A BETTER WAY TO EVALUATE THE TUNE OF A STICK SHIFT CLUTCH
by GRANT ROBBINS
Inventor of the ClutchTamer and Hitmaster 2-stage clutch control devices


The ClutchTamer / Hitmaster method of clutch tuning is all about INERTIA MANAGEMENT. In specific, managing inertia flow out of the engine's rotating assy during launch and after the shifts. These inertia discharges are seldom discussed in detail, likely because in the past there has been no practical way to quantify them or their effects on the car. This article is intended to change all that, to help racers comprehend what's really going on inside their bellhousing, and then to show how they might use that knowledge to make their car more efficient.


To understand how clutch engagement can affect your dragstrip performance, you need to gain a firm grasp of the forces at work and their effects on torque applied to the driveshaft. If you want to understand the ClutchTamer/Hitmaster INERTIA MANAGEMENT approach to clutch tuning, you need to get familiar with this simple formula-

(rpm/1000) x (rpm/1000) = units of inertia energy contained within an engine's rotating assy

This formula allows one to visualize the EXPONENTIALLY increasing amount of inertia energy stored within an engine's rotating assy as it gains rpm. It's also a way to visualize the amount of inertia energy that is being discharged as that same rotating assy loses rpm. Just basic physics that applies to the energy contained within most any common spinning flywheel. The formula does not assign a specific value to the energy, as that varies from engine to engine. But it does allow you to compare relative amounts energy flowing back and forth as an engine's rotating assy loses/gains rpm. This formula is structured so that an assembly spinning at 1000rpm will contain 1 x 1 = 1 "unit" of energy, at 10,000rpm that same rotating assy will contain 10 x 10 = 100 units of energy. Say you want to know how many units of energy are in a rotating assy spinning at some random rpm like say 6257?, just move the rpm number's decimal point 3 places to the left... 6.257 x 6.257 = 39.15 units of inertia energy.

To quantify how much energy 1 unit might represent, let's first remember that the entire assembly of pressure plate, flywheel, crankshaft, balancer, timing components, camshaft, pulleys, all those components bolted/meshed together become one big energy storing flywheel when in motion. Let's say it takes about 49 ft/lbs of torque applied for about 1 second to accelerate a rotating assy from 4500 to 5500, which means it also takes the same 49 ft/lbs of brake torque to slow that same rotating assy from 5500 to 4500rpm in one second.


Since 4500rpm equals 20.25 units of energy using the formula above, and 5500rpm equals 30.25 units, the difference between the two is 10 units. If 10 units = 49ftlb/seconds, then 1 unit is equal to one tenth of that, or 4.9 ftlbs of torque applied over 1 second.

Is this 4.9ftlb/sec number above correct for any engine? No, just ballpark for the engine in the following examples. If you're rotating assy is heavier the number will likely be larger, and just the opposite for a lighter rotating assy.

That 49ft/lb/seconds might not seem like a lot, but when that 4500-5500 or 5500-4500 rpm 10-unit inertia exchange happens over a tighter time frame, the numbers start to get much more interesting. Acceleration/deceleration times and the torque required to accomplish them are inversely proportional. For example, 10 units of energy could be expressed as 49 ftlbs applied over 1.0 seconds, or 98ftlbs applied over 0.50 seconds, or 196ftlbs applied over 0.25 seconds.


It's important to note that any time an engine is gaining rpm, it's rotating assy will absorb some of its torque, which in-turn reduces its net torque output. The amount of torque absorbed by the rotating assy depends on the rate that rotating assy is gaining rpm...
...If the engine is operating WOT steady state, 100% of its torque production is available to be measured by a dyno's torque absorber.
...If an engine is accelerating at its maximum WOT no-load rate, it will have ZERO torque output, as the rotating assy itself is absorbing all the torque.
Bottom line is the faster an engine's rotating assy is gaining rpm, the less net torque that engine will have left over to do actual work. The reduction of net engine torque might be relatively small in high gear where the acceleration rate might only be 350rpm/sec, but you will see a huge reduction of net engine torque at 3700+ rpm/sec 1st gear acceleration rates.


Now let's put these rough numbers to work on an actual dragstrip pass. Let's say the engine in this example makes a maximum of appx 425ftlbs of torque operating WOT steady state in the heart of its torque curve, that WOT 425ftlb maximum number will be our baseline for calculating torque applied to the transmission's input shaft...

Here's the calculations I used to estimate average torque during each part of this run...

RPM PULL DOWN DURING LAUNCH- NOTE- (inertia discharge rate during launch on this pass was controlled) The engine went from 7174rpm (51.47 units) to 4622rpm (21.36 units) in 0.597sec. That's 30.11 units of energy discharged in 0.597sec. Since 1 unit in this example is equal to 4.9ftlb/sec, 30.11 units x 4.9ftlb/sec = 147.5ftlb/sec. The energy discharge occured in less than a second, so 147.5ftlb/sec / 0.597sec = 247ftlbs of averaged inertia energy discharge. This torque is in addition to the engine's WOT baseline of 425ftlbs steady state torque production, which means 425ftlbs WOT baseline + an added 247ftlbs due to inertia discharge = average 672ftlbs applied to the transmission's input shaft during the intial 0.597sec during launch. Multiplied by the transmission's 3.17 1st gear ratio, average calculated torque to the driveshaft during launch was 2130ftlbs.

