Date: Tue, 8 Oct 2002 19:04:05 +0100
Reply-To: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Subject: Re: best speed for best gas mileage
Content-Type: text/plain; charset="iso-8859-1"
>The engine speed at which the maximum torque occurs is
not relevant. Slower = better mileage. That's it.
WRONG. Even though it may not be the engine speed to cruise at for best
economy, max Torque (= Min SFC) - it is certainly relevant.
Take the case of an engine with a cam with a lot of overlap. The torque
curve will fall quite dramatically beneath the maximum, as a lot of of fuel
and air are either coming out the exhaust or being spat back into the inlet
manifold, or the cylinder filling will be just plain poor, but the pumping
and frictional losses are the same as ever. As you venture into this region
you are obviously just wasting fuel, though the payoff with reduced drag may
mean driving below best SFC/Max torque speed.
However reduced the drag on the vehicle is as you cruise slower and slower
there will always come a point when the fuel consumption starts going up
(town driving?). In the case of a 'cammy' engine, that engine speed in top
(or even one down from top in windy/hill climbing adventures) will be higher
than you are suggesting. The VW flat fours are not 'cammy' as such, quite
the opposite and probably have quite flat torque curves, but the principle
remains that slower, doesn't always = better mileage. The aerodynamically
draggy nature also mitigates towards lower cruise speeds for best mileage.
30 mph is too slow for my 2.1 Syncro in top, 50-55 always seems a good max
range speed when low on fuel and looking for a filling station, maybe a bit
high, but I'd have to be desperate to start cruising about in 3rd for any
distance!
A good tip is to ensure that you try to follow the rules for constant energy
driving - slow down progressively when climbing hills, keeping the engine at
maximum torque speed or thereabouts, speed up going down hills and use that
speed to burn-off up the next climb if its a roller coaster road (motorways
are like that in the UK, sometimes for mile after mile, and the same twats
come roaring past you uphill and then lift off down the next slope to avoid
breaking the speedlimit, real or imagined - thinking they're so safe and
smart drivers). Being draggy, the vanagon will not gain as much using this
technique as a heavy (dense) but slippery vehicle with a good overdrive top
but nevertheless, theoretically its the way to drive roller coasters, going
very slow over the peaks with loads of potential energy just waiting to be
converted to kinetic on the way down the next slope. Of course, I might be
the twit here, pissing off people who just want to roll along at the same
speed come hill or high water - as in 'Cruise Controls' - about the worst
thing man has invented - 'Ok, go to sleep if you must, and while you're at
it, I'll burn more fuel keeping a constant speed than you could even if you
were trying' - seems to be the philosophy.
Clive
----- Original Message -----
From: "Anthony Polson" <acpolson@HOTMAIL.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Tuesday, October 08, 2002 5:52 PM
Subject: Re: best speed for best gas mileage
> Hi Nathan,
>
> As with any vehicle, the slower you go, the better the
> mileage. That's an inevitable consequence of the
> unchangeable Laws of Physics.
>
> Go at the lowest speed (in top gear) that the vehicle is
> comfortable with - I've heard it defined as the "lowest
> non-snatch speed in top gear".
>
> The engine speed at which the maximum torque occurs is
> not relevant. Slower = better mileage. That's it.
>
> The problem is, progress at 25 mph may be a little slow!
>
> Good luck!
>
> Tony
>
> '84 Vanagon Reimo
>
>
> >------------- Original Message -----------------
> >
> >Date: Mon, 7 Oct 2002 18:15:52 -0500
> >From: Dart 330 <dart330@HOTMAIL.COM>
> >Subject: best speed for best gas mileage
> >
> >I am curious at what highway speed my Vanagon will get the best gas
mileage
> >at. Would it be at the max engine torque rpm (2800 for my 1.9 I think)? I
> >am
> >planning a trip for Thanksgiving break and am considering slowing down
from
> >my usual 4000 rpm cruise to save gas money and enjoy the scenery a little
> >more.
> >
> >Thanks,
> >Nathan
> >85 Westy
>
>
>
>
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