Re: New Audi A6 2.7T 6 speed in the garage

[Paul Nicholson <paul@eisusa.com>]

At 12:40 -0500 11/16/99, Cobram . wrote:
>Paul Nicholson <paul@eisusa.com> writes:
>
> >you figures like 192 miles/gallon. However if you go down the hill 
> >with the car in gear and no foot in the throttle, it shows "---". it 
> >looks like it completely cuts the fuel under this condition.
>
>Hasn't changed a bit, my 1990 V8Q does the same thing.  The lines just
>mean it's over the calculation threshold.  If you look in the archives
>there was a thread a while back about some of the quirks of the onboard
>display.

The engine idles at about 500 rpm. When coasting down the hill out of gear I'm seeing 192 mpg. I'd expect to see less mpg if the engine is engaged braking the car at no throttle and running at 2500 or so RPM. That's why I thought they cut the fuel of under this condition.

When I used to do instrumentation and electronic consulting for Toyota Racing Development, I heard plenty of (old wives?) tales about people that decided to cut off fuel when the engine was braking and paid for it with burned pistons.

The explanation was that compression temperatures were much higher when there was no fuel in the cylinder to cool the air. This does not entirely make sense, as under this condition the throttle is closed and the manifold pressure should be very low, hence not a lot of air would be drawn into the cylinder on the intake stroke, and compression temperatures should be low.

Paul