A Long Tale of Many Tails

I've long wanted to see an account of the evolution of the Porsche 917 tail, in detail, in one place.  The story is too long for a magazine or a chapter in a book about the car in general.  It's an "into the weeds" story.   But it fascinates me because the 917 was the dawn of the aero era in racing sports cars.  (Can-Am Chapparals and Formula 1 were already experimenting with moveable wings, front and rear, but these "unlimited" experiments were outside the rules governing Group 5 Manufacturers' Championship sports cars.)

So here is the story of the 917's tail, based on my own reading and picture research.  One of several reasons the 917 is iconic for me is that it was the "last of the non-areo" racing sports cars--at least as introduced.  Two were licensed for road use.  The wisdom of a 917 road car is debatable, but at least you could drive the thing across a curb cut.

The 917 was a development of the 908, with a 50% larger 4.5 liter engine (later 4.9 and 5.0).  The body was a direct descendant of the 907/908 racers, both short and long tailed.  It was built to compete in the top class of the FIA Manufacturer's Championship (later the World Sports Car Championship.)  Twenty-five cars were built initially, to comply with FIA rules, with some of the first batch reserved for the factory team and the rest for private sale.  They were available with a short or a long tail (or both).  The 917 was shown at the Geneva Auto Show in March, 1969.  Porsche printed a catalog, and our first picture comes from it:

In testing it quickly became clear that the 917 was unstable at racing speeds.  But the source was not conclusively identified as aerodynamic lift until after the 1969 season was over.  Both the short and long tailed cars suffered.  It was capable of 190-220 m.p.h., 20% (or more) faster than a 908.  The Ford GT-40, the first racing sports car to run 400+ horsepower, had suffered from aero instability too.  Ford solved the problem with a rear spoiler and a blunt nose.  With power to spare, higher drag was a price Ford was prepared to pay.  But the Porsche Way, going back to the 356 road car--the first Porsche ever--was low drag.  A "barn door" solution was not going to come from Stuttgart.

At Spa-Francorchamps in the spring of 1969, Jo Siffert practiced in a short tail 917 but chose a 908 for the race.  The 917 was too much of a handful in Spa's fast sweepers.  The other 917 retired, far from the sharp end of the race.  Here's a picture of the short tail at Spa, which was no different from the original specification:

A footnote at the front: 917-002, Gerhard Mitter/Rudi Schutz at Spa in 1969 (DNF).  The prototype's NACA ducts next
to the headlights have been closed off, and the opening for the oil cooler made smaller: "max streamlining."  Compare
this with the picture below at LeMans, where the NACA ducts are open again and additional ducts have been added
next to the oil cooler for the front brakes.

Porsche was determined to race a long tail at Le Mans, where its low drag would pay off on the long Mulsanne Straight.  In 1969 LH (Langheck, Long Tail) configuration, the car was not materially different from the Geneva Show car (or the 907 and 908 LH's that preceded it).  But for practice at Le Mans (and the race), "dive planes" were added to the front fenders.  The car retained rear winglets connected to each rear suspension unit by a system of bell cranks, designed to offset lateral weight transfer by pressing down on the inside rear tire.  Drivers still reported instability, particularly in the long righthand sweeper before the braking zone for Mulsanne Corner, where the car "used all of the road." (It could not be depended upon to hold its line).  Aero downforce on the front wheels from the dive planes had not solved the stability problem.  Here's a picture of the factory Vic Elford/Richard Attwood 917LH at Le Mans in 1969.  It shows the rear winglets in operation:

In the race, John Wolfe was killed on the first lap in his privately entered 917 at White House (a fast sweeper).  The factory cars retired with mechanical problems.  The LH was a one-race car, conceived with the idea of winning Le Mans overall.  (Porsche already had many class victories.)  With Le Mans behind them, development of the LH became a low priority.


Meanwhile, Jo Siffert was campaigning a 917 Spyder in some Can-Am races in the U.S.  The Spyder was a topless regular 917 with a 4.9 engine and design cues from the Bergspyder (European hill-climb) program.  One of them was a body that minimized weight at the expense of streamlining.   Here is a picture of the Can-Am Spyder as it arrived in the States:

Being a continent away, out from under factory supervision, Siffert had latitude to experiment.  He lengthened and lowered the nose, added bigger dive planes, and tried the vertical "fences" McLaren was using.  He added higher rear spoilers behind each rear wheel.  Here is a picture of the Spyder as Siffert had developed it by season's end:

Back in Europe, the short tail FIA car had grown rear fender blisters to cover wider tires and traded in its moveable rear winglets for fixed spoilers.  Up front, bigger inlets were used for the oil radiator and brake ducts.  Dive planes had remained a regular feature since the Nurburgring.  It won the last race of the season at the Osterreichring, with Jo Siffert and Kurt Ahrens driving, but it was still a handful.  Here is a picture of the 917 short-tail in its final 1969 configuration:

