[A4] A4 clutchless shifting

Robert King gt40mkii at gmail.com
Fri Apr 20 15:43:17 EDT 2007

On 4/20/07, Tyson Varosyan <tyson at up-times.com> wrote:
> Stephen,
> I would be curious to see the numbers of your fuel mileage when turning your
> car on/off at lights. Fact is, unless you got some mondo long lights, I
> would bet that you come out losing money. Reason being, the startup fuel map
> on just about any car is SO RICH that you would be better off idling for 5
> minutes than cranking for 2 seconds.

Start-up fuel enrichment serves two purposes.  First, a slightly rich
mixture is easier to start.  Secondly, on a cold engine, the mixture
is run right to pass unburned fuel into the exhaust system (along with
fresh air injected to burn that fuel,) to light off the catalytic
converters faster.  Once the car has warmed up (usually above 160
degrees or so,) the cold-idle strategy isn't used again.

Since presumably while sitting at a light, the engine is warmed up and
cold-idle enrichment isn't used.  That leaves start-up enrichment,
which is only used until the engine starts, A few seconds at most.

OEM manufacturers want to get out of any fuel enrichment modes as
quickly as possible for several reasons: it uses extra fuel, emissions
are increased, exhaust components run at elevated temperatures during
rich operation (depending on the design.)  Ford engineers were able to
cut 40 seconds off their cold-idle strategy on the '86 Mustang by
splitting the catalytic converters in two and moving half of them (the
"pre-cat") as close to the engine as possible.  That also explains why
the A4's cat is where it is.

So, with a fully warmed-up engine, You're simply not using a rich
mixture for more than a second or two.  And even a rich mixture is a
relative term.  Have you logged your injector pulse widths on a cold
and warm engine at idle?  There's not much difference in the rich and
normal pulse widths.  If you like, I can monitor this and report back
on the pulse widths.

>  I don't think that the difference in
> fuel is that huge, but you also got to think about the extra residuals that
> you a coating your exhaust system with from a very rich combustion.

Soot, mainly, which isn't that much of a problem, given that a
properly-running engine in rich-mode still isn't all that rich.
Typically you don't see problems in the exhaust system or the cats
unless you're burning lots of oil, or are running horribly, horribly
rich.  (which I've never seen except on carbureted race cars.)

> makes the little fibers on the clutch stand up more making
> it more gripy and less susceptible to glazing.

Any mechanical engineer or materials engineer will tell you one of the
biggest factors in materials fatigue and failure is reversal of
stress, which is exactly what happens when you let the clutch out when
downshifting.  That makes it all more important to match revs

> The most important benefit is in handling. Hitting the brakes (even with
> ABS) on a slick corner can make the wheel lock and slide sideways.
> Downshifting on a turn keeps the drive train moving and even if you do lose
> grip, the wheel will not lock. By rotating it will not get stuck in one
> place, overheat and lose all traction in a split second - it will be much
> more predictable and gripy. It will also contribute (with proper steering)
> to pull you out of the spin.

OK -- now you are getting into my area of speciality -- racing and car
control.  I currently hold a racing license with the National
Autosport Association and I've been racing in the American Iron series
for three years.  Previous to that I was involved in non-competitive
on-track driving for 15 years.  I am also a driving instructor with
the Texas World Speedway Motorsports Club.

First off, you've no business hitting the brakes in a corner --
especially in a situation where doing so could mean loss of control of
the car.

So if you are in a situation where there's not possible loss of
control, it really doesn't matter what you do -- use the brakes,
downshift, or throw out an anchor.  As far as handling or car control,
it doesn't matter.

So let's assume we are at the limit of control -- where things do get
While downshifting while approaching and entering a turn can be
beneficial (see my earlier post about "heel-toe" downshifting,) it has
its own risks.

When letting the clutch out, if the engine speed isn't right, one of
two things will happen.  If the engine is too fast (common, especially
with beginners,) the car will try accelerate, upsetting the balance of
the car, perhaps enough to cause understeer or oversteer.  If the
engine is too slow, the car will try to slow down, again upsetting the
balance of the car.

In FWD cars, a mis-matched downshift can cause the car to understeer
badly, since the front tires have an additional load imposed on them
by the engine at a time when cornering alone is using up almost all of
the tire's grip.

In RWD cars, the same thing can happen causing the rear wheels to
loose grip, leading to a spin.

AWD cars are much less prone to either.  whether it understeers or
oversteers depends on the setup of the car.  My A4, when pressed,
behaves very much like a RWD car.  My missed downshifts have always
made the A4 light in the tail, threatening a spin.

And I've never recovered from a spin by letting the clutch out, FWD,
RWD, or AWD.  Spin recovery is a whole other bag of tricks.  :)

> Also, it is good for your engine to downshift. When downshifting your engine
> works as an air pump - no real ignition going on there and cools off very
> rapidly. If you have an EGT gauge, you can observe this very easily.

EGT is nice in that it reacts pretty rapidly to temps in the
combustion chamber -- good for tracking fuel mixture, though they're
not used as much these days since wide-band O2 sensors have come down
in price.

But what's the use in cooling down the combustion chamber? The most
benefit you'll get from that is that the exhaust valve will cool down
somewhat, but that's hardly significant in a street car.  If you're at
risk of burning the exhaust valve, and have to rely on pumping cool
air through the combustion chamber, then you've got some serious
problems with the engine.

Indeed, I'd be more worried about cooling off the catalytic converter
too much.  Cats work best at about 600 degrees.  Get them much cooler
than that and their efficiency plummets (get them much hotter and they
melt down -- remember that if you plan on putting in a bigger turbo
and plan on going half-assed with your fuel management!)

>  Guys on the track compression break not because their brakes are insufficient, but
> because they want to keep their combustion temperatures low - especially
> important on a highly tuned turbo cars.

I don't know of a single driver or driving instructor who relies on
compression braking to keep their engine temps under control.  If a
car is overheating, we short-shift.  Taking 500 RPM off the shift
point will do wonders on the track.

Like I said earlier, I wouldn't consider any reasonable, street-driven
car, to need this kind of band-aid to help the motor in any way.

We (competitive drivers and instructors,) just do not
compression-brake to any great extent -- period.  We use the brakes to
slow the car.  Anything else that slows it is incidental.

We do heel-toe downshift to avoid upsetting the car, especially under
hard trail-braking (turning and braking at the same time -- which
makes the car especially sensitive to changes in the balance of the
car.)  That's the only reason.

For novice and intermediate drivers we teach them to keep the car in
gear as long as possible, then downshift BEFORE the turn-in point, and
accelerate through the corner in the lower gear.  This is the easier,
most straight-forward, and safest technique for them.

>That being said, you should never rely on the thin edge given to you
>by downshifting on the street. There are too many variables there and you
>should always take it slow.

Agreed.  If you're driving anywhere near the limits we're talking
about here, you shouldn't be anywhere near the street.  Take it to a
race track -- its cheap, safe, and most of all FUN!

>  However, the skills learned, like proper
> downshifting, will help you on the street handle your car better.

IMHO, the only benefit I ever for from heel-toe downshifting on the
street is as practice for the track.  There's just no real need or
benefit on the street.

Instead use what ever technique is easiest on the car.  If you can
match revs easily, then go ahead, but if you're jerking the car all
over the place, or zinging the motor to 7k RPMs or above, don't
complain when something breaks.

-- RK

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