Testing The Traction On A GTO and Formula - Hookin' Up A Goat(And A Bird) On A Budget

Don't fear. When the dense smoke finally dissipates, the second part of the Mickey Thompson ET Street Tire install and test will magically appear, transporting you to a time and place filled with the intoxicating sights, sounds and smells of Pontiac performance, the hallowed shrine known world-wide as the dragstrip.

Whether powered via an LS1 or LS2 motor, it doesn't take long to obliterate the stock tires on a Firebird or GTO, especially when you are driving aggressively or trying to maximize performance on the track. For late-model GTO owners, the diminutive size of the 245/45ZR17 tires further exacerbates obtaining maximum traction as a result of the OEM design of the rear wheelwells.

In Part I, we showed you how to put a large, super-sticky drag radial tire under a late-model GTO without "rolling" the inner fender and potentially reducing the collector value of your Pontiac. To accomplish this, we mounted M/T ET Street Radial tires in a 275/40-17 size ($189 each) onto a set of used 17x9 WS6 wheels ($200 for the set) and machined them ($75) for safety purposes. A set of BMR's GTO drag bags ($99.95) were then installed to allow the rear suspension dampening to be modified, providing adequate tire clearance on the '06 GTO owned by Bob Cook from Allen, Texas.

In order to test the merits of the M/T ET Street tires vs. stock tires, we brought out the nearly stock subject automatic-equipped '06 GTO and the author's own modified 6-speed '00 Formula WS6. As you may recall from previous articles, this rare Bird was transformed into a fire-breathing monster with the addition of an STS rear-mounted turbo.

When the smoke clears and the tires are tested to within mere inches of their lives, will the on-track prowess of the M/T tires for stock and modified late-model Pontiac applications be revealed?

Strip Testing
Testing was conducted on a warm spring day at Texas Raceway in Kennedale, Texas, a modern 1/8-mile racing venue in the Dallas-Fort Worth area. Temperatures varied between 68 and 74 degrees with partly sunny skies, light winds between 6-10 mph and humidity of 54 percent. The Texas Muscle Car Club Challenge Series was holding the first race of 2007 and the organizers, along with Texas Raceway, graciously allowed HPP to conduct tire-testing between rounds of the event.

Rather than simply launching at a pre-set rpm on both cars and documenting the 60-foot time differences, it was determined that it was necessary to establish an rpm threshold to find out when the stock tires would start spinning, thereby reducing performance.

To that end, both sets of stock tires were set to 28 psi by using the same gauge and were tested at various rpm in order to find out where they would get "blown-off." Once the baseline was established for the adhesion limits of the stock tires, the strategy for testing the M/T tires was to launch at or around the rpm where the stock tires would spin. Then the rpm was raised to find out how much additional power the drag radials could hold before spinning themselves.

Tire pressure was varied between 12 and 18 psi for the testing of the M/Ts to account for differences in weight between the two cars and to find out if the tires performed better at higher or lower pressures.

2000 Formula WS6 Tire Test
The '00 Formula WS6 was relatively light, weighing in at 3,242 pounds with a half-tank of gas. Equipped with an STS rear-turbocharger, the LS1-based Formula had 392.4 rear-wheel horsepower and 409.3 lb-ft of torque on tap at the tested boost pressure of 5.0 psi. Shifting was done via a stock T56 manual transmission, and the clutch was previously upgraded with an Exedy Twin-Disc unit to withstand the added power levels. The 10-bolt rear end was beefed-up with aftermarket axles and an Eaton Posi unit (HPP, Nov. '06).

Unlike OEM-style clutches, a twin-disc clutch is not designed to be slipped to modulate power coming out of the hole. Its intended purpose is to provide firm, positive clutch engagement and to withstand high-rpm launches.

With the stock tires, the Formula bogged on launch at an rpm of 2,300 or less. Since a bog off of the line increases 60-foot times, the rpm was raised to 2,400, whereby the tires started to spin and 60-foot times degraded further. Unfortunately, without the ability to "slip" the clutch, finding the exact rpm that doesn't produce a bog or spin is elusive. As rpm was increased at launch to 2,500 or greater, the tire spin intensified. Once the spin worsened, the tires heated up, resulting in the Formula walking from side to side on every shift, leading to poor 1/8-mile times.

When the stock tire-testing concluded, the best 60-foot time that could be mustered was a 2.21, coming out at 2,300 rpm with an 8.43 1/8-mile time. On the other end of the spectrum, a 3,000 rpm launch roasted the tires, resulting in an abysmal 2.67 short time with a horrid 10.43 1/8-mile time.

After switching to the M/T tires and setting the air pressure to 14.5 psi, the Bird was launched at the optimal rpm for the stock tires (2,300), which resulted in a 2.26 60-foot time. Right away it was apparent that the Mickey Thompson tires were getting more traction as the bog coming off of the starting line was significantly worse. In order to compensate for the bog, the rpm was increased all the way up to 3,000, whereas the stock tires produced their worst 60-foot time at 3,000 rpm. The M/T tires put down a 2.29 60-foot time. Although not as fast as the best stock run, the results were encouraging.