1st GEAR RPM RECOVERY CLIMB- after the clutch locks up, the engine pulls from 4622rpm to 7629rpm in .816sec. The engine's rotating assy is now absorbing torque as it gets re-charged with rpm, 36.8 units x 4.9ftlb/sec per unit / .816sec = 221ftlb being re-charged into the rotating assy. 425ftlbs WOT baseline - 221ftlbs absorbed by the rotating assy = 204ftlbs being applied to the transmission's input shaft. Multiplied by the transmission's 3.17 1st gear ratio, average calculated torque to the driveshaft after 1st gear pulldown was 646.6ftlbs.

1st to 2nd WOT CLUTCHLESS SHIFT FLARE- the transmission exited 1st gear @ 7629rpm, then engine rpm flared to 7820 as the transmission crossed neutral. That flare represents a 2.95 unit increase in inertia energy, or 4.9ftlb/sec x 2.95 units = 14.46ftlb/sec of inertia energy added to the engine's rotating assy over the 0.043sec the transmission was crossing neutral. Power to the driveshaft was briefly interrupted, but the power being produced during that interruption was temporarily stored in the engine's rotating assembly as increased rpm. The added energy produced by the engine during the flare was then applied to the transmission's input shaft after the shift.

RPM PULL-DOWN AFTER 1-2 SHIFT- the clutchless 1-2 shift's inertia discharge rate controlled only by clutch clamp pressure. As you can see, the engine went from 7820rpm (61.15 units) to 5132rpm (26.33 units) in 0.163sec. That's 34.82 units of energy discharged in 0.163sec. Since 1 unit in this example is equal to 4.9ftlb/sec, 34.82 units x 4.9ftlb/sec = 170.6ftlb/sec. The energy discharge occurred in less than a second, so 170.6ftlb/sec / 0.163sec = 1046.6ftlbs of inertia energy discharge. This torque is in addition to the engine's steady state torque production, which means 425ftlbs WOT baseline + an added 1046.6ftlbs due to inertia discharge = average 1471.6ftlbs applied to the transmission's input shaft during the initial 0.163sec after the shift. Multiplied by the transmission's 1.96 2nd gear ratio, average calculated torque to the driveshaft after the 1-2 shift was 2884.3ftlbs.

2nd GEAR RPM RECOVERY CLIMB- after the 1-2 shift pulldown, the engine pulls from 5132rpm to 7522rpm in 1.428sec (before the 2-3 shift flare as the trans crosses neutral). The engine's rotating assy is now absorbing torque as it gets re-charged with rpm, 30.21 units x 4.9ftlb/sec per unit / 1.428sec = 104ftlb being re-charged into the rotating assy. 425ftlbs WOT baseline - 104ftlbs absorbed by the rotating assy = 321ftlbs being applied to the transmission's input shaft. Multiplied by the transmission's 1.96 2nd gear ratio, average calculated torque to the driveshaft after 2nd gear pulldown was 629.1ftlbs.

2nd to 3rd WOT CLUTCHLESS SHIFT FLARE- the transmission exited 2nd gear @ 7522rpm, then engine rpm flared to 7811 as the transmission crossed neutral. That flare represents a 4.43 unit increase in inertia energy, or 4.9ftlb/sec x 4.43 units = 21.71ftlb/sec of inertia energy added to the engine's rotating assy over the 0.064sec the transmission was crossing neutral. Power to the driveshaft was briefly interrupted, but the power being produced during that interruption was temporarily stored in the engine's rotating assembly as increased rpm. The added energy produced by the engine during the flare was then applied to the transmission's input shaft after the shift.

RPM PULL-DOWN AFTER 2-3 SHIFT- the clutchless 2-3 shift's inertia discharge rate controlled only by clutch clamp pressure. As you can see, the engine went from 7811rpm (61.01 units) to 5238rpm (27.44 units) in 0.142sec. That's 33.57 units of energy discharged in 0.142sec. Since 1 unit in this example is equal to 4.9ftlb/sec, 33.57 units x 4.9ftlb/sec = 164.5ftlb/sec. The energy discharge occurred in less than a second, so 164.5ftlb/sec / 0.142sec = 1158.5ftlbs of inertia energy discharge. This torque is in addition to the engine's steady state torque production, which means 425ftlbs WOT baseline + an added 1158.5ftlbs due to inertia discharge = average 1583.5ftlbs applied to the transmission's input shaft during the initial 0.142sec after the shift. Multiplied by the transmission's 1.34 3rd gear ratio, average calculated torque to the driveshaft after the 2-3 shift was 2121.8ftlbs.

3rd GEAR RPM RECOVERY CLIMB- after the 2-3 shift pulldown, the engine pulls from 5238rpm to 6987rpm in 2.138sec (before the 3-4 shift flare as the trans crosses neutral). The engine's rotating assy is now absorbing torque as it gets re-charged with rpm, 21.38 units x 4.9ftlb/sec per unit / 2.138sec = 49ftlb being re-charged into the rotating assy. 425ftlbs WOT baseline - 49ftlbs absorbed by the rotating assy = 376ftlbs being applied to the transmission's input shaft. Multiplied by the transmission's 1.34 3rd gear ratio, average calculated torque to the driveshaft after 3rd gear pulldown was 503.8ftlbs.

