Two wheel drive cars - and very powerful four wheel drive cars - all suffer from a performance limitation: tyre grip. That is, if there is enough power to spin the driving wheels, you're not accelerating as fast as you would if all that torque was being transferred to the tarmac. And, in the real world - as opposed to talk at the bar - a car that can spin its wheels under power is also not as safe as one that cannot do this. The latter point's the reason that most manufacturers of powerful cars these days fit OE traction control - a system that detects wheelspin and drops engine power and/or brakes the spinning wheel. OE traction control systems vary enormously in their state of software tune - some cut power so strongly that it becomes an absolute pain to drive these cars fast; others are so subtle in their action that only a flashing dashboard light shows that the system is indeed working.
In the aftermarket, traction control systems are available in two forms: as part of a topline fully programmable management system, or as a standalone system able to be fitted to cars using the original engine management system - or even fitted with a programmable ECU not having a traction control feature.
The best-known aftermarket traction control system is from UK company, Racelogic. Their system - while expensive - can be fitted to all late model cars, and is programmable in its behaviour. Further, it optionally also incorporates launch control, data logging and full-throttle shifting.
The Racelogic System
The Racelogic system monitors individual wheel speeds using the output of the ABS sensors, or new sensors fitted just for the traction control system. When it is sensed that the driven wheels are spinning a certain percentage faster than the undriven wheels, engine power is reduced by a fuel or spark cut. (The system cannot brake wheels.)
When the fuel cut procedure is selected, the Racelogic unit is wired so that individual injectors can be cut off in a rotating sequence. This means that cars with group-fire injectors need to have the Racelogic loom connected near to the injectors themselves, while sequential injection cars can be interfaced near to the ECU. When the fuel cut operates, the cylinder does not fire at all - so a lean air/fuel ratio burn does not occur. The fuel cut is rotated in order that the fuel lining the inside of the port isn't scavenged - if this happened, the next full combustion would be lean. In addition, it is important that the deactivated cylinder be rotated in a certain sequence if torsional affects on the crankshaft are to be minimised.
The alternative approach is to cut the spark, using an additional spark interface unit which is able to drive up to two separate coils. The spark cut is automatically cycled so that the plugs do not foul. Because raw fuel passes into the exhaust when the spark cut occurs, this approach is not recommended for cars fitted with cat converters.
The ABS inputs are normally accessed near to the ABS control unit. When a car has three ABS sensors rather than four (eg for the rear wheels just a single tailshaft sensor is used), the traction control system can still be fitted; however, a laptop needs to be used to configure the software as the TCS cannot configure itself automatically.
In addition to these connections, the unit needs engine rpm, 12V and ground wiring. If the system is equipped with full throttle shift control, a clutch switch input is also needed.
The driver control over the complete system is via a dash-mounted control box. When the launch control option has been specified, the box contains a launch control button, a LED, and a rotary knob with five positions. The positions are:
- 10 per cent slip
- 15 per cent slip
- 20 per cent slip
The LED is used both for diagnostic functions (eg to make sure that wheel speeds are being sensed) and also lights as a warning when the system is switched off.
The optional launch control enables full throttle to be used prior to driving off from a standstill. A secondary rev limiter is brought into effect, holding revs at a preselected level. The driver then dumps the clutch and the amount of wheelspin that results is controlled by the unit. Both 'wet' and 'dry' launch rev limits can be set, and in addition, the level of wheelspin at which the system switches from Launch Control to Traction Control can be adjusted. The optional full-throttle shift facility uses another rev limit that is imposed momentarily when the clutch is depressed.
While the system can be set up without the use of a laptop PC, better functioning will be gained by using the provided software to configure the unit. With the use of a laptop, data (eg the four individual wheel speed and engine rpm) can be logged and displayed in graphical form.
However, more importantly, the DOS software allows many changes to be made to the way in which the TCS operates. For example, each of the following parameters can be altered:
- Cut tables - where the severity of the engine power reduction can be altered for each of the following: the driver selectable 1,2,3 wheelspin thresholds; for the soft cut rev limit; and for the hard cut rev limit. This control determines how many misses per cylinder per engine rpm occur, eg each cylinder once every 4 rpm.
- Straightline override - the software calculates when the car is turning and when it is being driven in a straight line. Generally, more traction control needs to be added when the car is cornering. For example, if the straightline override is 5 per cent, this value will be added to the permitted slips when the car is being driven in a straight line.
- Maximum difference - sets the speed difference between the driven and undriven wheels that will be accepted as feasible, to allow the rejection of errors. For example, if the launch control allows us to 50 km/h speed difference, this figure could be set to 70 km/h.
In addition, there are many other adjustable parameters - incredibly, there's over fifty of them!
We watched Australian Racelogic distributor, ChipTorque, fit the system to an Impreza WRX. It was an unusual application - obviously if all four wheels are spinning at the same speed, the system will not be able to detect it. However, in viscous-coupled four-wheel drive systems such as that used in the Impreza, usually the front wheels spin before the rear wheels.
ChipTorque fitted the system in order to see how well it would perform in a constant four-wheel drive application, and Terry, the owner of the car, was happy to have the system for a trial period, free of charge. The Racelogic system was optioned with the launch control function, another key reason why it was fitted to the car. The WRX was already running a MoTeC programmable ECU, piggybacked on the standard Subaru ECU.
Fitting was straightforward, with power and injector connections made at the engine management control unit.
The wheels speed sensors were accessed at the underbonnet ABS unit. Here the headlight has been removed to allow easy wiring access.
The engine rpm signal was also made at the engine ECU.
Checking that all the connections were correct and configuring the software was done via laptop PC.
And how good was the Racelogic system in the WRX? Well, it worked - but whether it is worth the AUS$2970 (plus AUS$420 installation) is another question. With the launch control activated, the engine was developing boost before the clutch was dumped - but then with the Rex's fragile gearbox, how often do you want to do full-bore launches anyway?
As Terry, explained, "It would be perfect for a WRX with a dogbox - you could launch as hard as you liked, then."
In terms of stopping wheelspin, the street driven WRX doesn't get enough wheelspin to really act as a good test vehicle. However, ChipTorque claim that on cars such as aftermarket supercharged V8 Commodores and the like, the system works very well indeed.
We saw enough of the Racelogic system - especially in its range of software adjustments - to be confident that it could be very effective, especially on a powerful two wheel drive car.