"Jim Thompson" wrote in
message ...
On Mon, 13 Feb 2006 18:19:34 -0500, "Ed Huntress"
wrote:

"Jim Thompson" wrote in
message ...
On Mon, 13 Feb 2006 17:32:38 -0500, "Ed Huntress"
wrote:

"carneyke" wrote in message
roups.com...
Grahm,
Hot is hot, no matter how you "analyze" it. Today's cars generate
much
more heat and there is many more cars than 30 years ago. No science
involved Sherlock, just plain old common sense, I was blessed with
and
you.........

Too much "common sense." Too little understanding of physics.

That's the kind of "common sense" that had people telling us their
vehicles
got their best fuel mileage at 65 mph.

I had occasion to drive a 1977 280Z from Phoenix to Houston, solo...
the wife and children flew, so I conducted an experiment, driving one
hour intervals at fixed speed.

(I was working on fuel metering at the time, so I had a breadboard in
the car that gave me accurate consumption readings.)

The mileage did indeed peak pretty close to 65MPH... around 28MPG
IIRC, but had only fallen to 23MPG at 100MPH.

But the 280Z was fairly light (2355#), pretty aerodynamic, and was
rated at around 21MPG _average_. (1977 was when the 5-speed manual
tranny was added.)

To the degree that was an accurate measurement, Jim, it likely was a
factor
of the crappy thermal efficiency that resulted from running that car at
part-throttle and resulting low effective compression ratio, combined
with
the crappy combustion efficiency we got from carburetors or throttle-body

fuel-injection (or even some of the early direct-port injection) running
at
less than their top cruise flow-rates.

So, what did you get at 45 mph? It should have been much better, except
for
the factors above.

It actually was worse. The 1977 280Z had injection into the
(straight-6) manifold at multiple points with "air-door" control.

Instrumentation was a combination of a positive displacement flow
meter plus a calibrated float in the tank... a precursor to modern
"miles-to-go" systems... with a _discrete_logic_ "microprocessor"
doing all the data churning ;-)

All else being equal, light cars typically get their best mileage at just
over 50 mph, and heavy ones somewhere between 45 and 50 mph. There are many
factors involved, including engine design.

The 280Z had a pretty advanced FI system -- yours might have been an early
Bosch L-Jetronic -- and it was fairly slick. Even so, though, to get peak
mileage at 65 mph, where drag has already become a significant factor,
speaks more to how lousy the system worked at lower speeds. g The
semi-sporty nature of that engine may have involved a lot of valve overlap
or large exhaust valves, both of which knock your low-rpm, low-load mileage
to hell.

In any case, even getting peak efficiency at 50 mph tells us not that the
car has high efficiency at higher speeds, but that the car has poor
efficiency at lower speeds, mostly for the reasons we've discussed.

--
Ed Huntress