Two months later, in mid-October, Porsche scheduled three days of testing at the Osterreichring with John Horsman and some of his mechanics present for the first time.  Horsman was the Technical Director of J.W. Automotive (John Wyer's Gulf Oil-sponsored team), which had been chosen by Ferry Porsche to run the factory team in 1970-1971.  This test became a bone of contention between "the Porsche camp" and "the Wyer camp" and has found its way into 917 books and personal memoirs.  The nub of the dispute is the answer to the question "Who was responsible for the 'new' K tail?"  The dispute would be forgotten but for the fact that the new tail solved the 917's instability problem and transformed the car into an iconic winner.

The agreed facts are these:
1) Siffert's 917 Can-Am car was present at the test (as were two coupes), but Siffert was not.
2) Wyer was not present.  Porsche staff from Stuttgart flew in on the third test day, after the Horsman Tail had been tried, but left without comment (at least to him).
3) The new tail was fabricated overnight by Horsman's mechanics under his direction.
4) It eliminated aero instability.  The car was more than 5 seconds per lap faster after modification.  Testers Kurt Ahrens and Brian Redman (who was under contract to Porsche and would drive for Wyer under that contract in 1970) reported that they could now focus on reducing their lap times without worrying that the car might get out from under them.  Redman, who had struggled with the 917 throughout 1969, but had also teased Horsman about his ugly cobbled-together tail, said "Now it's a race car."

What is in dispute is the inspiration for the new tail.  Porsche partisans claim that Horsman got his idea from Siffert's Can-Am car (thus appropriating a Porsche concept).  Horsman says he ignored the Cam-Am car (which was turning times within tenths of the coupes) because his brief in 1970 would be the coupes for FIA races.  Instead, he says, he noticed that while the fronts and windshields of the test coupes were covered with dead gnats, there were none on the rear spoilers.  He concluded that air was not flowing to or over them.  Wyer felt so strongly that Horsman had been denied proper credit for the breakthrough that he said in his memoir that the revision should be called "the Horsman Tail,"not named after him and his team.  Later tensions between Wyer and Ferdinand Piech, Porsche's Racing Director (and "father of the 917") and their partisans, did nothing to moderate the dispute over who gets credit for the 917K.

Horsman says in his memoir that he was afraid that his idea would be stopped cold by Porsche's racing management because it was high drag and "Not Invented Here."  So, toward the end of the test day, he took pains to casually ask if his mechanics might borrow a coupe and some sheet aluminum.  This was the modification Horsman came up with:

Whatever his feelings about Not Invented Here and high drag, Piech adopted the Horsman Tail for all 1970 (and later) short tail 917's.  He could hardly argue with the lap times and driver evaluations in the test.  As modified by Porsche, Horsman's prototype became the iconic K (Kurz, short) tail.  The Porsche modifications were: abandon bodywork behind the rear wheels, abandon the rear window altogether, and mold a channel into the center of the tail for rearward vision.  Doubtless the absence of a  lower rear body on the Can-Am Spyder was the inspiration for that detail of the new K tail.  All 917K's were retrofitted with the new tail, including the two that had been sold to private entrants in 1969.


Purely as an aesthetic matter, the K tail has always appealed to me because you can see the frame, transmission, halfshafts, and exhaust pipes.  It makes the car looks so mechanical.  Here are two views of the tail that became iconic, especially in Wyer-Gulf livery because of the team's dominance of the 1970-71 seasons, and because of Steve McQueen's film Le Mans:

There is a bit of circumstantial evidence that Porsche did not finalize the new K tail until mid-winter of 1969-1970.  David Piper's 917 was the first customer delivery, in time for the Nurburgring in '69, where he raced it in standard "Spa" trim (with dive planes added).  But at Kyalami, in December (two months after the Horsman test), it looked like this:

Piper said in 2007 that he was experimenting independently with aerodynamics, based on his experience with his Lola T-70, which was stable at high speeds.  The front is straightforward: a late 1969 nose.  The rear looks like an original '69 short tail with a more finished version of the Horsman Tail grafted on.  It retains Porsche's rear bodywork, window, blisters for wider tires, and side-exit exhaust  for the front cylinders.  There are two possible explanations for the Kyalami 917: 1) Piper's consideration of the 917 compared to his T-70 led him to the same answer Horsman found; 2) he had "insider" connections with Wyer's team or Porsche or both.  My guess is that Porsche lent Piper their prototype Horsman Tail in exchange for his feedback.  He won Kyalami.  In 1970, Piper raced his car in the new, standard, K tail configuration.