3rd to 4th WOT CLUTCHLESS SHIFT FLARE- the transmission exited 3rd gear @ 6987rpm, then engine rpm flared to 7260 as the transmission crossed neutral. That flare represents a 3.9 unit increase in inertia energy, or 4.9ftlb/sec x 3.9 units = 19.11ftlb/sec of inertia energy added to the engine's rotating assy over the 0.042sec the transmission was crossing neutral. Power to the driveshaft was briefly interrupted, but the power being produced during that interruption was temporarily stored in the engine's rotating assembly as increased rpm. The added energy produced by the engine during the flare was then applied to the transmission's input shaft after the shift.

RPM PULL-DOWN AFTER 3-4 SHIFT- the clutchless 3-4 shift's inertia discharge rate controlled only by clutch clamp pressure. As you can see, the engine went from 7260rpm (52.71 units) to 5271rpm (27.78 units) in 0.164sec. That's 24.93 units of energy discharged in 0.164sec. Since 1 unit in this example is equal to 4.9ftlb/sec, 24.93 units x 4.9ftlb/sec = 122.2ftlb/sec. The energy discharge occurred in less than a second, so 122.2ftlb/sec / 0.164sec = 745.1ftlbs of inertia energy discharge. This torque is in addition to the engine's steady state torque production, which means 425ftlbs WOT baseline + an added 745.1ftlbs due to inertia discharge = average 1170.1ftlbs applied to the transmission's input shaft during the initial 0.164sec after the shift. Multiplied by the transmission's 1.00 4th gear ratio, average calculated torque to the driveshaft after the 3-4 shift was 1170.1ftlbs.

4th GEAR RPM RECOVERY CLIMB- after the clutch locks up, the engine pulls from 5271rpm to 7290rpm in 5.674sec. The engine's rotating assy is now absorbing torque as it gets re-charged with rpm, 25.36 units x 4.9ftlb/sec per unit / 5.674sec = 21.9ftlb being re-charged into the rotating assy. 425ftlbs WOT baseline - 21.9ftlbs absorbed by the rotating assy = 403.1ftlbs being applied to the transmission's input shaft. 4th gear ratio is 1:1, so 403.1ftlbs average calculated torque to the driveshaft after the 4th gear pulldown.

Here I added to the graph a binary trace of the above calculated numbers. This one shows relative averaged amounts of torque delivered to the transmission's input shaft by the clutch, notice that 3rd gear got the most abuse...

I am always amazed at how many think that they don't want or need any clutch slip after the shifts! The above graph illustrates how an aggressive clutch can easily kill a transmission, just look at that 1471ftlb 2nd gear and 1583ftlb 3rd gear torque spikes!!! But that doesn't tell the whole story. Here's another graph of the same pass, but this time torque at the input shaft gets multiplied by the transmission ratios...

That spike after the shift into 2nd is the kind of thing that breaks u-joints and ring/pinion sets. This clutch needs it's base/static clamp pressure reduced, which would in-turn decrease the inertia draw rate after those "clutchless" shifts. Adjust/shim it enough to increase the 1/2 shift slip time from 0.163sec to around 0.220sec, torque applied to the driveshaft during 1/2 pull-down would decrease to roughly the same level as launch. If you shift using the clutch pedal, an alternative to shimming the pressure plate is to install a ClutchTamer to add a little slip after the shifts.

Up to this point you might be skeptical of just how accurate these calculations might be, but notice how closely the above calculated average driveshaft torque numbers align with the pass's AccelG trace...

You can see that the launch pulldown produced some good long duration AccelG, but notice how the 2nd and 3rd gear pulldown spikes came up short for the relative amount of AccelG they produced. That's because a large portion of the energy released in those two spikes went into intense wheelspin instead of accelerating the car.

Might help to think of it like accelerating a glass by pulling on a tablecloth. Jerk the tablecloth too quick, the glass doesn't move. Pull slow and the glass moves, but not very quick. Happy medium for accelerating the glass as efficiently as possible lies somewhere in between.

You might also notice that the 1st gear part of the "G" trace initially comes up a little short compared to the calculated torque applied, but in the later recovery part of 1st gear the "G" trace exceeds what you might expect given the averaged calculated torque in that section of the trace. That's the result of a dead hook wheelie. Initially during the high torque part of 1st gear, the front end rose as it absorbed some of that torque, which took some potential away from the "G" trace. But when applied driveshaft torque dropped as rpm began to climb, the falling wheelie started returning the energy it had absorbed, which in-turn raised that part of the "G" trace above what the average calculated driveshaft torque predicted.


ADJUST THE SHOCKS or ADJUST THE CLUTCH? When a stick shift car does wild things on the starting line, first reaction is often to calm things down with shock adjustments. Be careful with that line of thinking though, as using the shocks to control what's really a clutch problem can leave potential on the table. The intensity of inertia energy leaving the engine's rotating assy during launch is dictated by how fast the clutch pulls the engine down. Stiffening the shocks may dampen the chassis' excessive reaction, but the clutch is still going to draw inertia out of the engine at that same excessive rate. Because shocks only serve to absorb energy without giving any back, tightening them should be one of your last resort solutions.

