Tech Talk: 2015 Subaru WRX STi in detail
Back in the day you walked to up a car, got in, and started steering. No longer. These days every new car comes with a slew of technology and for best effect you’ve got to set the thing up before you do any serious driving – Robert Pepper looks at the new Subaru WRX STi in detail.
THE NEW SUBARU WRX STI STILL HAS ALL THE usual electronic safety and convenience features, but because it is a track-focused car we’ll concentrate on what it offers for those of us who enjoy driving in circles at high speed.
Traction Control System (TCS) – detects a wheel spinning and brakes which sends torque to the other wheel. And you go faster. Given the tremendous grip levels of the STi this isn’t as important a system to be concerned with as it would be in two-wheel drive cars. And don’t confuse it with…
VDC – Vehicle Dynamic Control which is Subaru’s electronic stability control system (ESC). If you hook into a turn too fast you’ll either understeer (plough straight on) or oversteer (lose the back end). VDC uses sensors to detect impending doom and will brake individual wheels to keep you going where you want to. For track work older systems did get in the way because for maximum speed you’d need to set the car up on angles of slip that would trigger the system, but Subaru have done well to avoid this with the STi’s version, and to be honest that’s helped by the car’s inherent stability and grip. There are three modes of VDC:
- On – road use, interferes quickly with any slip
- Traction – reduced sensitivity, allows a bit slip but you won’t be drifting in this mode, and TCS is off
- Off –VDC and all other aids completely off, except for ABS.
VDC is a Good Thing. While everyone likes to consider themselves a car-control hero the reality is you can still go very quickly around a track with it on, safe in the knowledge that should you make an error the electronic will prevent a trip to the panel shop. If you’d prefer to rely on your own skills, and such is the spice of life, then it can come off entirely. And you’d certainly want it off for skidpan fun or dirt road dancing where sideways is the name of the game.
If you want the quickest track time then Traction mode will be the go rather than Off, and you should get to any correction before the computers do. In Traction mode you get the benefit of ATV (below) and if you get into slip angles such that VDC starts to operate you’re wasting tenths being sideways.
ATV – Active Torque Vectoring. When understeer under power (not braking) is detected the inside front wheel is slightly braked, which has the effect of tightening the turn. Some similar systems brake both inside wheels and apply extra power to compensate. Subaru do not. Is it good? Well, it’ll make you go faster for less effort. If that’s your definition of good, then fine. If you prefer to rely on skills rather than computers to combat understeer, then not good. Noticeable only when you’re at the lateral grip limit of tyres then you accelerate, not just when you floor it.
ABS – antilock brakes. Prevents wheel lock no matter how hard you brake. The STi also has EBD, which when you brake hard enough to activate ABS the EBD system will distribute brake force front and rear to the tyres best able to use it. ABS just stops wheel locking, doesn’t help work out which wheel is best to brake. EBD is good, as normal cars are front-brake biased so using more of the rear brakes means a better stop, and the potential to help rotate the nose under brakes.
MMDCCD – Only Subaru could create an acronym like that. It stands for Multi-Mode Driver Controlled Centre Differential. You can influence two characteristics here:
- Front/rear torque split – how much torque is sent to the front wheels vs the rear. The default is 50/50, but it can vary to 41 front, 59 rear. Rear-bias will promote oversteer (well, reduce understeer) and lighten the steering, front bias provides handling surety.
- Differential lock – the extent to which the front and rear driveshafts are locked together. When a car goes around a corner the front wheels turn quicker than the rear. In all-wheel-drives such as the STi the centre differential permits this difference in speeds yet drives both front and rear wheels. Unfortunately, there is the risk of the power going to either front or rear wheels which spin at the expense of the other set of wheels which have good traction. If the centre differential has a high lock setting this problem is avoided, so you get better traction but at the cost of understeer (but, there’s ATV to help when on the power). Conversely, a low lock will make the car more nimble in the corners but at the expense of traction.