Over the next two years, there were minor modifications to the Horsman Tail.  They increased downforce or decreased drag, or both, in minor ways.  Wyer himself used a center airfoil sometimes in 1970.  It was made available to privateers through Porsche.  He used a full-width spoiler at Spa-Francorchamps in 1970.  In 1971, Porsche developed a scalloped rear fender line with vertical fins.  It was made available to everyone, and several teams used it on fast circuits.  Pictures of these tails follow:

With the short tail's instability problem solved, Ferdinand Piech turned his attention back to the LH.  He abandoned the bell-crank winglets for stabilizing fins.  Kurt Ahrens wrote the experimental car off in a test at the Volkswagen Proving Grounds in March 1970.  He walked away.  (Ahrens probably logged more test miles, and heart-stopping moments, in the 917 than any other driver.)  Here is a picture of the "fins only" car:

Piech's solution to Ahren's crash in March was to add a full-width rear wing: the first true, "modern," aerodynamic device to appear on a 917.  Wyer refused to use LH's at Le Mans in 1970, considering them unproven.  His refusal, along with his turn-downs of mechanical updates, further annoyed Piech.  When Wyer refused to race the 4.9 liter engine without more testing, Piech gave it to Porsche Salzburg for the next race.  Wyer had been led to understand that his was the factory team.    But when he turned down an engine with 9% more displacement and power, it showed up in the cars of his chief rivals.


The 1970 Le Mans race did nothing to improve the Wyer-Piech relationship.  The winged iteration of the LH was stable, and 20 m.p.h. faster on Mulsanne than the K tails.  It took the pole and lasted well into the race before a mechanical failure, possibly related to the enclosed tail, retired it.  The race was won by a very standard 4.5 liter 917K.  It was Porsche's first overall victory at Le Mans.   Both Piech and Wyer could consider themselves vindicated by the results.  The LH had dominated the race while it lasted, but the race was won by an old, proven, car (although not Wyer's).  This is the 1970 LH pole-winning, race-leading car:

Piech further refined the LH for the 1971 Le Mans race.  The nose was widened and squared-off, with larger openings for the brakes and oil cooler, and the rear wheels were partially enclosed.  It was faster and more stable than the 1970 car (reaching 240 m.p.h. on the Mulsanne).  Wyer relented and entered two LH's, backstopped by a K.  Porsche's Martini & Rossi-sponsored Salzburg team entered one also.  All the LH's retired, again arguably from heat-related causes possibly attributable to the enclosed LH tail.  The race was won by a Martini & Rossi K with an experimental magnesium frame.  Wyer's K came second.  Once the LH's instability was fixed, it dominated Le Mans for two years--but it never won.  Another LH irony, in the context of the personal tensions:  Piech never won with a car dedicated to this single race; Wyer, who had the best record in endurance racing, agreed to run LH's in 1971--and never won LeMans for Porsche.  (He won the championship for them in both 1970 and 1971.)  Here is the LH in 1971 configuration:

An interesting blind-alley in 917 aero development was the Pink Pig, a.k.a. the Zuffenhausen Truffle- Hunter.  Piech hedged his streamliner bet with a body intended to manage the boundary layer differently from (if not better than) the LH while being a significant improvement over the K.  It was designed by the French consulting firm S.E.R.A.  Its top speed on Mulsanne was about 220 m.p.h., halfway between the K and the LH.  So it achieved its design goal (but failed to finish).  Alain DeCadenet said that it made his car far more unstable in its wake than either a K or an LH, which suggests that its aerodynamics were more "modern" than the normal cars.  It failed to acquire a sponsor, so Porsche painted it pink with a butcher's diagram of pork cuts.

The aerodynamic design and development of the 917/10 and 917/30 turbo Can-Am cars is outside the scope of this essay.  Suffice it to say here that it was significantly different, as Porsche (with input from Mark Donohue and Roger Penske) searched for ever more downforce to cope with 1000+ horsepower and the ability to reach previously unheard of straight-line speeds, even on short courses.  The Can-Am cars have more in common with today's Audis, Peugeots, and Toyotas than they do with the original 917.


Which brings us to where we came in.  The 917 was a bridge between the eras of streamlining (imagined or wind-tunnel tested) and modern aero downforce.  It was the big power of the 917 (compared to what had come before), combined with its low drag, that forced a search for stability before aerodynamic instability was fully understood.  John Horsman's "grand piano with the top up" was inelegant engineering.  But it created a beautiful car: a visual feast of mechanical bits to go with the sound of the flat-12 engine.

SOURCE NOTES:
1. Racing In The Rain, by John Horsman.
2. The Certain Sound, by John Wyer.
3. Piper's experiments: as recounted to Pilote by an individual who had a direct conversation with Piper.
4. Anything else is from Pilote's reading, research, and general knowledge.