Rather than stiffen the shocks to control excessive inertia reaction, you might want to try slowing down the rate that inertia is fed into the chassis. Easy to do if you have a 2-stage clutch hit controller. Far less wear/tear on drivetrain components, sidewalls will also last much longer. After you discover that you can control the rate that inertia energy is fed into the chassis, you will then soon realize that there's not much keeping you from packing even more rpm/energy into the engine's rotating assy prior to launch.

Also be aware that excessively stiff shocks will reduce your chassis' ability to maintain traction over bumpy surfaces down the track. Your tires might skip right over those bumps!


WHATS THE BEST LAUNCH RPM ?- As much as you can get away with! Drag race starts are unique in that you have the ability to store inertia energy prior to the start. The faster you can spin the engine before the clocks start, the more inertia energy will be available to help move the car with the clocks running. The key to making all this work is being able to control the rate that energy is fed into the chassis. It's a balancing act as you want to feed that energy into the chassis as quick as possible without pulling the engine down too far, but feed it in too quick and the results are either a bog, spin, or broken parts.

The general idea is to gear the car to keep the engine riding the plateau of its HP curve throughout the run. If hi-rpm clutch hit is going to pull the engine down about 2000rpm, and the engine's torque peak is around 5200, launch rpm should be at least 7200 so that the engine doesn't get pulled down below the rpm where it pulls it's hardest. This approach makes best use of your ability to store energy prior to the start, also helps keep your engine rpm up where it's making more average hp.

In the graph above, lines A,B,C, and D represent different draw rates that the clutch is pulling down a 600ft/lb engine after a WOT launch.

Line "A"- 48.6 units @ 150 unit/sec rate. 600ftlbs engine + 733ftlbs inertia = 1333ftlbs x 0.325sec = 433 comparison rating.
The clutch is pulling engine rpm down way too fast. Even if the engine still made 600ftlbs at 1800rpm, that's only 205hp at the low point of the bog.
Even if that 1333ftlb slam to the input shaft doesn't break anything, you still get a huge bog down to 205hp before the long climb to the 7200 shift point.
Obviously, this 60' is going to suck!

Line "B"- 44 units @ 80 unit/sec rate. 600ftlbs engine + 392ftlbs inertia = 992ftlbs x 0.55sec = 545 comparison rating.
The slower pulldown rate raised the rpm/mph sync point to about 2800 at 20mph, but still only 320hp at the low point of the bog.

Line "C"- 37.4 units @ 52 unit/sec rate. 600ftlbs engine + 253ftlbs inertia = 853ftlbs x 0.725sec = 618 comparison rating.
Much better pulldown rate, now rpm stays above 3700. The engine is now up to about 420hp at the low point of the bog.

Line "D"- 28.0 units @ 31 unit/sec rate. 600ftlbs engine + 157ftlbs inertia = 757ftlbs x 0.90sec = 681 comparison rating.
That extra slip time allows the engine to stay closer to its rpm sweet spot for HP production. The engine is now up to about 550hp at the low point of the bog.
You get a softer launch that contains more average thrust!

The ability to raise launch rpm without over-powering the chassis with inertia can be a big help for stick shift turbo cars that have trouble building boost on the line.


INERTIA DISCHARGE RATE DURING LAUNCH vs INERTIA DISCHARGE RATE AFTER THE SHIFTS- Clutch clamp pressure directly affects the rate that stored inertia energy is pulled out of your engine's rotating assy. The ideal clutch clamp pressure for shifting into high gear is not the same as the ideal clutch clamp pressure for launching the car. If you set the clutch up for best launch, it's going to blow thru the clutch in high gear. If you set the clutch up to hold after the shift into high gear, it's not going to slip long enough for best launch. For example, let's say you want to launch at 5000 without pulling the engine below 4500. The pulldown rate of that rpm trace is going to be pretty flat, basically pulling out 500rpm over 0.80 seconds. But then when you shift, you are going to be pulling down around 2000rpm. That same pulldown rate from launch would take about 3.20 seconds to pull down 2000rpm after a shift! That's waaaaaaaaaay too much slipping after a gearchange!!!

My solution to that problem- a 2-stage coupling...
...With an automatic car, some hi powered applications have taken things to the next level by dumping some converter charge pressure during launch. This basically causes the converter to momentarily cavitate, which in-turn temporarily reduces torque multiplication, while at the same time raising stall speed. This effectively turns the converter into a 2-stage unit that's temporarily looser during launch, but then tightens up down the track as converter charge pressure returns. Result is the engine gets to its happy place where it makes power much quicker during launch, while minimizing converter slippage down the track in high gear.
...To prevent kicking the tires loose after the shifts, some have starting dumping some converter charge pressure after the shifts as well. The momentarily softer converter allows the tires to stay hooked thru the shift, also the engine doesn't get pulled down as far after the shift. In the end the tires stay hooked thru the shift, and less rpm loss after the shift equals more average power.
...With a manual transmission car, you can turn your clutch into a 2-stage unit by adding a clutch hit controller. The 1st stage of a 2 stage clutch hit controller has the ability to temporarily hold back some clutch clamp pressure, which makes the clutch looser during launch, then the 2nd stage comes in to tighten the clutch up down the track. Keeps the engine from getting pulled down/out of its happy place during launch, while also reducing the intensity of the inertia draw.
...Much like some of those hi-powered automatic guys mentioned above, some stick shift guys are now momentarily re-activating the clutch's 1st stage after the shifts. This slows the inertia discharge rate after the shift down to a tolerable level, while at the same time raising average HP throughout the run by raising fallback rpm, which nets them some area under the engine rpm trace.


