PH Blog: freewheeling


Decoupling. Get used to it, it's a phrase you're going to read with increasing frequency over the coming years.

Decoupling is a simple concept - quite simply disengaging the gearbox input from the motor and allowing a car to coast without the interia of engine internals. It's nothing new, our ancestors spent endless hours coasting their early motor vehicles down slopes in an effort to reduce fuel consumption, but it does have a marked effect on efficiency.

Clever, yes, but it ruins motorway flow
Clever, yes, but it ruins motorway flow
I've just spent a week decoupling in a new Boxster, and I bloody hated it.

I hadn't realised just how much store I set by being able to accurately judge a car's coasting characteristics with a gear engaged - normally the longest ratio on a motorway. You know the stuff, maintaining progress without using the brakes by anticipating and keeping a view as far ahead as possible. Well, now I do, because de-coupling kind of wrecks normal progress using this method.

On level ground, or a slight downhill section, you feel the powertrain release pretty much instantaneously as you back off the throttle. The trouble is, your brain expects a certain amount of residual deceleration at this point, and it never arrives - it's a bit like falling of a track onto wet grass, that sensation of almost accelerating again as you hit the green stuff. Of course you can adjust for this, but it takes time and distance, which isn't always available on a British motorway. Also, if the car(s) ahead of you are coasting, you have so much less friction that you inevitably catch them and have to brake. Irritating.

Re-engagement is a little sloppy too: there's a definite jolt in the car, and you have to judge the reapplication of throttle very carefully - too much and you'll encourage the gearbox to kick-down, which is most unseemly.

Thankfully you can decouple the decoupling
Thankfully you can decouple the decoupling
The economy gains are undeniable. I was averaging an easy 35mpg in the 2.7 litre Boxster PDK, a staggering achievement from a 170mph roadster, but the cost to serene progress on the Motorway was simply too great.

The upside was I thought I'd found a real gremlin in the otherwise impregnable Boxster package, but that proved to be unfounded. By hitting the Sport button, you kill the decoupling antics and the Stop-Start function - and you get faster shifts and a raised rev-limiter. On balance, I'd live with the latter to avoid the former, but the Boxster would be pretty much perfect if there was simply a button to kill the coasting alone.

By now you know the rest about the Boxster. It's the best car Porsche makes.

Chris

Comments (145) Join the discussion on the forum

  • JerseyS2000 26 Jul 2012

    Interesting legal question - is driving whilst decoupled still deemed to be in full control of the vehicle by the BiB? I was under the impression that its verging on illegal to freewheel in a car??

    Ben

  • kambites 26 Jul 2012

    I'd imagine it'd be horrible when driving hard because you don't have engine braking to get the nose to tuck in; you'd have to use the brakes to steer the car instead.

    There may be an off-switch in the Boxster, but will there be in the next generation of hatchbacks that have the feature?

  • Fastdruid 26 Jul 2012

    No different to anything 2-stroke I'd have thought.

    Jump on a two-stroke and the lack of any engine braking can seriously put you off but you soon get used to it.

    Not quite sure if decoupling would be quite the same but would have to try it to tell.

  • thewheelman 26 Jul 2012

    This Decoupling idea, I really hope it doesn't catch on. It sounds terrible.

  • PSBuckshot 26 Jul 2012

    JerseyS2000 said:
    Interesting legal question - is driving whilst decoupled still deemed to be in full control of the vehicle by the BiB? I was under the impression that its verging on illegal to freewheel in a car??

    Ben
    Good question.
    When I was learning to drive coasting was a HUGE no no.

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