The Vicious Crashes and Terrifying Records of Peak Downforce

“Downforce is a mind fuck,” says Tommy Kendall. He would know. “Everything you’d experienced before says, ‘You can’t do this. You can’t go faster.’”

Nearing 60, Kendall walks with a pronounced limp, each step a painful reminder of the ­awesome risks that accompanied the weaponized airflow of Grand Touring Prototype (GTP) racing.

The lanky Californian terrorized American racetracks in the early Nineties driving IMSA’s most extreme machinery. His efforts netted lap records at multiple circuits, but Kendall paid a price for the experience.

Nine hours of emergency surgery was the cost after Kendall was cut from his crumpled Chevrolet Intrepid RM-1 in 1991. Surgeons did their best to reassemble his shattered legs and pulverized feet after the crushing weight of downforce—like an invisible elephant perched atop the roof—snapped the rear suspension and launched the Intrepid into the barriers at Watkins Glen ­International’s Turn 5 at about 150 mph. But flirting with calamity was part of the job.

Outrageous budgets committed to GTP by ­General Motors, Jaguar, Nissan, and Toyota delivered unhinged horsepower and downforce figures for three glorious seasons. But as ­Kendall and others found, when you bend the laws of ­physics, sometimes physics fights back.

In a rolling tale by GTP drivers, engineers, and officials who survived the era, Road & Track explores the rise and fall of IMSA’s downforce kings from the early Nineties.

Davy Jones, Tom Walkinshaw Racing (TWR) Jaguar XJR-14 driver: When I first drove the Jaguar XJR-14 at a test in Phoenix, I couldn’t hold my head up.

David Brabham, TWR Jaguar XJR-14 driver: The g-forces were pretty high. With full wing, it was like 10,000 pounds of downforce.

Davy Jones: We had to modify all kinds of head braces and shoulder straps just so I could drive the thing.

PJ Jones, Toyota All American Racers (AAR) Eagle Mk III driver: It was like 4 Gs under braking. At most tracks, we braked at the 1 marker. Your whole body got mushed into the seatbelts. It just pushed the wind out of you. I haven’t driven anything like it since, even Indy cars.

Mark Raffauf, IMSA technical chief: Tommy Kendall once told me if you didn’t get out of the Intrepid after driving it and feel sick, you weren’t driving it hard enough. That’s how brutal it was.

Trevor Harris, chassis designer, Nissan ­Performance Technology Inc. (NPTI): It wasn’t by ­accident. We were trying to get ever more downforce. The chassis was to follow whatever the aerodynamics wanted.

The revolutionary era didn’t come about by chance; it was the result of a series of innovations and a mounting quest for more speed. In 1981, GTP cars put hand-built low-slung prototypes in the spotlight. They looked spectacular and drove IMSA’s popularity to new heights.

By the mid-Eighties, turbocharging pushed GTP horsepower into four digits, but the explosive acceleration wasn’t matched with ­aerodynamic help. GTP cars were relatively light at around 2000 pounds, with maximum downforce figures in the range of 2500 to 3000 pounds at 200 mph. At most tracks, the GTPs behaved more like dragsters, unable to carry speed through the turns.

Geoff Brabham, four-time GTP champion from 1988 to 1991 with Nissan: When I first drove the GTP ZX-Turbo in 1986, we had 1100 hp. But you could hardly drive it.

This was GTP’s biggest issue. Designers spent the rest of the Eighties looking for downforce that delivered cornering performance to match the straight-line power.

As the Nineties approached, peak downforce rose steadily: 3000, 5000, then 7000 pounds. Simultaneously, cars like the GTP ZX-Turbo were fast becoming antiquated; their sleek bodywork minimized drag. To go faster, that design philosophy would need to be upended.

Bill Riley, co-creator of the Chevy Intrepid with his father, Bob: Engine power started to make it possible to pile on the downforce. We had the power to pull through the drag. That’s where it really started.

