Fast backwards

  • Image credit: Karen Ballard
  • Image credit: Karen Ballard
  • Image credit: Karen Ballard
  • Image credit: Karen Ballard
  • Imge credit: Karen Ballard
  • Image credit: Karen Ballard
  • Image credit: Karen Ballard
  • Adam Savage (left) and Jamie Hyneman (right), The Mythbusters Image credit: Karen Ballard
  • Mike Allen, PM Senior Editor, Automotive. Image credit: Karen Ballard
  • Image credit: Karen Ballard
Date:1 January 2011 Tags:, , ,

PM helps Mythbusters tackle a decades-old automotive legend: is the Porsche 928 more aerodynamic going backwards?

It’s 8 am on a Monday. Instead of riding the lift up to Popular Mechanics’s Manhattan office, I find myself in San Francisco, holding my welding helmet and keying in the entry code to one of several doors in a plain-Jane industrial park. A sign on the wall reads “M5 Industries”, a surefire giveaway to fans of MythBusters. This is the TV show’s home base, where I have an audacious mission: remove the body of a Porsche 928 and weld it back on – backwards. I have four days to complete this monumental build, and one to debug it, so Adam Savage and Jamie Hyneman can test the inverted car the following Monday.

The two hosts greet me as I open the door. Before expressing pleasantries, Adam says, with his typical enthusiasm, “Dude, you brought your own welding helmet! We got the right guy.”

Why me, you’re asking? A week or so earlier, I had picked up my ringing phone and, surprise – Jamie was on the line. Okay, Jamie and Adam often write for PM and have visited the office more than once. Last year a bunch of us from the magazine spent a few very hot days dirtbiking through Death Valley with Jamie – but since he lives on the other coast, we don’t chat every day. Off-camera, Jamie is just like he is portrayed on the show – all business – and he got right to it. “Mike”, he said, “we want to test the myth that a 928 is more aerodynamic going backwards, and to do that I need to figure out how to measure the aerodynamic efficiency of a vehicle.” I was intrigued. at myth has been around since the sexy V8-powered coupé debuted in 1978. “No problem,” I said. “Just park it in a wind tunnel and read the numbers off the screen.” Jamie, however, had a more ambitious plan, one that fit the MythBusters paradigm of an extreme build and a hands-on test. He wanted to actually build a backward 928 and measure its fuel consumption, which would reflect the amount of power used to push it through the air. If the car used less fuel going backwards, then that configuration would be more aerodynamic.

I thought that approach was too complicated and the results too difficult to repeat. But before we came to any conclusion, Jamie said, “You sound like you know a lot about this. Why don’t you come help us?” Me and my big mouth. I immediately booked a flight.

From my racing days I know a lot about the nitty-gritty architecture of the 928’s underpinnings. Removing the body in one piece so that it could be reattached without twisting up like a Pringle would be a real challenge. In fact, no one’s ever done it. The 928 is a unit-body car made from dozens of thin sheet-metal stampings, none thicker than a coin. The pieces are welded together, and they rely on one another for strength. It’s not like the old days, when cars had separate bodies that could be unbolted from a distinct frame.

For anyone mechanically inclined, M5 is like a giant playground. Jamie and I head straight to the shop. Television’s wide-angle lenses make the work space appear a lot larger than it actually is. Cleaner, too.

Beneath the glare of a dead mechanical shark hanging on the wall, I survey all the assembled gear. A penumbra of toolboxes, plasma cutters, air tools and metal-inert gas welders surrounds a nervous-looking 1984 Porsche 928 S that’s about to be disembowelled. Jamie introduces me to M5 fabricator Don Best, and the three of us discuss tactics.

Jamie wants to cut through the car’s floor pan, between the seats and the rocker panels, a method I’m afraid will make the chassis sag between the wheels like a banana. I want to separate the rocker panel into inner and outer halves, to keep the car’s structure intact. Jamie ultimately defers to my superior knowledge of Teutonic automotive architecture: We’ll do it my way.

The next two days are filled with the screech of cut-off wheels and Sawzalls, the hiss of the plasma cutter and the crackle of the occasional small fire as Don and I remove the interior, dashboard and extra wiring. Every couple of hours, the camera crew shows up, which usually burns about an hour as we pose and redo what we just finished fabricating. Occasionally we also have to desist from noisemaking while the crew tapes a vignette for a different segment. Flipping a car around tends to make a racket: quiet denotes lost progress.

