"Alex Esplin" <[EMAIL PROTECTED]> writes:
> The data points were gathered in my 99 Passat on the freeway between
> St. George and Provo, a trip I've made _many_ times. All other things
> being as equal as possible (temperature, wind, etc) setting the cruise
> at about 93 (which placed the RPM range right where the turbo started
> to produce good boost, but a few hundred RPM below where the variable
> lift on the valves started sucking in _lots_ of gas) produced mileage
> over 35 mpg for the tank I filled in St. George and topped off on
> arriving home.
93 mph must put you right in the optimal powerband for your engine in
top gear, and you must have a pretty low coefficient of drag to avoid
wasting huge amounts of power to maintain that speed. I don't think
the turbo had anything to do with it, because if you weren't
accelerating, the manifold was still at lower-than-atmospheric
pressure; i.e. the turbo wasn't providing any boost, and the engine
would have been perfectly capable of pulling that amount of air all by
itself.
If you have a boost gauge in your car and you did actually see boost,
then I guess your car is set up differently than mine, but I can
cruise at 93 mph without boost in an old 1.6l turbo engine, so I
expect your newer and bigger (1.8L turbo?) engine can as well.
Again, I have access to the fuel injection and timing maps of my
engine, since I have an aftermarket ECM, and the maps for turbo and
naturally aspirated cars are pretty much the same in the non-boost
areas.
As I pointed out in my earlier discussion of Miller Cycle engines, it
might be possible with a turbo, variable valve timing, and a clever
ECM to get greater efficiency at cruise from a turbocharged engine, I
think the gains are mostly at low RPMs where a turbo wouldn't be doing
much anyway, thus Mazda's choice of a postive-displacement
supercharger instead of a turbo for their Miller Cycle engine.
--Levi
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