CLAMP PRESSURE vs TUNING WINDOW- Matching a clutch's static clamp pressure to its application has a huge effect on how easy that clutch will be to tune. Say you install a 1200ftlb capacity dual disc clutch behind a 600ftlb engine, the tuning window for that application is going to be very narrow, which will in-turn make it very hard to find and consistently hit the sweet spot. Ideally, you want the clutch's overall torque holding capacity to be well matched to its application, which will give you the widest possible tuning window.

As you can see, proper clutch clamp pressure also greatly reduces the chances of breaking your transmission/drivetrain! If you shift using the clutch pedal and don't want to go to all the trouble of dialing in your clutch's static clamp pressure, installing my ClutchTamer product is an alternative that will allow you to soften the hit of your clutch after the shifts from your driver's seat.

Here's how that change in base pressure affects the torque applied to the driveshaft...

Notice how a reduction in clamp pressure effectively increases clutch slip time. Doubling clutch slip time will cut the intensity of the inertia spike in half.

Another important thing to note is how spring design affects clutch clamp pressure and tuning window as the clutch disc wears...
...Diaphragm PP springs are typically designed to go "over-center" around mid-point in their travel range. They start out gaining clamp pressure as the disc wears, then at about the mid-point of disc wear the pressure curve levels out and begins to gradually lose clamp pressure for the rest of the disc's life. Basically, you end up with about the same static clamp pressure at the end of the disc's life as you had when it was new.
...B&B and Long style PP coil springs gradually lose clamp pressure as the disc wears. That means you must start out with a wide margin of extra spring pressure when the disc is new, just to have enough static clamp pressure to hold at the end of the disc's life when it is thin.
Hypothetical example of why this is important to understand- let's say a given combo wants 2800lbs of single disc clutch clamp pressure to hold after the shift into high gear.
...With a typical diaphragm spring, you need 2800lbs of pressure plate with a new disc, in order to have about 2800lbs at the end of the disc's life.
...With coil springs, you might need 3500lbs of pressure plate when the disc is new, in order to still have 2800lbs of clamp available when the disc is used up.

In general, regarding clutches that are installed out of the box without any PP shimming...
...A diaphragm sprung clutch that is closely matched to its application will have a wider average tuning window over it's life, as clamp pressure varies less over the life of the disc.
...A coil sprung PP starts out with a narrower tuning window when new due to excessive clamp pressure, then that window gradually opens up as the disc wears. Generally, a well-matched out of the box coil sprung clutch will perform its best just before the disc is worn out.

If you want the clutch to perform at its best throughout the life of the disc, periodic maintenance shimming of the pressure plate is one way to keep clamp pressure in its optimum range for a wide tuning window, regardless of disc thickness.


SUGGESTED SEQUENCE OF CLUTCH ENGAGEMENT RATE ADJUSTMENTS WHEN USING A CLUTCH HIT CONTROLLER-
...1- Don't over-clutch the car! Install a clutch that has just enough overall capacity to handle all the power that you are going to put thru it over the disc's entire lifespan, as this will give you the broadest possible clutch tuning window.
...2- Line-lock and 2-step switches should not release from top of clutch pedal travel- With the ClutchTamer or Hitmaster controlling clutch engagement, your clutch pedal may not immediately return to the top of its travel when the clutch pedal is released. If you use an oem style upper clutch pedal switch to release the 2-step and/or line-lock, the result could be a second or so delay of when the pedal contacts the clutch switch. Late release of the 2-step will likely only cause a performance issue, but late release of the line-lock will cause a safety issue. Please make sure that your line-lock is not releasing AFTER the clutch hits!
...3- Ballpark the pressure plate's "static" clamp pressure- This adjustment controls the rate that inertia is drawn out of the engine's rotating assy after the shift into high gear. Shimming/adjusting allows you to make small incremental adjustments over the life of the clutch disc, as a way to keep static clamp pressure in its optimum range if static clamp pressure is not adjustable.
...4- Adjust the clutch pedal stop- Before you adjust the clutch's 1st stage draw rate, it's very important that you install a clutch pedal stop and verify its proper adjustment. By proper adjustment, I mean a setting that allows enough clutch dis-engagement to achieve clean hi-rpm shifts, but also enough dis-engagement to be able to put the transmission into gear without grinding when the clutch is warm. Using a pedal stop helps ensure a consistent release point, which in turn helps ensure that you get repeatable results. If for some reason you have to change your pedal stop height after you begin the tuning process (maybe you find the clutch isn't releasing cleanly which causes a shifting problem), you may have to go back and repeat the tuning process all over again.
...5- Dial-in the clutch's 2nd stage of full clutch clamp pressure- The only way to do this proper is to make a high rpm shift into high gear, analyze the results, and go from there. I generally adjust for about 0.30 to 0.40sec of clutch slip duration after a WOT 3/4 shift. If you do this, be sure to do it BEFORE you adjust 1st stage inertia draw.
...6- Adjust Launch RPM- Changing launch rpm can affect your clutch's 1st stage inertia draw rate!!!!!! Even without any sort of "centrifugal assist", a typical 10.5" diaphragm style pressure plate can gain around 350lbs of clamp pressure just by going from 4000rpm to 7000rpm. For best ET, you generally want to stage at least 2000rpm above your engine's torque peak. You can choose any launch rpm you want (typically the higher the better), but it's a good idea to keep launch rpm consistent from run-to-run if you want consistent results.
...7- Adjusting the clutch's 1st stage inertia draw rate-The HitMaster 1st stage control valve has one simple dial type adjustment for controlling the intensity of the initial hit....
...Rotate HitMaster dial clockwise = slower inertia draw rate.
...Rotate HitMaster dial counter-clockwise = quicker inertia draw rate.
Add counter-clockwise turns to the HitMaster valve adjustment with each run, until either the tires start spinning or the 60's stop improving. On the other hand, if the tires spin instantly on the hit, keep adding clockwise turns to the HitMaster valve adjustment with each run, until either the tires stop spinning or the 60's stop improving.
...8- Adjusting the 1st stage duration- Generally, you want the duration setting on the clutch's 1st stage timer to expire just short of kicking the tires loose. If the tires dead hook the launch and then spin shortly after the hit, extend the timer setting 0.10sec and make another hit. On the other hand, if it's not kicking the tires loose when the timer expires, you might want to shorten the timer setting 0.10sec per pass until it does. In general, if you are shifting using the clutch pedal you want the minimum timer setting that does not knock the tires loose.

