximum from everybody at all times.’ In spite of a nagging hangover, he more than met their expectations. After extensive changes were made in the chassis setup to moderate a high-speed oversteer at his specific request, Donohue sliced the record to 49.7 seconds.
A visit to Stuttgart originally scheduled to last three days extended to three weeks for Donohue. Immersed in their own worlds of business and racing in America, neither Penske nor Donohue had been aware of the engineering advances Porsche had made under Piëch, Bott and Mezger. They discovered that the car the other Can-am competitors derided as ‘junk’ was ‘like a fairytale,’ as Donohue said. ‘From an engineering standpoint it’s really clever and the pieces in it are fantastic.’
Under Peter Falk, the head of vehicle testing, Helmut Flegl was made Can-am project engineer and technical liaison to the Penske team. Donohue and Flegl worked methodically on the large and small Weissach skid pads to explore all the chassis variations of the existing unblown 917/10. By 12 November
Mark further reduced the lap record for the Can-am Track to 49.3 seconds.
After that session Mark told Pat Bedard of Car and Driver that Flegl was ‘one of the smartest “in-between” guys there is. He thinks exactly the same way I do, only he can’t drive the car. But because he can’t drive the car he can keep an open mind and because he is familiar with a lot of other drivers and other cars, he has insights that I don’t have.’ For their part the Porsche men rated the collaboration with Donohue ‘fantastic’ and added, ‘He was clearly the best test driver for this project.’
While Donohue was in Germany in November, his chosen chief mechanic for the Can-am effort came to Weissach for six weeks of indoctrination into the mysteries of the 917/10. ‘I learned about the engine, transmission, suspension and even welded on one of the chassis,’ recalled John ‘Woody’ Woodard. As nursemaid to the complex Can-am Porsche he was assisted by Heinz Hofer, Greg Syfert and the manager of the Penske shop, Chuck Cantwell.
After the October-november tests at Weissach, Porsche decided to continue the development of the 917/10 in unsupercharged form so the car would be in ideal trim when the blown engine was judged ready. Since the frustrating failures of the summer of 1971 Mezger and Schäffer succeeded in making the ‘turbomotor’ much more reliable.
One area that had not given trouble was the bottom end. This was as Mezger’s group had calculated it should be. ‘In the case of the 5.0-litre engine,’ Mezger reported, ‘the maximum connecting-rod bearing load of the naturally aspirated version at its power peak of 8300rpm is at about the same level as that of the supercharged engine at its
8000rpm peak power speed.’
No changes had to be made to the forged one-piece crankshaft, the bearings or the connecting rods. The steel shaft that took the power from the central drive gears to the clutch had to be enlarged from 22 to 24 mm in diameter. Even so, it twisted 20 degrees under the impact of the peak torque of the turbocharged engine. New pistons with almost flat crowns reduced the compression ratio to 6.5:1 to prevent detonation at the high combustion pressures reached at full boost.
For the first time in the history of the 912 engine, its cooling had to be increased. This was done by exchanging the two bevel gears in the blower drive so the fan turned at 1.12 times engine speed instead of the previous 0.9 of crank speed. This elevated its air-pumping capacity by 30 per cent to 6600 cubic feet per minute at the cost of an 80 per cent rise in the amount of power required to drive the fan.
To improve the engine’s mid-range running when the
“THEY EXPECT THE MAXIMUM FROM EVERYBODY AT ALL TIMES…”
turbocharger was off boost, its inlet timing was made milder by using the same cam lobe form and the same 10.5mm lift for the inlet valves as already used for the exhausts. ‘On a turbo,’ said Hans Mezger, ‘we found that you didn’t need that wide opening period for the intake valve. You got the air in with pressure, not just by tuning the opening time.’
With the new valve timing of 80°/100°/105°/75° the reduction in the effective overlap at top dead centre was much greater than the small difference from the unblown timing data would suggest. Mezger: ‘The smaller valve lift and less overlapping helped to improve the throttle response.’
The blown engine’s cylinder heads and valve sizes were the same as those of the unsupercharged Type 912. Porsche successfully introduced inlet valves made of titanium for the first time, each of the 47.5 mm valves weighing only 2.4 ounces. The stems of both inlet and exhaust valves were hollow for sodium cooling. Exhaust-valve stems were chromed to reduce the seizing in their guides that resulted in many destroyed test engines. The exhaust-valve guides were shortened so they would absorb less heat, increased in running clearance and given direct delivery of lubricating oil through a special drilling.
