[Vwdiesel] Diesel Braking

Lee Hillsgrove hillsgrove at adelphia.net
Sat Jan 3 12:49:40 EST 2004

> > A gas engine must contend with manifold vacuum when attempting to fill
> > cylinders.  This vacuum, (@ 25 lbs. or a bit less than 2 atm.), causes
> > resistance to engine rotation.  If the cylinders were able to get all
the air
> > their capacities allow, they only have 1/3 to 1/2 the numeric
compression my
> > diesel has.

 Did you mean 25 inches of mercury, not 25 PSI? That's more representative
of the kind of vacuum you'd expect to find in a gasser intake manifold.
 It's a physical impossibility to have any more than 1 atmosphere worth of
vacuum. Most you can get is 29.92" at sea level and less the higher you go

 As an amateur radio operator, I've had the chance to travel up and down the
Mt. Washington auto road in our club's role in providing communications
support for bicycle and foot races up the mountain. I can definitely say
that neither my '86 NA diesel nor my '98 TDI Jetta really offered much by
way of compression braking. I can also say that even with Mintex Redbox pads
and Zimmermann cross-drilled rotors, you cannot expect to drive down the
length of the road without stopping to cool your brakes, unless you drive
very slowly. Ask me why I know. :-p
 The mountain runs "stages" up the auto road - 3/4 ton Chevy and Ford
passenger vans with big-block engines and automatic transmissions. They lock
them in low gear on the way down and don't use the brakes all too much. I
asked the driver once how often they had to service the brakes and he said
it wasn't all that often. I bet if they were diesel and did not use engine
brakes it would be a fairly regular event.
 BTW, they convert some of the stages to "snow coaches" in the winter, with
tracks replacing the tires. They don't run to the summit with those, but
with all that drag I bet they don't need much by way of braking action,



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