If you launch your car on a variety of surfaces, you will likely find that different surfaces may require a different 1st stage control valve setting. When you find a good setting for a particular surface/condition, we strongly suggest you record those conditions/settings for future reference. The least expensive way to reference a particular setting is to use a caliper and measure/record the length of exposed threads between the 1st stage control valve's knob and nylon guide housing.

Another method to reference a particular setting, is to install a pressure gauge to measure psi at the slave/throwout bearing. Basically idle the car in neutral, then record the psi number at the "hit" during a simulated launch. We define a simulated launch as simultaneous release of both the clutch pedal and line lock button, with the engine idling and the transmission in neutral. The object is to cycle the components while the clutch is rotating, an idling engine makes the results more consistent by minimizing any added effects due to centrifugal forces.

The below graph shows the centrifugally increasing hydraulic throw-out bearing pressure of a typical diaphragm pressure plate.
...Red- engine rpm (ranges from idle to 6900)
...Green- throw-out bearing Psi (ranges from 444psi to 600psi)

For this test, the clutch pedal was held in a single position against its travel stop, then hydraulic pressure data was logged as engine rpm increased. As you can see, centrifugal forces have little effect on throw-out bearing Psi below 2500rpm. While the pressure plate's pressure ring position did not change, notice that Psi increased from 447psi @ 2500rpm to 600psi @ 6900rpm due to centrifugal forces acting on the diaphragm's levers. That's a 156psi gain due to rpm only, with no change in lever position. This shows that throw-out bearing Psi is not an accurate indicator of lever position above 2500rpm. For that reason, we suggest that only Psi data from around idle speed be used for setup/tuning comparison purposes.

For clutch tuning our Shop Mule test car, we added a pressure sensor to the end of our hydraulic throw-out bearing's bleed hose. That pressure sensor's data is displayed to the driver by a Racepak Intelli-gauge, allowing for precise on the spot 1st stage clamp pressure changes without consulting a laptop. The Racepak data recorder is set to initiate a test session when triggered by the line-lock button. After data collection starts, we then put the shifter in neutral and "dry-snap" the clutch pedal @ idle prior to staging. That gives us a recorded snapshot on that run's graph showing the throw-out bearing's Psi release curve, leaving no doubt about what clutch control adjustments might have been in place for that pass.


WHAT DOES LAUNCH EFFICIENCY LOOK LIKE ON THE GRAPH? One of the simple things you can do when looking at a graph is to grab something straight and apply it to the screen, just a simple straight edge aligned with the upper section of the 1st gear engine rpm trace...

If you apply that straight line to most dead hook graphs, it will likely cross the vertical "O" line somewhere between zero rpm and 50% of launch rpm. On this graph where the car is set off at 7174rpm, that means the line will likely cross the vertical "O" line somewhere between zero and 3587rpm. If it crosses around or below zero, launch efficiency was poor. On the other hand if it crosses at about 3587rpm, that's about as good as it gets. Notice this straight line hits the vertical "O" line at about 2837rpm. That's pretty good, about 79% of its 3587 ballpark potential, but there's still room for improvement.


WHERE AN RPM FLARE DURING LAUNCH CAN WORK TO YOUR ADVANTAGE- maybe you have a car that is set up for a sticky track that is attempting to run on a poor surface, basically needs more rear weight. A slight rpm "flare" prior to launch pulldown can give RWD cars a little extra time to transfer weight before pulldown inertia hits the transmission's input shaft. If you are using a 2-step activated by a clutch pedal switch and a lower 2-step setting, locating the switch to release the 2-step before the clutch engages is one way to create a launch flare.


