Estimated reading time: 2 minutes
By Evan J. Smith
“Customer states, car will not do donuts,” said Justin Starkey, president of VMP Performance, facetiously. “This is a high-powered, wide-bodied Shelby Mustang that we’re talking about. The customer’s actual concern is that the car will not do donuts, so it came back to us for a diagnosis and repair.”
The Mustang in question is a Shelby GT500 Super Snake producing about 700 horsepower at the wheels. It has custom bodywork, expensive aftermarket wheels, and is worth well over $100,000. At VMP we know the importance of donuts, and we value customer satisfaction, so we got right to work.
“I thought about optimization and wondered if the issue was fuel related,” Justin said. “This Mustang has an aftermarket fuel system that we utilize to support higher power levels. We’ve experienced similar issues in drag racing, so that gave me a clue. In a donut you’re in a continuous corner, using a lot of power, so it could be easy to starve the fuel pump due to high g-loads.
With a stock fuel system, the pump, regulator, pickup, and filter are located inside a bucket that fits in the tank. So, the pickup is always submerged in fuel unless you run the tank very low. “With an aftermarket setup, one side of the saddle tank is used for the pump and pickup, so you run the risk of uncovering it (starving the pump) under high g-load. This is especially true with dual- and triple-pump arrangements,” Justin added.
“So, the first thing that came to mind was fuel starvation. We got the car to VMP Performance, and with the customer’s approval, we replicated the issue. I drove, and Joe Goodnough, lead calibrator at VMP, data-logged.” Just as the customer described, the Shelby Super Snake died out when a donut was attempted in the counterclockwise direction. “We saw a total loss of power from lack of fuel, which was a dead giveaway,” Justin said. Thanks to the computer information, it only took them a few seconds to figure out what was going on.
“We then did clockwise donuts to confirm our suspicion, and the Mustang had no issue.” The Shelby smoked the tires and laid down donuts at will. “After a couple of loops, we filled the tank with 93-octane and came back to try it again. This time we could do donuts in both directions with no problem,” Justin said. Problem solved!
A lesson can be learned by anyone with an aftermarket fuel system in a S197 or S550 Mustang. It’s highly recommended to run at least a half-tank of fuel to ensure the pump pickup remains submerged in fuel. Running the pump dry is never a good idea, as you can experience a hiccup and loss in performance.
“Usually, when this happens it’s so abrupt that the car will just stumble, but if you have so little fuel that combustion can’t occur then you won’t generally blow up,” Justin explained. “It’s like a rev limiter. There’s no fuel, so nothing can happen. Of course we try to avoid that,” he added. While you may not hurt the engine, it kills performance and that’s never fun.
