Stick Shift Adjustable Clutch SetupCounterweight or No Counterweight???

One of the bottom-line requirements of any stick shift drag racing clutch- it must have at least enough clamp pressure to keep from blowing thru the clutch after the shift into hi-gear. That's the point in the run where the clutch sees its maximum load. What many don't realize is that any clutch clamp pressure, beyond what's required to hold after the shift into high gear, is actually counter-productive. That's because excess clutch clamp pressure reduces shift recovery efficiency. The root of the problem is there's a big inertia energy release that must go somewhere quick any time the engine gets pulled down by the clutch. If the clutch pulls the engine down too fast, that inertia energy exiting the engine's rotating assy will hit the drivetrain with excessive impact, which in-turn leads to spin/bog/broken parts problems during launch. Then after the shifts, the clutch pulling the engine down too fast can either knock the tires loose or lower the engine's recovery rpm. Managing the rate that rotating assy inertia energy hits the drivetrain is the key to putting that energy to work as efficiently as possible, and you do that by adjusting the clutch tune. Since there is more than one way to set up an adjustable clutch, the question here is- "Counterweight or No Counterweight?" For the purpose of this discussion, let's suppose a minimum of 2200lbs of clutch clamp pressure is required to hold a naturally aspirated engine's output as it falls back from 8000rpm to its torque peak of 6000rpm after the hi-gear shift. Below are examples of two different ways to get that 2200lbs of clutch clamp @ 6000rpm out of the same adjustable clutch. Nice round calculated numbers that do not account for pressure creep due to centrifugally induced parts deflection, just to make the critical differences easier to grasp... Example 1- adjustable clutch with a typical "counterweight" clutch tune- let's set this centrifugal assist clutch up with 900lbs of static clamp pressure. With 900lbs of static, it would need an additional 1600lbs of centrifugal assist by 6000rpm in order to achieve the 2200lbs of clutch clamp required to hold the above engine @ 6000rpm (maximum torque). Basically, 900 static + 1300 centrifugal @ 6000 = 2200lbs @ 6000. While the 900lbs of static clamp pressure will stay the same as rpm climbs, that centrifugal assist contribution of 1300lbs @ 6000 will grow to 2311lbs @ 8000 shift point. That 2311ftlbs of centrifugal, combined with the 900lbs of static, equals a total of 3211lbs of clamp by the 8000rpm shift point. That's over a thousand pounds of excess clutch clamp pressure pulling the engine down way too fast after the shift.
With the counterweight clutch tune, clamp pressure grows beyond what is needed, which reduces efficiency after the shifts. What the counterweight clutch tune offers in return is improved 60', due to the ability to launch the car consistently from near the engine's torque peak. The reason a counterweight clutch tune is quicker than a no-counterweight clutch tune? the 60' improvement more than offsets the loss of shift efficiency. Example 2- adjustable clutch with a "no-counterweight" clutch tune- let's remove all the counterweight from the levers, and then increase the static clamp pressure enough to achieve 2200lbs of clamp @ 6000rpm. First you must consider that Soft-Lok levers, even without any additional weight, still add around 524lbs of centrifugal assist @ 6000rpm. That means you would only need 1676lbs of static clamp to get the required 2200lbs of overall clamp by 6000rpm. By the 8000rpm shift point, the levers by themselves, even without any weights, will still add another 408lbs of centrifugally induced clamp, bringing the total clamp at the 8000rpm shift point to 2608lbs. That's 408lbs of excess clutch clamp pressure at the shift point. Less excess clamp pressure after the shifts results in the clutch pulling the engine down slower after the shifts, which increases the engine's average power production while also reducing power wasting wheelspeed spikes. The problem with a simple "no-counterweight" clutch tune? they are very hard to launch efficiently. The root of that problem is too much clamp pressure during launch, which in-turn pulls the engine down too fast during launch, resulting in either engine bog or excessive tire spin. As a result, lower launch rpm is required to launch consistently, which in-turn hurts the 60'. To sum up the above comparison... ...the "counterweight" adjustable clutch tune gives you more launch efficiency, but with less efficient shift recovery. ...the "no-counterweight" adjustable clutch tune gives you less launch efficiency, with more efficient shift recovery.
At this point, you can see why counterweight clutch tunes have ruled for so long. It's basically because a better 60' time more than offset the loss of shift recovery efficiency........UNTIL NOW!!!
With ClutchTamer/Hitmaster, "no-counterweight" beats "counterweight"!!! With the ability to adjust inertia draw rate during launch independent of rpm, the "no-counterweight" clutch tune now becomes the winner. You get more launch efficiency than a counterweight clutch tune, without losing any shift recovery efficiency!!! ...higher launch rpm equals more stored inertia prior to launch, which can then improve 60' after the clocks start ticking. ...better recovery after the shifts, which in-turn raises the engine's average HP output. ...less intense energy release after shifts reduces energy wasting wheelspeed spikes....reduced wear/tear on drivetrain components. ...smoother energy discharges makes it easier to take advantage of more efficient radial tires. ...eliminates the need to switch back/forth between street and race adjustable clutch settings for street/strip cars....improved consistency for stick shift bracket racers. Tuning the clutch for a dead hook on poor surfaces eliminates the need to adjust clutch settings when you encounter sticky tracks. Obviously there is a point with the hi-horsepower engines we have today where static clutch clamp pressure alone may not be enough to hold the power, as the clutch pedal itself may become too stiff to operate with consistency. Those applications may require adding some centrifugal assist, along with lower launch rpm, to reduce clutch pedal effort down to a manageable level. In my opinion for the typical application below about 1200hp, the old counterweight clutch setup has become outdated tech.