Fired by the same ignition system used on the earlier twelves, the two spark plugs per cylinder were of a platinum-tipped Bosch design. Bosch also provided the fuel-injection system. The diameters of its twelve pump plungers were enlarged to satisfy the blown engine’s greater appetite. The pump’s control mechanism was made responsive to boost pressure as well as to engine speed and throttle position.
No part of the turbocharged 912 was subject to more changes than its inlet manifolding. One feature that eventually stabilised was the positioning of the injection nozzles as close to the inlet ports as possible. Slide-type throttles were replaced by butterfly throttles. Some test engines had one big throttle for each cylinder bank and in others – eventually in all – each inlet tract had its individual butterfly. At the forward ends of the two log manifolds a small pipe between them balanced their pressures.
Long feed pipes came forward to the manifolds from the two turbochargers, were placed above and flanking the gearbox. Carried by a structure composed of the exhaust pipes between them and some additional bracing, they were suspended from the frame at the rear. The turbochargers were like those made by Eberspächer for diesel truck engines with one exception – their shafts ran on ball bearings instead of bushings. Each had its own oil supply from the engine. Exhaust gases entering the turbine side were as hot as 1800° F at full load. The compressed induction air fed to the engine reached a 300° F temperature in the absence of any form of intercooler.
A key variable in any supercharged engine is the amount of boost pressure used. This is the pressure achieved in the inlet manifolds, through supercharging, over and above that of the atmospheric pressure. In the early engines the boost was moderate for a racing unit – between 13 and 15 pounds per
square inch. Still, this was enough to extract more than 800 horsepower from the 4.5-litre 912. At about this level, an eighthour durability run at full power was successfully completed in early December 1971. It was successful as far as the engine was concerned but one heard that the dynamometer was not in the best condition afterward!
Thus it was not without confidence that Porsche shipped Team Penske one of its latest turbocharged 4.5-litre engines at the end of January 1972 for installation in their car. This, Donohue felt, had to be the solution to their lack of convincing speed. It was an awesome presence, said Woody Woodard: ‘I was stunned by the monster size and complexity of the flat twelve-cylinder with twin turbochargers. It looked like it would be more suited for an airplane racer than a race car.’
By the time they returned to Road Atlanta it was the last week of February and it was cold. They had to tow the car to get the engine started and then, wrote Donohue, ‘We damn near couldn’t keep it running. I tried to drive it a few laps and discovered that the throttle worked like an ignition switch – it was either wide-open power or off. It wouldn’t run at any partthrottle condition.’ ‘The boost was very sudden,’ Flegl confirmed. ‘Coming out of a turn, if Mark stepped on the throttle too early he’d lose the car. Starting was very problematic – sometimes we had to tow the damn thing for 40 yards!’
Only by taking terrible risks on a lap that he could not duplicate was Donohue able to lap as fast as he had a month and a half before with the unsupercharged engine. Finally a turbocharger impeller failed and its pieces went into the cylinders, wrecking the engine. ‘Mark wanted to know how turbocharging could work on the Offenhauser but not on the 917,’ said a frustrated Flegl. ‘How could we control the boost?’ Their cooperation faced its first major challenge.
Another engine was shipped over, together with Flegl and Schäffer, in time for the car’s first showing to the press and its sponsors at Road Atlanta on March 20. It had what Donohue called a $3000 paint job and it looked magnificent. But the engine, though improved, started and ran little better than it had in February.
Donohue and Flegl agreed that the driver should come to Germany for further tests at Weissach, which he did starting on 9 April 1972. On the less demanding track there Willi Kauhsen had lapped at 49.1 seconds with the turbocharged test 917/10, which was 0.2 seconds faster than Donohue’s best with the unblown car. Yet Donohue, struggling with its balky throttle response, could do no better than 49.7. To Porsche it seemed that the driver had to try harder to adapt to the engine, while to Donohue and to Penske, who watched some of the tests, it seemed that the engine should be improved.
In April the fuel-injection system was completely recalibrated from scratch. ‘On a naturally aspirated engine,’ explained Helmut Flegl, ‘fuel feel is determined by throttle position and engine rpm, but turbocharging adds a third element which we realised we weren’t taking into account. I had to force the engine guys to run the dyno to show fuel input right through the rev range, not just above 5000rpm. From these readings we shaped a cam to control fuel admission according to boost level.’
The engine’s fuel requirements were determined in both its blown and unblown modes of operation through its full speed range. Based on these findings Bosch supplied a new space cam that supplied the fuel dosages needed. Taking advantage of the pump’s fitting that normally sensed atmospheric pressure, its control mechanism was made responsive to boost pressure as well as to engine speed and throttle position.
