Still on my Challenge “Top 5.” The car was inspiring, terrifying, and HILARIOUS to watch on the auto-x course. And the team was super cool. Wish we’d had the chance to duke it out with them on the strip :/
Never underestimate the power of a plan, particularly in competition. Every team that competes in our $200X Challenge events does so with some degree of premeditated intent. One peek at the Spotter’s Guide shows that the complexity of these schemes varies greatly from one team to another.
For some, the game plan consists simply of finding a cheap car, putting some cheap parts on it, and having a blast at the event. Way at the other end of the spectrum are the grandiose machinations dreamed up at 2 a.m. after one too many beers. These complicated projects commonly end up as barely operational metal abominations or hilarious, long-term garage art.
There is a sweet spot, however. We’ve noticed that the top finishers at our low-buck Challenges typically start with a solid, realistic plan and do their best to stick to it.
Considering the tight budget at a $200X Challenge, a great deal of flexibility is often required in the execution phase. Still, the closer the end result is to the initial concept, the better. Like a professional photographer composing the shot in his mind before he triggers the shutter, masterpieces are the result of foresight, solid technical skill, and a measure of good fortune.
At last October’s Kumho Tires Grassroots Motorsports $2007 Challenge presented by eBay Motors and CDOC, most of the Challengers showed up the Thursday before the event to register and to turn in their build books. These books typically contain pictures of the team’s build, along with receipts and spreadsheets detailing the budget. Some go as far as to include backstories and build diaries.
Build books come in very handy when we’re writing about the cars for the magazine, but to be honest, registration is a hectic scene. With the line backing up, we're more worried about getting everyone the right wristbands than we are about flipping through the build books. As a result, the books have to wait until Friday for analysis.
In a way, putting them aside preserved the amazement we enjoyed when Team Cheaparral rolled their No. 66 Corvette off its trailer on Friday morning. In addition to the usual collage of receipts and photos, the Cheaparral build book was full of atypical entries that would have spoiled the surprise.
For example, there was a very professional spreadsheet with 12 potential engine options for the V8, ranging from carbureted with nitrous injection to twin-superchargers, complete with estimated horsepower figures, costs, quarter-mile times and efforts required. Several of the pages were devoted to defining the task leaders, members and schedules for no fewer than nine project subteams; a space shuttle launch can’t be much more complicated than this.
Then there was the occasional math-heavy page complete with titles like “Estimate Fan Requirements for Zero Speed Downforce System.” Exactly what kind of project was this, anyway?
In the simplest terms, the Cheaparral 2J-2 is a 1986 Chevrolet Corvette powered by a twin-turbo V8 and modified with a special undertray capable of generating 1000 pounds of vacuum downforce at any speed—the name Cheaparral being an homage to Jim Hall’s famous Chaparral 2J sucker car. The Cheaparral dominated at the autocross and blew the judge’s minds in the concours, and the whole enchilada was built for just $1920.70.
The Cheaparral team obviously spent a ton of time on this project even before they had a car to wrench on, but the effort was not an abnormal stretch for team leader Stoyan Lokar and the dozen or so fellow Procter & Gamble engineers who contributed to the project. For them, the Cheaparral was a way to have fun and do some team building while flexing their motorsports muscle.
“We develop machines that make P&G products like Tide, Crest, Pringles and Duracell batteries,” Stoyan explains. “We do a lot of different things. We talked about all the build-off shows on TV, and we said, ‘Hey, we could do something like that.’”
While the team was still brainstorming about the type of project they’d like to do, one of the team members brought the GRM Challenge to their attention. In no time, the Challenge was near the top of the spreadsheet listing potential motorsports projects.
The biggest draw of the Challenge was the competition aspect. “Not only would we get to build something, but [we would be] comparing it to other people doing the same thing,” reflects Stoyan.
The team ultimately had to choose between building an Ariel Atom-type naked car or a $2007 Challenge car; their Car Project Decision Analysis sheet shows the Challenge winning by a 13 percent margin. (Seriously, we’re not kidding about all the spreadsheets and tables.)
