Re: High RPMs at highway speed?

[Scott Fisher <sefisher@cisco.com>]

Sam Clough asks his original question again, after we went off on an
interesting digression:

> Why do our cars run such high RPMs on the Interstate? 

The main reason is so that the engines are in the strongest part of the
powerband at cruising speeds, to make passing safe and easy.  This is a
factor of the cam timing, port velocity, and other design parameters,
which in turn are all conscious decisions on the part of the engineers.  

You might gain an appreciation of the cars' design by asking the same
question from Ingolstadt's point of view -- why do U.S. and Japanese
cars run such low RPM on the Interstate?  There's no real "right"
answer; it's more a case that each band of engineers is working from a
different perspective.  And examining that perspective is something I've
had a lot of fun doing for many years; I hope I can convey a sense of
that fun ain this posting.

> In the A6 at 80mph it's turning 3,600.  

Ah, then Audi is becoming more attuned to the American market.  In my
'83 Coupe, 80 mph is 4000 RPM.  Note also that in an enthusiast-oriented
transmission, the gear ratios are frequently set up so that when you
shift out of gear N at the peak horsepower, you end up in gear N+1 at
the peak torque.  If I shift out of gear N at 5400 RPM, I end up in gear
N+1 at... you guessed it, 4000 RPM.  Hence my observation that 4000 RPM,
as a cruising speed, is set up for optimum passing response.

> None of the American or Japanese cars
> I've ever driven turn anything like this.  Most of them seem to just be
> getting to 3K at around 80/85. It seems to me that a German car designed for
> the Autobahn would have a higher top gear.

The Japanese have done their homework re: catering to the typical
American consumer, who mainly cares about getting across the
intersection before the Joneses can in their Land Crusher 6000 (though
why they do this, only to potter through the freeway on-ramp at half the
speed posted on the yellow sign, is still a mystery).  Low-end torque,
off-the-line response and an absence of a need to shift gears have been
the hallmark of U.S. cars.  Not that there's anything necessarily wrong
with that; it's what the typical U.S. vehicle was engineered to do,
because it kept costs down and made customers happy.  The Japanese
figured this out some time ago and (with the notable exception of
certain VTEC engines from Honda) have moved the torque curves downward
and the sales graphs upward ever since.

Japan, like the U.S., has restrictive speed limits.  A Japanese car
doesn't need to turn more than 3000 RPM or thereabouts at freeway
speeds; in fact, turning lower RPM at freeway speeds is one way of
reducing fuel consumption.  Neither does a U.S. car, not when the legal
limit is 70 and 80-85 is considered fast.

In Europe, the circumstances are and have been very different.  To take
an extreme example, I own a 1967 Alfa Romeo GT 1300 Junior, equipped
with a 1290cc engine that produces about 100 bhp at something like 6200
RPM.  I've seen 8000 RPM on the tach at competitive events (or rather,
I've seen the tach pegged; if it was over 8000 I have no way of
knowing).  My usual shift point in that car, just driving to work, is
between 6000 and 7000 RPM.  

Why is the Alfa motor built like this?  Because it was legal, when the
engine was designed, to travel at unlimited speeds on the Italian
autostrada, but gasoline cost $5.00 a gallon and there were progressive
taxes on engine displacement.  Little engines don't use much gas, but
they also don't produce much power unless you rev the hell out of 'em,
so you end up with a small, low-tax, fuel-efficient engine that, if you
rev the hell out of it, acts like a race motor.  Of course, it's also
because the Italians are all crazy, which may also have some bearing on
why I own two Italian cars (and one drawn by an Italian).

> If I drove this car like I
> assume it was intended to be (ie Autobahn) and assuming the gearing between
> USA and European cars is similar, I'd be cruising at or over 5k. 

And your point is...? :-)

> I mean how long can the motors last being driven that way?  

250k to 300k or more, apparently.

> It just seems to me that
> with the gearing on the car I'm putting unnecessary wear on the motor with
> long highway drives.

No, you're simply looking at cars built from a different engineering
paradigm.  Japanese and U.S. cars are built with the primary intention
of making money for the manufacturer; they are engineered to wear out
just slowly enough that the purchaser thinks they were a good deal and
will buy another of the same marque, preferably a more expensive
version, but wear out quickly enough that the manufacturer makes a good
turnover on new product.  It's critical to understand that modern
automotive engineering is a servant of the bottom line, which is why all
the neat car companies have been bought up by all the boring car
companies over the last 20 years.  You'll never achieve a 20% market
share selling to the 98th percentile.

Furthermore, there's a different customer mentality in Europe.  Our
European listers can chime in with local examples of this, but I in
conversations on other marque lists I've learned that it's a Big Event
when a European family buys a new car, because it happens far, far less
frequently than it does in the U.S.  (Japan has laws which heavily
penalize cars over three years old, so that's a completely different
story.)

> Am I crazy, or are these cars so bulletproof that they
> are immune to the wear affects of higher RPMs?

I'm not a licensed mental health professional, so I can't comment on
your first question, and if you are, welcome to the club. :-)  But for
the second, it's true at the very least that European cars are designed
from a different engineering paradigm.  They're expected to run at
higher RPMs, so their components are all built with that in mind. 
They're expected to last longer, so the metallurgy and component
manufacture are planned with that in mind (but they're also expected to
be maintained, because of the proportionally higher initial expense). 
They're not expected to provide neck-snapping acceleration, so they can
be geared and cammed for optimum higher-end operation.  And they're not
meant to be replaced as often, because of price and often taxes, which
means the owner must amortize the cost across a longer span of years
than the typical US/Japanese car. 

In short: you're probably okay driving 3600 RPM on the freeway, but
change your oil and timing belt when you're supposed to. :-)

--Scott Fisher