A DEEPER DIVE ON SHIFT RECOVERY DIFFERENCES

With centrifugal/counterweight, the part of the rpm trace where the engine is being pulled down by the clutch is shaped like a backwards "J". At first the clutch pulls the rpm trace almost straight down. Then as the clutch loses some rpm/clamp, the counterweight relaxes and the bottom part of the J starts to form a curve as it levels out. Eventually it curves enough and as the curve begins upward travel, that's where the clutch locks up. That initial sharp pulldown is evidence of how quick inertia is initially being discharged from the engine's rotating assy with the backwards J shape. With base pressure only and no centrifugal/counterweight, the pulldown part of the engine rpm trace is pretty much a straight line from the top of the J to the point where the clutch locks up. Adjusting clutch base pressure is how you change the angle of that straight line as the clutch is pulling the engine down. If you look at input shaft rpm data, note that as soon as the transmission is in the next gear, input shaft rpm drops straight down as dictated by the ratio change. No curve at all, basically a hard "V" at the bottom. When you look at the backwards "J" shape, remember the top of the J is the point where the clutch begins to pull the engine down. If you overlay that input shaft trace over the engine rpm trace, the area between those two traces is clutch slip. Then where the two traces meet up after the toe of the J, that's the point where the clutch locks up. The area between the two traces where the clutch is slipping represents the engine making more RPM/HP than it would if it had instead got pulled all the way down to input shaft rpm with no clutch slip at all. If you further overlay a straight line no-counterweight trace over the backwards J, adjusted to get the same duration of clutch slip as the backwards J, that straight diagonal line would go from the top of the J, to the toe of the J, where all three traces meet at the point of clutch lock-up. That straight diagonal line represents rpm loss at a slower rate than the backwards J shape, indicating a softer hit on the gears/drivetrain/tires than the backwards J shape. Another thing to note is that the straight line has even more area between it and the input shaft trace than the backwards J shape. The additional area between the two traces represents an increase in engine RPM/HP with the straight-line shape over the backwards J shape. With the same duration of clutch slip, the straight line maintains a higher average engine rpm over that same duration. This picture makes it easy to see how the straight line energy discharge can raise average RPM/HP over the backwards "J" shape, without increasing actual clutch slip time!!!

The difference in overall power production with a straight-line drawdown is short duration and might be small, but the added power is really noticeable in lower powered applications. Several very successful NHRA stocker customers are currently setting their adjustable clutches up this way. It's basically the same way my Coyote Stock customers were setting up their diaphragm clutches, difference is the Coyote Stock guys typically had to shim their rules required diaphragm pressure plates to adjust base pressure, while the adjustable clutch NHRA guys adjust base by cranking on the adjusters.