With their course set, the team dug in and started their $2007 Challenge project in earnest. Er, that is to say, they did more research. First, they compiled a table of the top, middle and bottom percentile performers from previous Challenge events. Using this information, they came up with target scores for the autocross, drag and concours segments that they felt would result in a victory.
Armed with a specific goal, the team started hunting for cars on eBay and in the local classifieds. They pondered everything from the BMW 2002 to more standard Challenge fare like the Mazda RX-7 and Porsche 944.
Lightning struck when they spotted a rolled, salvage title 1986 Chevy Corvette in the Tradin’ Post classifieds for $2000. As a bonus, the Vette was not far from the team’s home base in Cincinnati, Ohio.
“The Corvette was probably the eighth or ninth car that we ended up looking at,” Stoyan recalls. “It was already February. We realized we needed to get the car fairly quickly. The body was in terrible shape, but the engine was intact and it started right up. We were able to negotiate on the price—the guy’s wife really wanted to get it out of the barn. He came down to $1400 eventually.”
At this point in the story, it’s obvious that the Cheaparral team can kick butt with spreadsheets and planning. However, it’s easy to assume that they might falter when confronted with the cold steel of a physical build, particularly one as complex as this Vette. As it turns out, these Procter & Gamble wrenchers are good with crazy ideas, but they excel at bringing them to life. Having access to industrial-grade designing and machining tools doesn’t hurt, of course.
With the heavily damaged Corvette in the shop, the team had about six months to execute the remaining five phases of their multistep build schedule. The plan would take them from disassembly and cleaning through final testing and then down to Florida for the Challenge. Along the way, each team member had various tasks to keep the project on track.
It took about six weeks just to get the car running and repaired well enough to obtain a title and registration. Once the wheel bearings, A and B pillars, windshield, lights, and jagged body were addressed, the project moved into the modification phase.
“We were focusing on three main areas,” says Stoyan. “One was how we would get more power—that [Corvette] engine was a dog that year. Second, how do we get rid of a lot of weight? And third, how do we improve handling?”
An untold fourth category—one that we see on a lot of Challenge cars—was to implement something that would make every team member chuckle a bit when they saw it. For Team Cheaparral, the idea came to them as they finished off a pony keg of Blue Moon Belgian White ale. Not coincidentally, that very same pony keg was under the knife in no time, and it would serve from that point forward as a fuel cell and a punch line for their creation.
For power, the team was hoping to improve the peaky torque curve of their early injected C4 Corvette. After going over a dozen engine options, the team settled on the relatively inexpensive but complex dual turbocharger option.
“We picked the small Garrett T25 off Mitsubishis,” Stoyan explains. “They’re very cheap since everyone is trading them out for bigger ones. They were spinning up instantaneously—as soon as you hit the pedal they’re right there.” The turbos ended up costing just $71 for the pair.
It took a bit of trial and error to get the oil pressure for the turbos just right, and in the process they fried one of the turbines, breaking its shaft in half. Fortunately, no fragments went into the engine, and they had plenty of time to source and install a replacement.
The team made their own turnbuckle-adjustable wastegates and settled on 6 psi of boost. They figured this pressure was a tad risky considering the cast pistons, but still doable. They expected the setup to net them more than 300 horsepower on the stock internals at a low 4000 rpm redline. Torque from the big, turbocharged engine was not going to be a problem.
Reducing the mass of the Corvette was fairly easy in comparison. The team did it the old-fashioned way, simply removing anything that wasn’t necessary for the competition. Since taking the car apart and cleaning everything was an early step in their plan, it was merely a matter of not putting the heavy bits back in. This allowed them to keep the competitive weight right around 3000 pounds in autocross trim, or 2800 pounds in drag trim.
That 200-pound difference is the kicker. It accounts for the Cheaparral’s secret weapon, an ingenious, low-buck, vacuum-generated downforce assembly.
“For the handling, we were looking at all the usual things, like suspension and ground effects,” Stoyan recalls. The team realized that they couldn’t generate enough speed at an autocross to make the usual aerodynamic aids worth their cost or complexity.
“Then we thought, ‘Didn’t Chaparral do something with a blower?’” Stoyan recalls. “One of the engineers, Cliff [Papsdorf], started doing some calculations to get various g numbers. It looked like it was doable.” The team spent time modeling the airflow and then built a test mule for proof of concept.
