The clutch. There is no more critical component of a drag racing vehicle, and in no other motorsport is it so important. Whether it is a Pro Stock, Pro Modified, or a 7,000 horsepower nitro-burning Funny Car or Top Fuel dragster, nothing else in drag racing determines the outcome of more runs down the track. Too little clutch, and the car won’t “get up on the tire” or make a fast run. Too much clutch, and the end result is likely tire shake in a Pro Stock or Pro Mod, and tire spin in the fuel classes. Go to any drag strip around the country, or the world for that matter, and you are bound to hear racers and crews talking about the clutch in some sort of lingo: “we were light on that run,” “she was locked up on that one,” “it was pretty soft,” or “welded it that time” are just a few of the terms you are likely to hear.

It is the key component that seperates winning from losing...

The clutch. There is no more critical component of a drag racing vehicle, and in no other motorsport is it so important. Whether it is a Pro Stock, Pro

Modified, or a 7,000 horsepower nitro-burning Funny Car or Top Fuel dragster, nothing else in drag racing determines the outcome of more runs down the track. Too little clutch, and the car won’t “get up on the tire” or make a fast run. Too much clutch, and the end result is likely tire shake in a Pro Stock or Pro Mod, and tire spin in the fuel classes. Go to any drag strip around the country, or the world for that matter, and you are bound to hear racers and crews talking about the clutch in some sort of lingo: “we were light on that run,” “she was locked up on that one,” “it was pretty soft,” or “welded it that time” are just a few of the terms you are likely to hear.

To the average person, drag racing is just mashing the gas and driving in a straight line, right? And what is all this talk about the clutch? Don’t you just let it out to get the car going and push it in to change gears?

Sometimes, professional drag racers at all levels wish it were only that simple.

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It almost can’t be stated any better than the way Texas Top Fuel driver Bill Ancona did: “In every class, the race is won or lost in the bell housing.”

In drag racing, the clutch acts as the buffer between the power produced by the engine and the tires trying to apply it to the track surface. These cars, especially the fuel cars, are making so much power that it can’t possibly be put to the ground all at once. There has to be something to take that power and apply it progressively through the RPM range or through the run, and allow the car to launch from a dead stop with a percentage of the engine’s power then apply 100% of it when possible.

If this were a completely technical article, it could go on for days and still barely scratch the surface of the complexity and intricacies involved in the mechanics and application of the clutch in drag racing. Instead, it should prove far more interesting and accessible to give a simple overview of how both Pro Stock/Pro Modified and Top Fuel/Funny Car clutches work, the basic similarities and differences between the two, and the way the two types are actually driven inside the respective cars. In basic terms, both clutches work by a set of levers that apply clamp force through the pressure plate, therby compressing the clutch discs and floaters together.  The levers are in turn engaged and disengaged by the throwout bearing. This acts as the gateway for the power of the engine’s crankshaft to move through to the transmission, driveshaft, third member, and eventually the rear tires. In Top Fuelers and Funny Cars, no transmission is used, so the power goes directly to the driveshaft and third member.

“Both clutch styles rely on RPM to ‘activate’ them,” said Ancona. “The faster you spin both of them the greater the application of the counterweight that is attached to the levers.”

But, as Ancona added, “That’s about where the similarities end. To begin with, the Top Fuel clutch actually works completely backward from the Pro Stock or Pro Mod style clutch while at the same time, in principal, performing the same function. By that I mean there are some differences in how the clutch is applied, however the end result is the same. Both apply friction (clamping force) via levers and counter weights.” Ancona stated, “The Pro Stock or Pro Mod clutch uses springs (typically four or six) to apply base pressure when the clutch pedal is initially released. In contrast, the Top Fuel clutch uses similarly placed springs to keep the clutch slipping with the pedal out. As you can see, these two functions are completely opposite.”

In a Pro Stock or Pro Mod style clutch, also known as a pedal clutch, releasing the clutch pedal pushes the throw out bearing in. But in a fuel clutch, also called a glide clutch, the process is vastly different. Ancona explains, “The throw out bearing is controlled by a series of flows that are activated by air-over-electric timers. Based upon a pre-set timing sequence that begins off a wide-open throttle switch on the gas pedal, the throw out bearing moves more (or less) quickly, which applies more (or less) clutch as the car moves down track. The speed of the throw out bearing also determines how much the clutch will wear, so you are constantly in a state of chasing the wear, compensating for it, and at the same time applying clutch pressure, which eventually results in lockup. Locking up too soon and/or with an abundance of power results in tire smoke.”

