Detecting tire slip is done a variety of ways. Some units use wheel speed proximity sensors mounted on the wheels to detect a difference in wheel speed between the front and rear tires, while others compare ground speed to rear wheel speed. Still others monitor a speed signal indicative of engine RPM and use software to check the rate of change of this RPM-if it exceeds a certain rate of change (indicating the tires are slipping too much), then engine power is affected. For example, flywheel teeth can be counted to get this rate of change of engine speed, as can transmission gear teeth, or closely monitoring a tachometer signal.

Once tire slip is detected, electronic methods are used to reduce the engine power to the wheels. As one ETC manufacturer stated, "You can take away about 15 percent of an engine's HP with no indication it is being done. You can take more but you get into indicators like sound (engine note changes), temperature of the exhaust (increasing), or fire coming out the exhaust pipes (unburned fuel}."

Also, engine cylinders may be randomly dropped (controlled misfiring), ignition timing can be retarded a little at a time depending on the amount of tire slip, or electromechanical actuators may be applied once excessive tire slip is noted. For example, Racetronics (ph: 610-759-8217) has actuators that apply/release the rear brakes, or manipulate the throttle to manage the engine's output for best traction. Davis Technologies (ph: 828-645-1505) and Tri-Mark Performance (ph: 608-643-0088) market ETC using management of ignition timing, or misfiring.

ETC PACKAGING EVOLUTION
Four to six years ago a typical ETC unit could have been made by cannibalizing various MSD Ignition circuit boards out of their factory packaging and retrofitting them into one of the main ignition control spark boxes. This may still be done at the more local levels of racing. For instance, an off-the-shelf timing retard circuit board, or RPM limiter (controlled misfire) circuit board, might be piggy-backed inside the spark box, activated by a push button on the steering wheel, and then the racer could serve as sensor/activator.

An ETC packaging method of a few generations ago. The extra ignition timing retard circuit board (right hand) is cannibalized from an off-the-shelf MSD Ignition product and installed inside the main ignition control box. This ETC unit established how much timing was retarded by plugging in retard "chips" disguised as RPM-limit chips.

A next step was to control the activation / deactivation of the timing retard circuit, or RPM limiter circuit via a "window switch," which turns the circuit on at one preset RPM and turns it off at another. The racer didn't have to be involved. Sometimes this timing retard circuitry, or controlled misfire circuitry, would be extracted from the factory packaging and installed elsewhere in the car (say for example, in the seat foam), if tech inspectors decided to open up spark boxes and see if they were just a bit too stuffed with electronics.

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