‘This was the breakthrough we’d been looking for,’ said Flegl. With this, he and his colleagues the Porsche crewmen felt, they surely had something that would satisfy the demanding American, the man they called ‘a real driver-engineer.’ They put the pump on an engine, the engine in the car and went back to the Weissach track.
After some initial adjustments, Donohue wrote, ‘Suddenly, it was right! It started, idled, accelerated and had immense torque over a wide throttle range! Almost immediately I was down to 48.9 seconds. I came in and said, “I am quite happy with this fuel pump.” They were elated.’ ‘It was only six weeks before Mosport,’ Flegl recalled, ‘the 1972 season opener. Mark leaped on the telex to tell Penske.’
‘They were so pleased they never let go of that pump,’ Donohue recalled, ‘which became known as the “happy pump”. They kept it at Bosch and it was used for the calibration of all other fuel-injection systems we used. Whenever there was a problem, they would always go back to the “happy pump”.’
Now some of the other improvements that Valentin Schäffer made to get faster response from the turbocharger began to bear fruit. One in particular was decisive. A butterfly valve that opened
“I DISCOVERED THE THROTTLE WORKED LIKE AN IGNITION SWITCH…”
to the atmosphere was installed between each log manifold and the delivery pipe that brought compressed air from its turbocharger. This valve was connected to the throttle linkage in such a way that it opened fully just when the throttles closed.this system vented the pressure in the manifolds so the impellers of the turbochargers would find it easier to keep spinning because they wouldn’t be pumping against a dead end. In this manner turbocharger speed was kept higher through a turn and more ready to rev up when the throttles were opened again. Each delivery pipe was fitted with a small pressure-relief valve to vent any excess boost.
Before the first Can-am race another novel device was fitted at the suggestion of Mark Donohue. Four suction-operated air valves, looking like tiny top hats, were fitted to the top of each log manifold. Because the throttles were quite a long way from the blower air inlets, the driver was concerned that the engine’s lowspeed response might be hampered by a lack of atmospherically inducted air. These valves admitted extra air for that purpose. When the boost pressure rose, the eight valves automatically snapped shut.
Supercharging pressures were raised during development. Normal boost was stepped up to between 18 and 20 pounds per square inch. At that level the 4.5-litre engines used in the
Interserie in 1972 produced 840 to 850 horsepower. The lack of mechanical troubles with this engine encouraged the engineers to try the turbo-blowers on the 5.0-litre version. This worked sensationally well. Visitors in May saw such an engine taken momentarily to a power reading of more than 1000bhp on the dynamometer.
Durability testing reassured Porsche that the 5.0-litre was sound enough to be relied on in the Can-am series. Three of the engines used in the Penske cars in 1972 registered peak outputs between 894 and 918bhp at 8000 rpm. ‘That doesn’t mean,’ cautioned Ernst Fuhrmann, ‘that it developed the same power when installed in the car, where for the entry of the induction air is not so ideal as on the test stand. Thus effectively it was rather less than 900 horsepower.’ One curve published by Porsche showed 910bhp at 7800rpm on a boost of 19psi and a peak torque of 707lb ft at 6400rpm. Such an engine weighed 617 pounds.
The 5.0-litre’s performance in the car was even better than its sensational specifications because the larger displacement further reduced the turbocharger response time. Said Fuhrmann, ‘With a lot of detail work we have been able to reduce the delay to a few tenths of a second, but it is not yet quite so precise as an unsupercharged engine. That means that the driver must still take into account a small delay.’ In the 5.0-litre size the base engine also had more power to offer at part throttle when the boost level was low.
One of the revised turbomotors was airlifted to Philadelphia for installation in the Penske test car at Newtown Square. The 917/10 then went to Riverside for evaluation. ‘It was an unbelievable transformation,’ said Woody Woodard. ‘The car was a rocket right out of the box, so we were feeling very good.’
Also feeling good were the Porsche designers and developers, who moved during 1972 to new quarters at Weissach. Although farther from the production lines where their creations were made, they were now nearer the proving grounds where their brainstorms met their nemesis – or went on to glory.
In this case glory was the result. Equipped with a more rugged transaxle that had four forward speeds and no reverse, the turbocharged Porsche 917s went on to overpower the opposition in both 1972 and 1973, Porsche’s final year in the Can-am series.
After Donohue was benched by a crash in the first season George Follmer became champion, while Mark recovered to win that honour in 1973 with a Porsche that Woody Woodard said
‘had 1550 horsepower on tap if we wanted it. However, in the race we’d dial it to 1100 horsepower and leave it there or maybe dial it down some more.’ That got the job done. PW