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Billet Clutch Tech Talk: Q & A

Black Star Billet Clutch
Our technicians have been diagnosing drag racers' clutch problems for more than two decades. Some of the questions most-often asked of Titan's clutch department appear here, along with representative answers.

Q: At what point should I step up to a slider clutch and a planetary-type transmission?

A: Either when your current clutch- or converter-and-trans combination is either not able to hold the power (as evidenced by breakage or burned clutches in the transmission, etc.), or when you're making enough horsepower to make a car like yours run two- to four-tenths quicker and three to 15 mph faster, were it equipped with an adjustable slider clutch and stronger transmission.

Q: My car leaves the starting line perfectly, but on the shifts, it spins the tires and shakes slightly. How come?

A: This is where a lot of e.t. is lost because the clutch drives the engine to a lean condition and knocks the tires loose, too. The basic cause is the slowing rpm of the engine (and fuel pump and pressure) with the gear change, while adding a lot of load. To keep this load vs. fuel delivery closer to being in synch, you can adjust the clutch to the point that it comes loose slightly on the shifts. Since every car acts a little or a lot differently, this is something that each race team must find for itself. The key is to add static pressure and subtract counterweight until the clutch is, let's say, 40-percent static and 50-percent counterweight at 8000 rpm (or whatever rpm is appropriate). You want the clutch to slide (with the springs controlling how much) on each gear change. There are lots of variations on most parts of a race car. The clutch is one of the most misunderstood — yet most critical to elapsed time.

Q: My nitrous Pro Mod car backfired and blew the hood off on my first pass. Where do I start?

A: The load-to-fuel/air/nitrous ratio must stay within certain parameters. Unsupercharged nitrous motors tend not to have the torque of a blown motor until they are up on the cam. We usually start with a lot less static pressure and less counterweight, as these cars typically have lower starting gears and a four-speed or five-speed transmission (vs. the three-speed used in blown cars). This lets the motor run more freely to the counterweight and lets the engine get up on the cam before adding a ton of load.

Q: I have a 250-cubic-inch V8, a five-speed trans with a low starting gear, and a two-disc clutch with almost no static and almost no counterweight. Why does the car always bog off the line? (Once it's rolling, it flies!)

A: Sometimes you need enough rotating mass (i.e., flywheel weight) to be able to use some of the rotating energy stored in the flywheel/clutch assembly to get the car to move and keep the engine up on the cam. Since you have a multiple-disc clutch, you could run thicker or heavier floaters, and run a steel (or titanium) flywheel, instead of aluminum.

Q: I have an old Vega Funny Car with a blown, iron big-block, an eight-inch converter and a Powerglide. How much better would my car run with a clutch and Lenco-type trans?

A: It's relatively easy to go two-tenths quicker and five to 10 mph faster. With a clutch that's correctly tuned, we've seen combinations like yours pick up half a second over the best e.t. with an automatic transmission.

Q: I've got a 580-cubic-inch Hemi, blown on alcohol, in my '33 Ford coupe. I've just changed from a torque converter and Turbo 400 to a Titan slider clutch and three-speed Lenco. I'm leaving at 5000 rpm. The car shakes the tires instantly. What do I do?

A: Tireshake is caused by an out-of-balance combination of the supercharger (if blown), the fuel curve and the clutch setup. Usually, tireshake is symptomatic of an engine that wants more to burn (fuel) or less load (same thing). This lean condition — i.e., too much load for a given fuel/air ratio — can happen in a very narrow rpm band; once in this "zone," sometimes the car will start coming apart before you can drive through it.

Let's say we want to change the clutch to try to take some load off of the engine and eliminate tireshake. The most common mistake made is to lower the static pressure of the clutch. This often makes the problem worse. Let's consider what makes the clutch lock up, and what makes it slide.

When we send out a new clutch, it has to be plenty "safe"; i.e., equipped with lots of counterweight, so it will not be damaged by being too loose and burning up on the first pass. All manufacturers of tuneable parts must start safe. (Can you imagine buying a fuel injector and melting all eight pistons on the first run?) We usually start with a moderate amount of static pressure and a lot of counterweight (centrifugal force). Let's say that, when shipped, this new clutch has 10 percent of its clamping force from static (springs) and 90 percent from counterweight at, say, 8000 rpm. The amount of counterweight varies the total clamping force and the rpm at which it overcomes the amount of torque the engine is able to make (that day). The amount of static (spring pressure) varies how quickly the engine is allowed to run up to the counterweight rpm from the leaving rpm.

The sliding is done as an engine runs toward peak-torque rpm. If the clutch is not locked up at or before peak-horsepower rpm, you will have a very-high-maintenance clutch (i.e., new parts required every run). So, in our example, we would suggest increasing the time that the clutch is sliding by doing one, two or three things: (1) add some static (perhaps 150 to 300 pounds); (2) remove some counterweight (maybe six to 12 grams, total); (3) slightly lower the rpm at which you leave the starting line. (Another idea: You might try richening the injector, too.)

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