To control the centre diff there are a few modes. The Auto mode starts off at 50/50 front/rear, but the computers vary the front/rear torque split as they see fit, as well the diff locking extent. The Auto+ mode biases drive towards the 50/50 side of things, and the Auto– mode biases the drive towards 41/59, or a more rear-drive orientation. The computers also decide how far the centre differential should be locked.
You can also manually control the centre diff lock with six stages from pretty well open to nearly locked. To see the difference try a tight turn on gravel at walking pace with the windows down – with the diff locked as far as it’ll go you hear a distinct crunch as the front and rear axles try and turn at the same speed, and extra power is required to overcome the drag. This also has the side effect of making the steering feel heavier. Switch the control back so the diff is free, and the car becomes nimble again and will turn corners readily. When manually controlling the centre diff the torque is split 50/50 front/rear.
MMDCCD set to Auto
Centre diff set halfway to its maximum locking position.
Centre diff set to maximum lock.
SI-Drive – in automatic Subarus this changes gearshift pattern and the throttle sensitivity. The STi is manual so here it just changes the throttle sensitivity – there’s no more power, but a quicker response to accelerator input. The three modes are I (Intelligent) which is for smooth, economical road driving, to S (Sport) and S# (Sport Sharp). The differences are noticeable, but not earth-shattering.
SI Drive set to Intelligent and Sport Sharp. The graph shows the difference in how quickly the power is applied, not the total power.
LSD, Torsen LSD – limited slip differential. A normal differential allows two wheels on an axle to turn freely while being driven, permitting the inside wheel to turn a shorter radius than the outside wheel. Problem is that if one starts to spin – say the inside one on a tight corner at speed – the other wheel gets very little drive. So the LSD semi-locks the two wheels on an axle together, helping prevent that initial onset of wheelspin. You might think that LSDs have no place with traction control, but they do because the LSD helps prevent wheelspin happening in the first place whereas the traction control deals with it when it does spin. And also an LSD helps break traction on the rear if you’re really keen on a bit of sideways action. Some form of LSD is one indication of a true sportscar instead of a car with a pretty body. Many LSDs have pre-loaded clutches, but the STi uses at the rear a Torsen LSD (TORque SENsing) that uses gears to even up the drive between wheels on an axle, and a conventional helical LSD at the front.
Viscous coupling – a device that distributes engine torque to the front and rear wheels. This is what most Subarus use, but the STi has a centre differential that is computer-controlled by the MMDCCD to vary the amount of lockup and torque distribution.
One catch is the dash light – with all these electronic systems there’s only one dash indicator light to indicate the systems are operating, the VDC light. So when that flashes it might be any of the above electronic systems operating, not necessarily stability control.
The image below shows the gearshift, the controls for SI-Drive and the MMDCCD control – press the Auto/Manu button then use the rocker switch to select Auto, Auto- and Auto+ or your desired level of centre differential lock.
The VDC button is located to the right of the steering wheel under the boot release and is marked OFF. One press to put the car into Traction mode, denoted by a green Traction icon and a yellow VDC icon, and a five-second press to turn the car into Off mode which is denoted by the yellow VDC icon by itself (refer to the Auto image above).
So with all that said, how do you set up the car for a given situation? One beauty of these systems is that you can choose your own way of doing things, but here’s some ideas to get you started.
To set a quick laptime on a track day I’d go with VDC Traction mode (one press of the VDC button) as you get the benefit of Active Torque Vectoring. VDC in this mode is late to intervene so shouldn’t slow you down. For sideways fun turn VDC to Off with a five-second button press. For novices leave it all on, you’ll still be quick and have a bundle of fun.
For a spirited on-road drive I’d leave everything on as if you approach the limits where the electronics intervene that would indicate speeds of irresponsibility.
On dirt I’d lock the centre differential up as far as it would go and switch all VDC to Off because you otherwise won’t be able to get the slip angles dirt-road driving demands.