“Let's talk a little bit about why you want a proper limited slip and why you do not want to weld your differential.” Those are the wise words spoken by Grassroots Motorsports’s Tech Editor JG Pasterjak in the latest installment of GRM’s Path to the Podium YouTube series, which takes an S197 Ford Mustang and turns it into a Club Spec contender.
SCCA’s Club Spec category is a build once, run everywhere philosophy when it comes to competition. Club Spec cars are designed with spec (or near spec) kits people can outfit their cars with, and then compete in SCCA Autocross or SCCA Time Attack with the same set of rules. There are currently two chassis classed in Club Spec, the NC Mazda Miata and the early S197 Ford Mustang GT, with the 718 Porsche Cayman joining the fold in 2027.
SCCA's Club Spec webpage contains class rules, buyer's guides, and more.
GRM is building an S197 Mustang GT with the intent to take the company’s Video Editor Chris Tropea (who is relatively new to driving on the track) to the 2026 Tire Rack SCCA Time Attack National Championship in October.
This is the third video in the multi-part series (you’ll find the first video here, and here’s the second).
While Club Spec includes the word “spec” in the name, the rules actually allow for a certain level of flexibility, and this is exactly the case with the S197’s differential. There, the rules list two clear options.
“In Club Spec Mustang, we have a few options,” Pasterjak explained. “We can use our regular stock clutch-type limited slip. We can actually replace those clutches with a higher performance carbon fiber option rebuild kit, because ultimately, what's going on here is that the clutches … are worn out. The diff isn't bad … those clutches, just like the clutch that connects your engine to your transmission, get worn out, and it can no longer couple the torque from one side to the other.”
GRM chose the other option.
“We opted for a more permanent solution. Which was an Eaton Truetrac diff,” he said.
The Truetrac utilizes gears rather than clutches, making this a relatively service-free option.
“I think for our situation where we've got a bunch of drivers of different skill levels, you're not going to notice that I'm driving with a different type of differential,” Pasterjak noted. “You're just going to notice the car comes off of corners really well [and] the car goes into corners really well.”
The video includes not just theory, but also installation, a track test, and then sorting through data obtained during the time on track.
Check out the video:
Photo courtesy GRM’s YouTube channel
