With the introduction of the sporty '05 Mustang came something Mustang fans weren't used to-a hydraulic-clutch mechanism. Rather than having a cable connected between the clutch pedal and the fork to release the clutch disc for shifting, the S197 uses hydraulic fluid. This makes the clutch silky smooth but can cause problems in high-performance applications. So, we'll ask, is your transmission grinding and hard to shift under aggressive driving situations? If so, the transmission in your modified S197 Mustang probably isn't broken; it may be in the clutch setup instead.
We wanted to throw that out there because there have been a lot of misdiagnosed transmission problems lately. There's an epidemic in the growing S197 community-grinding gears that are hard to engage and disengage. In our search for a solution, we spoke to Justin Burcham of JPC Racing. "Everybody thinks it's a problem with the clutch or the transmission," he says. "It's actually a problem with the hydraulic clutch-release bearing, which will eventually lead to transmission problems. Ultimately, the synchro rings get beat up." Unfortunately, there's no way of knowing how many cars experience this problem.
The grinding and difficult shifting is due to the hydraulic clutch bearing that's used to control the clutch application (either in or out). The pedal isn't connected mechanically as in previous generations of Mustangs, which utilized a cable that connects the clutch pedal to a fork inside the bellhousing. The new cars feature a hydraulic unit that Ford engineers designed to make the Mustangs easier to drive. Much like the brakes, fluid is used to activate a mechanisim; in this case, the throwout bearing that presses on the pressure plate to control release and engagement of the clutch disc (or discs in a multidisc setup).
Since a clutch upgrade is a popular modification, many S197 Mustangs now have an aftermarket unit. Poor performance can result due to the OEM hydraulic clutch release system. "The car will shift fine under normal driving conditions, and the driver won't notice the issue until the engine is at a high rpm, or when racing or beating on the car," Burcham says. "When the driver goes to put it in gear or shift aggressively, the transmission will grind and, at times, not let the shifter go into gear because the clutch is still engaged.
"The factory release bearing was designed for the OEM pressure plate. When you add an aftermarket clutch, there's an increase in centrifugal clamping force on the pressure plate in order for the clutch to attain a greater holding capacity. Thus, the spring that holds the clutch is stiffer, and in turn, the factory release bearing can't deal with the increased load."
According to Mike Norcia of RAM Clutches, the static clamping pressure of the clutch (at idle and low rpm) isn't that much greater than the stock one. The RAM 11-inch clutch has 2,400 pounds of pressure compared to the stock 11.5-inch unit's 2,250. As the rpm increases, however, the centrifugal force from the levers pushes the pressure-plate ring down harder. This is where the extra grip comes from, in addition to the better clutch disc material. Norcia adds, "The position of the lever in relation to the centerline of its mounting point (called the plane of rotation) is such that the upward angle promotes the lever forcing itself outwards as rpm increases. In addition, the centrifugal pressure isn't linear. It starts out shallow, around 5,000 rpm, and basically goes up by the square of the engine rpm from there. At 7,000 rpm, the pressure curve is at about a 45-degree relative angle. At 10,000 rpm, the line would be shooting almost straight up."
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