When it comes to the wear endured by the two styles of clutches, differences abound. “The Pro Mod clutch works best when little or no plate material is worn off on a run, and the Top Fuel clutch depends on a predetermined amount of clutch to wear off on a run. On a typical, successful quarter-mile pass, a Pro Mod clutch should wear four to seven thousandths of material off, whereas a Top Fuel clutch will wear of 110 to 125 thousandths.”

Pat Norcia of Ram Clutches offered more explanation of the wear endured in the Pro Mod and Pro Stock classes, but from the extremely important perspective of slippage. “Slippage time or amount largely depends on the track conditions,” he said. “When the track temperature is low, you can lock the clutch up sooner. However, the racetrack does the opposite in that as it cools, the clutch will normally slip longer. This is why you see crew chiefs nervously pacing the starting line on an evening session where the temperature is dropping quickly, or looking for a friendly cloud during midday eliminations that might drop the racing surface temperature five to ten degrees.”

In comparison of the clutch slip on a successful run in each class, Norcia added, “Slippage time, on average, for an IHRA Pro Stock is typically seven to eight tenths. For a Pro Mod, it’s four and a half to six tenths.” But since there are differences in engine combinations in Pro Mod, there are differences in slippage time as well. Norcia continued, “The nitrous Pro Mod time is less due to the second nitrous system coming on, transferring more weight to the rear tires and wheelie bars. The clutch normally locks up right when the second system engages. Blower cars have a little longer slip time, usually eight tenths to one second, because they are trying to harness more power and, with their three-speed transmission, will be in low gear much longer than a Pro Stock or nitrous car.”

It may not sound like much, but the differences in all these numbers are huge. Top Fuel and Funny Car clutch discs slip and wear to the point that they may go back in the car for a second pass, but are usually trashed after one run.

Nothing in fuel racing is cheap, and the same goes for the clutch. When asked about the cost of a complete clutch, Ancona said, “The clutch is anywhere from $10,000 to $14,000 and is comprised of the flywheel, stands, and hat.” That doesn’t even include the constant replacing of the clutch discs and floaters. “In small quantities, clutch discs cost about $135 each, and there are five in most Top Fuel clutches. Floaters cost $45 each, and there are four of those in the clutch. You do the math,” Ancona suggested. This doesn’t hold as true for Pro Stock or Pro Modified clutches, as they can go many more runs due to the nature of the way they are run, with minimal slippage.

There are also several prominent differences in the way the two different styles of clutches are tuned and applied.

In most forms of high-horsepower drag racing, to get the car off the line successfully one of two things almost certainly has to occur: either the clutch has to be slipping or the tires have to be spinning. Charles Carpenter, driver of the Embee Performance 1955 Chevy nitrous-assisted Pro Mod, explained this concept in depth. “In Pro Modified, either slip or spin can occur; on a good race track, the tires have greater traction with the track surface, so they are going to spin less,” said Carpenter. “Because of this, the clutch has to slip. If neither happens, tire shake is the inevitable end result. On a hot, greasy, or just generally bad track, the tires are much more likely to spin, so the clutch is much more likely to lock up.” In comparing this to Top Fuel, Carpenter added, “This is not necessarily the case in Top Fuel. Those guys can only spin the tires so much before they go completely up in tire smoke.”

“In both Pro Mod and Top Fuel, I always say there are two ‘make or break’ points in a run,” Carpenter went on to say. “In Pro Mod, it’s the one-two shift point. 99% of the time if the car makes it past that point, it’s going to complete the rest of the run. In Top Fuel or Funny Car, it’s when they go to apply all their clutch or power, which is usually somewhere around the 330-ft. mark. If they don’t go up in smoke at that point, most of the time they are going to be OK the rest of the run.”

Where tuning the clutches is concerned, the two drivers both weigh in on how they dial in a perfect clutch run, in hopes of seeing that beautifully aligned data graph on the computer monitor back in the trailer after the run.

Carpenter said, “When I am tuning the clutch in my Pro Mod car, we only have a few grams of counterweight that we can apply in either direction.” Going into more detail, he added, “That little bit we take off or add on to the clutch can make a huge difference, especially in my car, as I use an eight-inch clutch. A lot of guys are using ten-inch clutches, which have a slightly larger ‘window’. The eight-inch is smaller and lighter, but has a narrower window in which we can work.”