Don and I become good friends, which is easy when you spend 12 hours a day for a week underneath the same car, covered in slag and grinding dust. I’m not the least bit surprised when we’re still hard at it the following Sunday.

The next morning at the test site, I wriggle through our project car’s window and blast down the runway. True to its breeding, the 928 feels rock-solid at 225 km/h, meaning we’ve properly welded it back together and the chassis still has enough rigidity to keep the suspension stable. Hot air off the radiator blasts my face and dirt vacuumed from the surface fogs the cockpit. Before the next run I empty three cans of construction foam into the leftover voids, but this doesn’t keep the dirt out, and soon a thick layer of dust and debris carpets the floor.

Adam is standing nearby, obviously excited to try our new toy. “Can I drive it?” he asks. My primary mission accomplished, I hand over the keys.

Three, two, one – lift!
Turning a Porsche 928’s body around to test the myth that the vehicle is more aerodynamic backwards started with two days of cutting the body free. Then Jamie, Adam and the MythBusters crew helped us lift it off. We needed lots of hands to distribute the weight and keep the fllmsy body from distorting – the body of the 928 wasn’t designed to stand free of the chassis.

Keep it straight
I taped the doors and rear hatch shut (above, left) to give the body as much structural integrity as possible during its delicate removal. The MythBusters, who once made a boat of duct tape, heartily approved. Above, right: Adam (left) inspected the Porsche’s insectlike intake system, which, thankfully, didn’t require reworking. Behind them were shelves stocked with 600 crates of supplies. Sadly, none contained a “Porsche reversal” kit.

Nose amputation
The front end of the engine bay had to be shortened so the stubby rear cargo area could fit. That meant the bumper supports, fans, a/c condenser and headlight frames all had to go. We eventually had to add more than 180 kg of ballast to equal the stock car’s weight. To ensure the front and rear ride heights were retained, we used wheel scales to set the front-to-rear weight bias close to the original 50:50 proportion.

It's not really a convertible
The most complicated cuts were at the bottom of the A-pillar and firewall confluence, since the area is so hard to access and contains several sheet-metal panels. We welded in a steel bar to support the dash. The computer and ignition boxes were removed so the MIG welders wouldn’t fry the boxes’ delicate insides. To preserve structural integrity, we cut the rocker (at the bottom of the photo above) in half, along its length, instead of cutting it out. Removing the body left several gaping holes (above, right) in the structure that we eventually welded shut.

Going together
Above: It was Thursday, four days into the build and just three days until testing. We needed every minute to fiddle, trim and modify the chassis to accept the backward body. The rocker panels mated up directly, but in many other places we had to MIG-weld 20-gauge-steel scab patches to bridge gaps.

It's alive
M5’s forklift proved to be a perfect hoist for countless test fittings. On day six there was a resounding cheer when we finally got the body to fit. With the hardest part over, we hooked the engine to an 8-litre fuel cell and turned the key. Nope – no immediate joy. We soon deduced that the car’s burglar alarm wasn’t happy with some of the cuts we had made to the wiring harness. Grounding a couple of wires that used to go into the door’s wiring fixed that, and the car fired up. A few more loose ends later, we were ready to test – just in time.

Adam: "This is so wrong"
Now that we had our backward Porsche, the producers secured an unmodified silver 928, and we set out for the favourite MythBusters test site, the runways of the decommissioned Navy base in Alameda, just across San Francisco Bay. The plan was to run the cars side by side up to 100 mph (160 km/h), shift into neutral and coast to the end of the runway. The car with the lower drag coefficient (Cd) would take longer to slow down, pulling ahead of the high-drag car and finally putting an end to this automotive mystery. Why 160 km/h? Because aerodynamic drag climbs with the square of velocity, so the higher the speed, the bigger the difference, if any. We also pointed the cars into a 25 km/h headwind – higher airspeed equals more drag – and added 14 kilograms to our bizarro Porsche so the two cars weighed the same. Adam drew the short straw, so he drove our project car and endured the heat that poured from the radiator onto his balaclava-ed head. Jamie piloted the unmodified Porsche. My job wasn’t over: As the guys blasted down the runway, I manned the radios.

Related material
Video:
To watch a video of Mythbusters Jamie Hyneman and Adam Savage answering questions about the Reverse Engineering Shoot. [click here]