So close but yet so far, Project Fox developed a knack of running consistent 10.0s at LMP.
With our test session coming to a close, we were tapped out of ideas on how to improve the car while at the track. Clearly, the engine and chassis combination had leveled off in the 1.5-second range, and all it needed was a bit more grunt coming out of the hole. Unfortunately, the converter and footbrake combo wouldn't oblige. If not for the nasty headwind, or the extra 20 pounds of processed Big Macs and cheesecake hanging off the driver's midsection, it's quite possible that the Mustang would have picked up the two-hundredths of a second it needed to post a single-digit pass that day. We were that close. Lame excuses aside, the truth of the matter is that whether Project Fox ran a 10.01 or a 10.99, it's still just a 10-second car.
On the cusp of 9s, we determined that the cheapest and easiest way to knock our 60-foot times down was with a transbrake. Based on how hard similar combinations to our Mustang launch, with an optimized chassis tune and torque converter, Project Fox should be able to pull 1.3- to 1.4-second 60-foot times all day long. To get Project Fox part of the way there, we ordered a transbrake from TCI. By engaging First gear and Reverse at the same time with a solenoid, valve, and an electric switch, a transbrake offers several advantages over footbraking. The most obvious benefit is that it holds the car stationary in the staging beams, allowing an engine to rev to a higher rpm for a more aggressive launch off the line. Furthermore, transbrakes eliminate the loading of the chassis and suspension that results from holding back driveline torque with a footbrake. The result is cleaner, more consistent launches. The TCI transbrake bolted right up in a couple of hours, and then we headed out to San Antonio Raceway for another round of testing.
It doesn't look much different from a standard Turbo 400 valvebody, but the TCI transbrake
Expecting great things, we staged the car, stalled up the converter to 3,500 rpm, and popped the transbrake button. Presumably due to the additional shock of the harder launch, the Mustang shook the tires so violently that we had to get out of the gas. The car registered a miserable 1.84-second 60-foot, and we didn't bother driving it out the top end. Of the dozen or so passes the car had logged thus far, this was the first time it experienced tire shake, which happened to coincide with launching off the transbrake for the first time. It didn't take a genius to figure out that the two were related, so we convened with the Bill Buck Race Cars crew to try to correct the situation.
Although vehicles as diverse as street/strip machines, 2,500hp Outlaw drag cars, and 6-second Pro Stock machines have all been battling tire shake for decades, eliminating it is still somewhat of a black art. After a tire's sidewalls distort and wad up at launch, the energy released as they "unwind" back into shape sends violent shock waves through the tires and chassis. This affect is greatly exacerbated with drag radials compared to bias-ply slicks due to their stiffer sidewall construction. With marginal track conditions where a slick would still hook, radials have a greater tendency to go into tire shake. In essence, the chassis and engine tune in a car that experiences shake isn't so far off that it blazes the tires, but it does allow for enough spin to create the condition in the first place. That's just a fancy way of saying that Project Fox's setup was close, but not quite all the way there. "Tire shake can be caused by not having enough power, having too much power, or hitting the tires too hard out of the hole," says chassis expert Bill Buck of Bill Buck Race Cars. "Drag radials and slicks require different setups to prevent tire shake. With drag radials, setting the rear shocks too soft or running the air pressure too low is usually the problem."