164 Terminal Speeds 19 1/2: This Time, It's Really Terminal.

["Rich Wagner" <drsavage@xxxxxxxxxxxx>]

Hi Gang:

As I alluded to in my last post, I've completed my analysis of the 164
terminal speeds.  This morning, Colin emailed me the engine power curves
for the base, L, and S models.  Using the data from the analysis I posted
earlier this week, I laid a set of power required curves down over the
engine power curves to see where they intersect.  The intersect point
indicates the cars' actual terminal speeds, given their gearing.  If any of
you would like to see the graphs, let me know, and I will email you a set. 
Perhaps Colin can post them on his 164 page.

The result, for the base and L models was very interesting.  The power
required curve intersects the power available curve at its highest point. 
I don't believe this to be a coincidence.  To reach its absolute terminal
speed (the maximum speed the car should reach, based solely on it's
available power), the gearing must be arranged in just such a manner.  I'm
sure the Alfa engineers chose the overall fifth gear ratio accordingly. 
This also validates, to some degree, the engine curves and aerodynamic data
that John scared up for me.  For those interested, it also indicates that
the driveline of the 164 is about 75% efficient--a figure which I'm betting
will apply well to any roadgoing system.  (Expect a straight-cut race
system to be more efficient.)  The final results:  the 164 base and L
models should reach a speed of 145 MPH without hitting the rev limiter. 
With performance mods (real ones, that is) they can go faster.

The result for the S model is even more interesting.  As we all know, the S
has more power available, and so should have a higher terminal speed.  Not
so.  In fifth gear, the base and L models will stop accelerating right at
their 5,700 RPM power peaks.  The S model is geared much shorter than the
base and L models.  In fifth gear at 5,700 RPM (also its power peak), it
will only be going 131 MPH.  That's a gearing constraint, and can only be
changed by changing the gearing.  We believe that the rev limiter for the S
is set at 6,200 RPM.  At 6,200 RPM, the S will be going 143 MPH.  Unless
the rev limit is changed, it simply can't go faster.  Now, don't go out and
start playing with your ECU.  Even with a mod to its ECU to extend the rev
limit, the car will stop accelerating at 147 MPH, 3 MPH short of its best,
because of the short gearing.  Given the correct gearing, the S has the
potential to go about 150 MPH.  On a terminal speed run in your S, you will
hit the rev limiter.  This makes the base and L models a few MPH <faster>
than the S.  It appears that the Alfa engineers chose shorter gearing for
the S to provide snappier acceleration--at a slight cost to terminal speed.
 A terminal speed run will not tax the engine in your S to its limit.  At
its power peak of 5,700 RPM, the engine will still have about 25 unused
horses on tap.  The result again:  the S will top out at 143 MPH against
the rev limiter.

So, the S model is a real drag racer compared to the base and L models. 
With more power and shorter gearing, it will easily walk away from the base
and L under acceleration.  However, with the pedal to the floor and plenty
of straight, flat highway, the base and L will eventually walk away from
the S.  Course, the difference is literally a walk, 2-3 MPH.  Given the way
it accelerates, and a terminal speed that is, to the human, the same as
that for the base and L models, the S must feel like a LOT of car.  I'm
sure the Alfa engineers planned it that way.


Rich
Montrose, CO
'82 GTV6 Balocco

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End of alfa-digest V7 #474
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