As a good New Englander, you probably own a car that is “good in the winter.” That may mean simply fitting snow tires, driving more carefully in the snow, or adding some mechanical advantage – such as front wheel drive or all wheel drive (or four wheel drive – yes, it’s technically different, I know). Whatever your solution to the problem of slippery surfaces, how much do you think about how your vehicle does what it does?
Porsche recently introduced their latest Carrera 4 and 4S models with, beyond the standard 992 changes, new tech: a liquid-cooled front differential for the all wheel drive system. As an owner of a 2007 Targa 4 with AWD (not to mention a string of Cayenne’s), I was curious why they would bother investing time in a system that most of us won’t ever really think about. Liquid cooling implies that the goal is to disperse heat, right? Why is there heat here to worry about? I was curious so I did what anyone does these days and Googled it.
If You Can’t Take The Heat
As I’m sure you know, when a car turns, the wheels on one side go faster than on the other. This is because the wheels on each side are suddenly following two different arcs with two different radii. All that high school geometry is coming back to you, right? But why clear those cobwebs? Well, if you solidly connect all those wheels together, they literally get themselves into a bind. The binding pressure builds up and has to go somewhere. To get out of this bind (known as torque-wind), the parts might deflect and bend or they will break. This, of course, is bad. We need to add something with a bit more give – a “differential” – a brilliant little bit of engineering that allows a wheel on one side to temporarily “slip” a bit and go a different speed. The price you pay for this controlled slippage without breaking? Usually that energy from the “slip” is dissipated as heat, often through a clutch pack or viscous coupling device of some sort. There is always a price to pay.
It was a great story arc that went from an origin of concerns about the fuel economy of four-wheel-drive systems born in the 1970’s and how saving every drop of fuel became a primary design goal – sound familiar? The Borg Warner chaps decided that being able to disengage a four wheel drive system’s front wheels manually at the hubs just would not do. Would you want to have to hop out in the snow and mess with turning some big dials on the front of your car? How plebeian. Our computer overlords could surely do a better job of doing this for us.
Gone are the inconveniences associated with part-time-four-wheel drive. Drivers don’t have to ruminate over their choices; they don’t have to wonder whether they’re in four-wheel drive; they don’t need to possess a shred of knowledge about the technology.
In the 1980’s they set to work fitting a dodgy station wagon with a prototyped computer-controlled torque sensing/splitting central transfer case that did away with the center differential completely – by using sensors to react to wheel spin by sending more torque to the front wheels via an electronically-controlled clutch (clutches are basically two spinning discs that rub together to different degrees to vary the rotational speed differences between them – but they get hot with all that friction). The computer to control the new transfer device took up the whole back seat.
Eventually they found that constant measurements of the change in RPM between various bits told them what they needed to know about slip to tell the central clutch pack to engage and grab some power for the front wheels. They packaged it up and sold it to Ford. And shrunk it so it didn’t take up the whole back seat anymore. As a side note – the clutch material was paper-based – odd choice for a heat-dependent system, no? But they tested it and raced it and it worked – after they made it bigger…to handle the heat.
Porsche’s Brand Of AWD
About the same time Borg was getting Ford on board, Porsche was putting an all-wheel-drive system into their 959. Interesting case study in economics right there if you consider the cost of an ’80’s Explorer versus a 959… Porsche’s system was similar, though, in that it used lots of computer-controlled sensors and a computer-controlled clutch pack to go from a pretty even front/rear torque split to as much as 20/80. As far ahead as Porsche was in some ways, the 911 had to make do with a passive viscous-coupling system for all wheel drive up until the computer-controlled clutch pack system went into the 911 Turbo in 2006. The fancy computers didn’t go into the base Carrera 4 until 997 gen 2 in 2008, after the Cayenne helped push the technology along a bit. [ed: some great member feedback – Nick D. pointed out that the 964 C4 had a fully mechanical AWD system with electronic controls that were very similar to the technology in the 959. The 993 went to a more simple and lightweight viscous-coupling system that was much more rear biased but slower to correct for wheelspin. Thanks, Nick!]
Once the technology was in place, what has changed since is…power! (In Jeremy Clarkson voice, of course) The latest base 992 is up to 379bhp and 331lb-ft of torque. My 2007 only had 325bhp and about 270lb-ft. These new turbo base cars are really bringing the torques! I’m betting that this new power, along with ever better and grippier tires, is the reason Porsche is adding some extra cooling to the package. More power means more slip means more heat.
So where did we wind up? All that torque wind-up wound up getting me all wound up. And all this so you won’t wind up in a ditch. It truly is all about where you wind up.