WHY A WOT SHIFT FLARE IS BETTER THAN A WOT IGNITION CUT SHIFT- two completely different approaches to WOT shifts.
...WOT ignition cut is about momentarily cutting engine power, for the purpose of reducing engine rpm, to facilitate a WOT clutchless shift. Easy to accomplish with a strain gauge shift knob, making it an obvious solution for "quick shifts". Problem with that line of thinking is that not only was power momentarily cut, but you also wasted a lot of stored inertia energy by using internal engine friction and pumping losses to quickly reduce engine rpm. That "smooth as butter" clutchless WOT ignition cut shift might seem quick, but it comes at a power production cost. Basically, with a cut it takes longer to produce "X" amount of power.
...WOT shift flare is about turning the engine loose to produce/store as much energy as possible, as quickly as possible, even while the transmission is crossing neutral. The flare is the product of adding energy to the engine's rotating assy, and clutch control is the key to harnessing that flare's energy and applying it to the transmission's input shaft at a rate that the chassis can handle. The advantage over the ignition cut method is that no energy gets wasted motoring the engine during a power cut.


TRACTION CONTROL vs 2-STEP REV LIMITER vs 2-STAGE CLUTCH CONTROL- two completely different approaches to getting a car down the track.
...Traction control is about momentarily/selectively reducing engine power, typically via reduced engine timing. Easy to accomplish with modern ignition controllers, making it often the obvious answer for keeping the tires stuck at points where data says they spin. The power reduction may be temporary, but the cost of that temporary reduction of power is that it extends your engine's power production timeline.
...2-step rev limiter is a way of limiting the quantity of inertia stored in the engine's rotating assy prior to the start. In that regard a 2-step is a form of traction control (power reduction). Traction control should be your last resort when there's nothing else left to adjust.
...2-stage clutch hit control is about allowing the engine produce/store as much energy as possible prior to the start of the clock. During launch, the clutch's adjustable 1st stage of limited clamp pressure draws stored energy out at a controlled rate that doesn't upset the chassis. After a timed delay, the clutch's 2nd stage of full clamp pressure is allowed to come in to insure lock up.

What would happen if the engine in that graph above were capable of raising the launch rpm from 7174 to 9000, with shift points/etc all remaining the same?  The answer is the amount of energy available for the G surge would almost double, from 30.03 to 59.57 units of inertia. If you draw that 59.57 units in the same 0.589sec time frame that the 30.03 units were drawn out, the hit on the input shaft almost doubles in intensity, which would likely either break something or knock the tires completely loose. But if you use a 2-stage clutch hit controller to draw that 59.57 units at same draw rate as the 30.03 unit hit, the intensity of the hit does not increase but the G surge then lasts almost twice as long. The engine's power curve does not need to extend to 9k to exploit this, just capable of spinning 9k on the starting line without flying apart. Just an example of what the ability to control inertia draw rate can allow you to do.


THE DOWNSIDES OF ADJUSTABLE CENTRIFUGAL ASSIST- Rpm controlled centrifugal assist is generally used to prevent the clutch from pulling the engine down/out of its power range during launch. When adjusted as designed, centrifugal assist will automatically relax enough to let the clutch slip before the engine gets pulled below its torque peak. They also raise fallback rpm over that predicted by ratio changes after the shifts, which serves to increase the area under the engine's rpm trace. More area under the engine's rpm trace generally translates to more power produced in a given time frame.
...On the street- over about 1000hp, an adjustable clutch's drag strip tune is generally stiff enough for casual street driving. Mash the throttle to pass someone though, that race clutch tune is going to slip unless you downshift to pick up some centrifugal assist. In my opinion, 1000+ hp on the street looks kinda silly when it has to downshift to pass someone. To prevent that embarrassment, many street/strip racers find it necessary to adjust their adjustable clutches back and forth between street and race clutch tunes. That usually means jacking up the car to make the adjustments, which in-turn means a good jack and jack stands are required. Keep in mind that the engine has to be rotated to at least 6 different positions to facilitate the adjustments, also the danger of being under the car and forgetting to place the transmission in neutral before rotating the engine. That's a lot of extra hassle for a guy that wants to drive his car to the track.
...On the track- An adjustable clutch that actively relies on rpm to increase its torque capacity is less than ideal for drag racing. They were an improvement over earlier non-adjustable versions mainly because the clutch can be adjusted to slip below the engine's torque peak, which in-turn prevents the clutch from pulling that engine down out of its power range regardless of launch rpm. This also made it possible to adjust launch intensity simply by varying launch rpm. The downside is centrifugal assist clutch forces you to compromise on how much rpm you can bring to the starting line.

There is also significance to different shapes produced during the pull-down/inertia draw sections of an engine rpm trace...
...Backwards "J" pulldown is the basic shape you get from typical adjustable centrifugal assist pressure plates.
...Linear "\" pulldown is the basic shape you get from typical non-centrifugal assist pressure plates.