In IHRA, and NHRA Pro Stock in particular, the window gets even narrower. “The NHRA cars use six and seven-inch clutches, whereas in IHRA they use eight and ten-inch units,” stated Norcia. “NHRA cars can use the smaller units due to their smaller displacement, lower torque, and higher RPM engines.”  

Norcia also believes in taking a look beyond just the clutch when it comes to determining a window. “In terms of looking at the broader view on the clutch, I always keep in mind the transmission and rear-gear ratios, four-link, and the shocks. Taking this one step further, you need to evaluate the weather conditions you'll be racing in, as well as the track temperature and condition. This holds true whether it is a Comp Eliminator car, a Pro Mod, or a Pro Stock. These details just tend to be more scrutinized in Pro Stock.”

“Evaluating all these factors will allow teams to reach their potential each time out and eliminate the hit-or-miss performance that often times occurs,” Norcia said.

“There really is no window when using a contemporary Top Fuel clutch due to the fact that we utilize a cannon to control the throw out bearing movement,” said Ancona, referring to his Top Fuel dragster. “The pedal clutch has a window to work within because after the clutch pedal is released, centrifugal forces (among many other factors) go to work until there is lockup and or it beaks loose again, locks up, repeats, etc.” Ancona continued, “By ‘no window’ what I mean is with a pedal clutch the window is a period of time or spot (by virtue of the engine RPM) that the clutch will lock up, basically no matter what other circumstances are employed. This is not true with a Top Fuel clutch that utilizes a cannon. With the cannon controlling the throw out bearing, you can force the clutch to slip virtually forever.”

Beyond all these differences in the way the pedal and glide clutches work, there is yet another difference that is just as important: the way they are actually driven when out on the racetrack.

In Pro Stock, Pro Modified, and many other racing categories, the way the car is driven is much the same with respect to the clutch. In the more traditional sense, the clutch pedal is depressed when the car is sitting motionless and to move the transmission from forward to reverse, and is released to launch the car at the starting line. If the car is equipped with a Lenco-type transmission, the gears are changed in the car without pressing the clutch, and the clutch pedal isn’t pressed again until the end of the run. Watching a number of in-car cameras, you may have noticed the drivers jerk their foot off the gas and stab the clutch as quickly as possible at the end of the run. This is done to separate the engine from the drivetrain as soon as possible to avoid over-revving and damaging the engine.

Things are vastly different inside a Top Fuel or Funny Car. To illustrate this, Bill Ancona takes us through a typical run in a fuel car. He begins, “When you start the car you have the pedal out, but your foot on the pedal. As the engine comes to life you quickly depress it just to the point where the clutch is disengaged. This sometimes causes the car to roll or rock a little if the timing of this action isn't perfect. You let the clutch out and start the car moving forward, but it’s slipping a ton.”

Beginning the burnout, Ancona explains, “You hit the throttle, the primary levers engage the clutch, and the car spins the tires. When you let off, the clutch goes into a slip mode again, but it is also dragging. As you come to a stop you put the pedal in, stop the car, change the reverser direction, let the pedal out, and the car begins to back up. After the car is rolling backward, you put the pedal in again and coast back to the starting line area. This is done so as not to build too much heat in the clutch. Then you stop behind the starting line and do basically the same thing as out on the track.”

To get the car up to the staging beams, Ancona continues, “You put the clutch in, shift back into forward and after you get the signal that you are cleared to stage, bump the pedal out to get the car rolling forward towards the pre-stage beam. Once pre-staged, you deck the clutch and wait for the opponent to do the same. When both cars are pre-staged, you hesitate a very small amount and pull the fuel pumps on full, grab a handful of brake, hesitate another very small amount, and let the clutch out completely. The car will pull against the brakes, and you work the brakes to bump the car into the stage beam.” Now for the fun part: the launch. “At the first flash of anything amber, green (or the flash from someone taking your picture),” Ancona jokingly adds, “you mash the gas. You never touch the clutch pedal again. Not even as you roll off the track and come to a stop.”

While drag racing may appear simple to the average person, and the clutch may appear simple to the casual drag racing fan, the truth is that this is definitely not the case. The margin for error in drag racing is extremely small, and it gets even smaller when it comes to the clutches in professional-level cars. And somewhere in that small margin, there is an almost innumerable amount of differences between the pedal style and glider style clutches.

“In short there are not many similarities between a Top Fuel clutch and a Pro Stock or Pro Mod clutch except they both move the car by slipping followed by locking up,” Ancona surmised.

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