“We got some industrial blowers that we use for various machines, hooked up a five-foot-diameter blower with a 50-horsepower motor, and did some tests to see how close to the ground it had to be and what kind of force was needed,” he explains. “We knew it was feasible, but we had to figure out how to get it into a car.” Finding a compact but powerful blower within the allotted budget was the biggest difficulty the team faced.
“We at one point thought about building our own blower, but as the leader I was saying, ‘No way in hell.’ The clock was ticking,” Stoyan recalls. They were a week away from giving up on the idea when Cliff, prowling the Internet late at night, found their solution in an unexpected place—the back of an M1 Abrams main battle tank.
“He found this tank exhaust blower being sold for scrap metal surplus,” Stoyan remembers with a smile. “He called the company that builds the blowers to get some fan curves and he ends up talking to the engineer who designed it.”
Amazingly, this compact, high-flow, lightweight alloy vane-axial, two-stage fan cost the team just $26.50. “I’m sure the government pays dearly for those. We basically picked it up for the scrap metal value. It had met its service lifetime.”
The blower was far from pristine, however. The team suspects it had seen duty in Desert Storm, as the unit was packed with sand and generally in rough shape. They polished the vanes, rebuilt the bearings and did their best to cope with galling on the shaft. Stoyan estimates that 20 man-hours went into refurbishing the blower.
Using computer-aided design programs, Cliff and the others came up with plans for the under-car skirt design and how to mate the blower to a used snowmobile engine. The blower is fairly exotic, but the rest of the assembly is primarily made of plywood and off-the-shelf hardware, like casters. These more mundane elements had the important duty of keeping the whole assembly rolling and preventing it from sucking itself completely to the ground.
In operation, the setup is surprisingly simple. A big square tray is attached to the bottom of the car with a simple suspension. The tray rides on casters that keep it the right distance off the ground for the target vacuum, and a flexible skirt creates the airtight seal between the tray and the bottom of the Vette.
When the blower is activated, air is sucked out of the tray’s cavity, creating a region of reduced pressure between the car and the ground. As a result of the vacuum, the underside of the car wants desperately to snug up to the asphalt.
At an operational vacuum of 10.0 inches of water, the Cheaparral’s sucker design yields an awesome 1000 pounds of downforce with an inertial penalty of just 200 pounds of hardware. The whole system can be removed or installed at the track in about an hour.
There must be something to all those schedules, tables, spreadsheets and charts, because Team Cheaparral was able to complete their project on time and under budget. “We were essentially complete about two and a half weeks before the race,” Stoyan says. “We were able to get a couple of weekends to give Danny [Popp] some seat time.”
From the moment the Cheaparral rolled off the trailer and people realized what the undertray was for, the car was surrounded by Challengers poring over its secrets. The concours judges took note of the engineering genius, as well as the fit and finish of the build, and awarded it first place in the concours. And when the Vette pulled up to the line for its first autocross run, all ears perked up at the unmistakable high-pitched whine of the snowmobile engine driving the tank blower.
A 30-foot tornado of sand and debris shot up from the passenger-side blower as driver Danny Popp muscled the Cheaparral to a 33.208-second time—good for fastest time of the day—on his very first run at the autocross. Unfortunately, a dash screw had shaken loose on the bumpy trailer ride down from Ohio and found its way into the throttle pedal pivot. This caused the throttle to stick, which botched the car’s second and third runs. The blower motor cut out on the final run, but the team had earned enough points to take the overall $2007 Challenge victory.
Not surprisingly, the overall project’s six-phase time chart in Team Cheaparral’s build book already had small blocks of time set aside for a “victorious return” and “Corporate Engineering Technology Lab celebration” in the two days following the $2007 Challenge. Talk about sticking to the plan.
From the very start, Team Cheaparral wanted the end result of their project to benefit charity. To achieve this goal, they anticipated auctioning off the final result or selling it to a museum and giving the proceeds to a local cause. That’s still the eventual plan, but in the meantime their radical sucker Vette is going to be a teacher, too.