The backwards "J" parts of an engine rpm trace need to be broken into two different average pulldown rates, necessary to emphasize the significance of its extreme initial discharge rate. These shapes fall almost straight down initially after the shift, which indicates a quick/intense release of inertia energy from the rotating assy...so intense that it typically shows up as a large wheelspeed spike on the driveshaft rpm trace. As rpm gets pulled down far enough to relax the counterweights, the engine rpm trace begins to transition into the backwards "J" shape. This lower curved part of the backwards "J" is where the effective clutch slip occurs that actually raises hp. The upper vertical part of the backwards "J" was the clutch not slipping enough, causing some fallback energy to be wasted in a blip of tire spin. While counterweight does effectively raise fallback rpm over what is predicted by the ratio change, that steep initial drop is far from optimum.

The linear "\" pulldown also features the advantage of raising fallback rpm over what is predicted by the ratio change, but also offers two additional benefits...
...the shape adds even more area under the engine's rpm trace vs the backwards "J".
...the linear inertia discharge rate is smoother, does not create that large disproportional initial spike.
The downside to the linear "\" shape is that when the clutch's inertia discharge rate is optimized for launch, the clutch will slip waay too much after the shifts. That problem is easily fixed by adding a 2-stage clutch controller.

There are guys now using clutch hit controllers on their adjustable clutches, and some are winning races at the national level. The hit controller adds another layer of adjustability that they did not have before, making it possible to leave with as much rpm as they desire without any negative side effects. They are also abandoning adjustable centrifugal assist and its backwards "J" pulldown shape by removing the weights, then cranking in just enough base pressure to take advantage of the linear "\" pulldown shape. No longer is it necessary for them to "drive into the clutch". The linear "\" shape is also less likely to knock the tires loose after the shifts, and adds even more area under the engine rpm trace after the shifts. Look for guys with sky high launch rpm that don't rip up the starting line, they also just may be dead hooking radials thru the shifts.


IS A HEAVY DRAG CAR REALLY MORE LIKELY TO BREAK A TRANSMISSION?- Most stick shift drag racers subscribe to the conventional wisdom that says the heavier a drag car is, the more abuse it will inflict on its transmission and drivetrain. My viewpoint is the only way adding weight makes an actual difference is if wheelspeed is part of the launch. Adding weight typically equals more weight on the tire, requiring you to hit that tire harder with inertia to get the wheelspeed you need, which in-turn means the transmission will then see a higher peak load as a result. But all that changes when you install a 2-stage clutch hit controller, as the goal with a clutch hit controller is not controlled wheelspin, but instead controlling the rate that the clutch draws inertia against WOT. Since a heavier car accelerates at a slower rate, it's clutch will also need to draw the engine down at a slower rate to keep rpm in its optimum range.

Here's a crude theoretical comparison based on Wallace calculator "ideal" numbers. One 3500lbs and the other 2500lbs, both with the same 620whp. Wallace says the 3500lb car runs 10.00 @ 132.5 with a 1.39 60', while the 2500lb car runs 8.95 @ 148.04 with a 1.24 60'. Both have same tires, same 1st gear, and both geared for 7500 at the stripe- 4.72 gear for the 3500lb car and 4.22 gear for the 2500lb car. Both are also using clutch hit controllers tuned for a 7500 hit that draws down to 5500 before the clutch locks up. Based on averaged acceleration rates over the first 60' of each car, the 3500lb car takes 0.81sec to draw 2000rpm worth of inertia, while the 2500lb car accelerates quicker and takes only 0.755sec to draw the same 2000rpm of inertia. The heavier car spreads the same inertia draw over a longer time period, which in-turn effectively reduces the peak impact value of the inertia that gets passed along to the input shaft.

A key thing to note is that although both cars in the above example have the same 620whp and transmission, rear gears are different to achieve the same 7500 at the stripe-
...4.72 for the 3500lb car that goes 132mph
...4.22 for the 2500lb car that goes 148mph

If one were simply increasing weight with the same rear gearing, then that conventional wisdom would apply. But when you gear for the stripe as drag racers do, the heavier/slower car gets more rear gear which in-turn decreases the load on the transmission. When you remove wheelspeed from the stick shift launch equation, in-turn allowing both clutches to be tuned to draw the same amount of inertia, that conventional wisdom goes right out the window.

The most surprising thing that you will find with our ClutchTamer & Hitmaster products is how much more power you will be able to put down without breaking parts!


Here's one way you can fill in the gaps in your power delivery profile- NITROUS!

On the above graph, in blue you see the effect of adding a 250hp shot of nitrous to 1st and 2nd gears, the shot has been delayed just enough to take over just as the inertia surge starts to fade. The net effect is that instead of driveshaft torque dropping all the way down to 647ftlbs, nitrous comes on and torque only drops to 1547ftlbs. Because nitrous flow rate does not vary with rpm, the amount of torque added by nitrous drops off as the engine gains rpm. Torque with the addition of nitrous starts out at 1547ftlbs, then gradually drops to 1201ftlbs as the engine reaches the shift point. As you can see, there is enough room in that gap between inertia surges to dead-hook a 400hp shot!

Here is a chart that shows the amount of torque you might expect from different size shots as engine rpm increases...


Phone number for ClutchTamer / Hitmaster tech info- 360-391-1208
email address- grant@clutchtamer.com

CHANGING THE GAME ON LAUNCHING YOUR STICK SHIFT CAR!!!