Tuesday, June 9, 2015

SERA And The Porsche 917

One reason I "go on" about the 917 is that it was a case-study in race car development.  When the aero lift was sorted at the end of 1969, the Wyer-Gulf team, under the impression that it was the factory team, said "Let's get on with refining pace and especially reliability.  This is about the World Endurance Championship and LeMans."  That was Ferry Porsche's idea too, and the reason why Wyer had been hired: to reduce the cost of Porsche's racing budget.

Ferdinand Piech, Porsche's Racing Director, was having none of that.  Every aspect of the 917 was continuously and relentlessly developed: weight-reduction, body, frame, brakes, power-train.  Up to and including the 12-cylinder turbo and 16-cylinder atmospheric engine for Can-Am racing after FIA regs made the 917 obsolete for the WEC.  Or, as Piech said when lifting a dust-cover to show Wyer the 16-cylinder, "Auntie doesn't know about this."  He meant his Uncle Ferry (and had mixed up his English nouns).

Most of the facts and illustrations in this post come from Walter Naher's excellent book, Porsche 917 Archive And Works Catalog.  From it we learn that SERA consulted on the 917's aerodynamics from the beginning, not just on the "Pink Pig" LeMans car of 1971.  SERA (Societe d'Etudes et de Realisations Automobiles) was an engineering consulting firm, specializing in aerodynamics, owned by Charles Deutsch.  He had started it after his (mostly racing) car-building partnership with Rene Bonnet was dissolved in 1961.

Before Charles Deutsch ran SERA, he and Rene Bonnet collaborated in building and racing DB-Panhards at LeMans.
With its small 750 c.c. Panhard engine, the DB won the Index of Performance (or Thermal Efficiency) several times
in the 1950's, primarily because of its slippery shape.  The Index itself was a complicated formula that boiled down
to going the furthest with the least expenditure of energy.  The car pictured is the 1955 winner.

A 1:5 model of the 917 as tested in various configurations in the SERA wind tunnel in
February, 1969, two months before the 917 was homologated by the FIA. 

For Porsche, LeMans had always been the crown jewel of the World Sports Car Championship.  While the 917 was built to contest the Championship, a LeMans win was as just as important.  And the key to winning LeMans was high sustained speed down the four-mile Mulsanne Straight.  The key to that was low drag.  So the 917's shape was copied from the very successful 907 (2-liter) and 908 (3-liter) long tail cars.

Even as Porsche was testing a 1:5 (and later a 1:1) model of the 917 in the Stuttgart wind tunnel, the 1:5 model was sent to SERA in February, 1969.  For stability, Deutsch suggested (and tested) a tall dorsal fin like a jet airliner's tail.  He also suggested two smaller fins (as shown in #4 above).  A tail configured like the 908 LH's, as used at LeMans later that year, was also tested (as shown in #3 above).  None of these configurations showed a conclusive advantage in the wind tunnels.  And, while the 917 LH was homologated with the 908 LH-like tail, it remained aerodynamically unstable throughout 1969.

Apologies for the crummy scan: the picture was next to the binding of Porsche 917 Archives and Works Catalog.  But the
important part is clear.  This is the "SERA nose," which was tested at Zeltweg in October of 1969 and later, but was not
used on 917's entered in races.  It slopes more and has bigger ducts than the "production" noses.

Although Walter Naher doesn't make it explicit, Porsche must have contacted SERA in the summer or fall of 1969, asking Deutsch to take a shot at a nose with better downforce.  The result was tested at Zeltweg in Austria in October.  Also at the Zeltweg test was the 917 Spyder Jo Siffert had run, and tinkered with, in the Can-Am series in the States.  For the last race, Siffert had come up with a McLaren-like "chisel nose" that came to a sharp point, with vertical splitters and large "dive planes."

The key to the 917's aero stability was at the rear, and was unlocked with the Horsman K tail (at the Zeltweg test) and a large wing across the full width of the LH tail (in the spring of 1970).  The "production" nose of the 917 was new but evolutionary, borrowing from both SERA and Siffert.  It had the flatter bottom and more squared-off corners of the SERA nose, but the smaller oil-cooler inlet of the Siffert nose.  Unlike either, the leading edge was "stood up" vertically with the brake ducts adjoining the oil cooler.

Above and below: the famous SERA-Porsche collaboration; the Pink Pig.  Also known to the Porsche mechanics as "Big
Bertha" and "The Zuffenhausen Truffle-Hunter."  Officially, it was 917/20.  Without sponsor livery for LeMans 1971 (its
only race), Porsche Design came up with this tongue-in-cheek cuts-of-pork livery.

Shortly after the 1970 LeMans win, Porsche scheduled a meeting with SERA to discuss body developments for 1971.  The goal was a short-tail coupe with the same downforce as the production K tail but lower drag.  A shorter nose, like the 908/3's, and other detail modifications were also desired.
It was decided that SERA would compete with Porsche Design, and they came up with radically different designs.  SERA's won the competition because its open wheel wells were better for tire changes.  It had slab sides below the centerline, rounded wheel-arch contours, and a vestigial front splitter.

At the 1971 LeMans Test Days, it was "deja vu all over again" for the Pink Pig: unstable at high speeds and under braking.  Between then and the race itself, the car was tested in a wind tunnel and found to have drag higher than both the LH and the latest finned version of the iconic K tail, doubtless due to the Pink Pig's larger frontal area.  The stability problem was solved with much stiffer springs and a much higher rear spoiler and "Gurney Lip."

In the 1971 race, the Pink Pig qualified with a time of 3:21, two seconds off the pace of the latest, magnesium-framed conventional 917 K and seven seconds slower than the latest version of the LH.  It retired from 6th place well into the race when it was crashed at Arnage Corner.  In the Porsche episode of his Victory By Design TV series, Alain de Cadenet, who raced at LeMans that year, said that the vortex of low pressure behind the Pink Pig nearly sucked him off the road.  It was way more pronounced than the conventional LH's and K's.

So the Pink Pig was a failure as measured against its design goals.  But it was a harbinger of aerodynamic features to come: slab sides, a front splitter, and contoured wheel arches.

1 comment:

timo nassi said...

what is big Red box in back side?

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