GTPs made downforce in the nose, splitter, underwing, diffuser, and rear wing. The underwing is where GTP designers focused their ­attention and made mind-boggling gains with ground effects.

Raffauf: The side pods had inverted wings, and the undersides were contoured like upswept ­tunnels. You’ve got to get the air under the car to create an upside-down airfoil. The tunnels made the whole car a wing.

John Ward, designer of AAR’s Eagle Mk III: Ground effects, used properly, are the most effective way to make downforce.

Raffauf: The concept wasn’t new, but applying it to a full-size car opened a lot of opportunity to explore. The rules back then were the opposite of what they are now. Unless it said you couldn’t do it, you could.

Trevor Harris and NPTI aerodynamicist Yoshi Suzuka are credited as the fathers of GTP’s maximum-­downforce breakthrough. Their Nissan NPT-90, conceived and built in Southern California, was a hulking statement of intent whose enlarged underwing generated hellacious suction.

Geoff Brabham: And we ran a huge rear wing on the back. It was like a barn door.

It worked. Nissan clinched the 1990 and 1991 championships with the first model to produce 8000 pounds of downforce.

The NPT-90 was IMSA’s first drag-be-damned GTP car. But the Riley family reset the bar with the Intrepid they drew for GM-factory affiliate Pratt & Miller Engineering. Among IMSA’s downforce monsters, the Intrepid was King Kong.

Riley: It had 10,000 pounds of downforce.

With 800 hp and gobs of torque from its naturally aspirated Chevy V-8, the Intrepid’s ­argumentative visuals—a symphony of vertical battering surfaces—articulated GTP’s bold new direction.

Kendall: It just strikes you as a blunt instrument. Once Bob Riley made the decision that the downforce was worth it, he went all in.

Compared with the NPT-90, the Intrepid’s nose was steeper; everything that could produce downforce was dialed up to cartoonish levels, including the voluminous underwing and its giant ground-effect tunnels.

Riley: A lot of dogs could have lived beneath those tunnels.

The Intrepid’s maiden track test was a revelation.

Riley: Tommy radioed and said, “I have a flat. I’m just gonna stop on track because I don’t want to hurt anything.” So we went out there, and the cords were sticking out of the sidewalls! Like, it burst the sidewalls of the tires. The Goodyear engineer said, “We’ve never seen that before.” We were blowing sidewalls out of the tires that everybody else was using.

Goodyear had to start making stronger tires because of the Intrepid.

Kendall: People laughed at the Intrepid early on, thinking we’d gone overboard on downforce. Watkins Glen was a good example. Bernd ­Schneider was on pole in a Porsche 962, and he was 34 mph faster through the traps than me. Schneider did 212, and I was 178. We were hardly accelerating on the straightaway.

Despite the massive top-speed disparity, the Intrepid’s peerless cornering put it third on the grid, just 0.242 second shy of Schneider’s streamliner.

Riley: The Intrepid didn’t have the power the turbo cars had, but we had so much more downforce, it could make up the lap time.

In 1991, the Intrepid recorded six poles from 13 races. No IMSA track was a better incubator of GTP’s evolving downforce and resultant warp speeds than Connecticut’s tiny 1.5-mile bullring, Lime Rock Park.

All American Racers’ Drake Olson was the quickest qualifier at the 1990 Lime Rock race in his Eagle HF-89 Mk II with a lap of 48.701 seconds.

Kendall: Our lap was 44.647 seconds.

On his debut with the Intrepid in 1991, Kendall knocked 4.054 seconds off of Olson’s year-old standard.

Kendall: That qualifying run at Lime Rock is one of the absolute highlights of my life. Probably the most extreme lap I’ve done. I remember leaving the track and thinking, “There’s never been a bag of skin and bones that’s gone around Lime Rock faster.”

Unbeknownst to Kendall and the Intrepid team, new monsters lurked.