“We’re going to a local school to talk to the students about the physics that went into the sucker system, to explain what centripetal force is, what friction is—bring what they’re learning in class to a real-world example,” says team leader Stoyan Lokar. “The teacher thinks the kids are gonna eat that up because it’s such a cool-looking car. It’s a goodwill ambassador to encourage others to use creative science.”
If you’ve been reading GRM for a while, you’re probably familiar with the name Danny Popp. Back in June 2004 we featured his A Street Prepared-winning Z06 Corvette, and more recently Popp took a first in class and second overall—just behind Tommy Archer—in our 2007 Ultimate Track Car Challenge.
Popp was actually pondering the possibility of a C4 Corvette for a $200X Challenge when he was contacted by Cliff Papsdorf, one of the Cheaparral engineers, to consult on their own $2007 Challenge Corvette. When Cliff explained that they were going to attempt to make the Vette a sucker car like the Chaparral 2J, Danny decided they were a bunch of loons.
“I was right,” laughs Danny, “they certainly are loons, but they’re very innovative loons.”
When the build was nearing completion, Danny was invited to test drive the car. “It was significantly later at night than we had planned, so we could no longer set up an autocross in the lot. But we could set up a skidpad.” One 250-foot diameter skidpad later, Danny put in some baseline runs without the aid of the blower. “I was quite pleased. [The car was] a little loose but still good.”
Then it was time to try the skidpad with the sucker motor on. “Cliff fired up the snowmobile motor and I was in sensory shock,” Danny recalls. “That thing was loud. The only thing you could hear was the [snowmobile] motor running—it made shifting by ear impossible.”
What the system lacked in NVH comfort, it made up for in performance, cutting 1.5 seconds—about 10 percent—from the skidpad lap time. “Incredible. I’ve never seen anything like it,” Popp says.
Three days later they set up a 30-second autocross course. “My dad brought his B Street Prepared Corvette over to baseline the car against—LT4, six-speed, 315s up front and 335s in the rear. Fast,” Danny explains. Without the sucker motor, the Cheaparral was about 1.6 seconds behind the BSP Vette. With the fan running, the Cheaparral bested the race car by 1.4 seconds, meaning the blower was worth about 3 seconds—10 percent—at the short autocross.
So what’s it like to drive a vacuum-assisted twin-turbo Corvette?
“I had to drive off feel only. There’s no noise other than the sucker,” Danny says. “It took some time to get used to just how capable the car was for a beater car. The biggest area of improvement was obviously handling—the braking potential was never realized at the speeds we were traveling. Acceleration was not harmed whatsoever.”
The only element that was negatively affected was the power steering, he continues. “It was marginal to start with, and it did not like the additional 1000 pounds of downforce. It was not linear, either. These are the kinds of surprises that you don’t like, but there was no money in the budget to fix it.”
Popp insists that little snags like the loose screw in the throttle pedal prevented a demonstration of the Cheaparral’s true handling potential. “There was more—a lot more,” Popp assures us. “At least two more seconds on the autocross course.” Considering Popp’s level of experience, the unusual grip level of the car and the fact that he set FTD on his first run, we’re inclined to believe him. “Maybe next year,” he grins.
Still on my Challenge “Top 5.” The car was inspiring, terrifying, and HILARIOUS to watch on the auto-x course. And the team was super cool. Wish we’d had the chance to duke it out with them on the strip :/
I took from this, "they made one solid run with the sucker system working." Yep, sounds about like Chaparral, and despite the troubles, it's still likely my favorite challenge car.
I think ol' Jim would have been proud.
Now I just need to get the idea of doing this myself with an auto-x car out of my head... the math alone is likely out of my wheel house. I do know a few engineers though... =P
So nobody else has done a similar car since? Or did it suckcessfully 
emulate the Chaparral 2J by also getting its brand of innovation banned?
We have to run a blower at our autocross courses to clean off the racing line, so GRM just needed to let this car run first.... Of course it probably could use a rock-collector bin too.
In reply to PT_SHO :
IIRC they did run pretty early, and the rooster-tail was berkeleying glorious.
This is the car that started my obsession with the challenge and building cheap cars.
Thanks for the reminder to stick to my plan.
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