Re: da numbuz-

["Rich Wagner" <drsavage@xxxxxxxxxxxx>]

In AD7-467 John Hertzman wrote:

> In AD7-466 Rich Wagner, with his slide rule unhostered (bet that dates
> me!)

Maybe not.  I'm 35, and I own a few of them!  I also made my own circular
slide rule, which, believe it or not, I carry with me in my car.  (Makes me
sound like the biggest geek on this side of the continental divide.)  Comes
in handy for doing division that I can't do in my head.

Anyway, in AD7-466, I thew down the gauntlet, and challenged John to
produce aerodynamic data for the 164.  To my chagrin, he did:

> the penetration coefficient Cx = 0.305 for the base
> and 'L', and 0.300 on the 'S'. Effective drag surface is Cx.s =0,61 sq.m.


Thank you, John.  I'll never question your archives, or their
administrator, again.

For kicks, let's keep this a group project.  From John's numbers, I've
produced the following data for the 164:

Speed (mph)	Drag (lb)	Power (hp)
=====================================
120		242		77
125		262		87
130		284		98
135		306		110
140		329		123
145		353		136
150		378		151
155		404		167

Note that "Power" is the amount of power required <at the wheels> to reach
these speeds.  It is not the power required at the crankshaft of the
engine.  Gearboxes, given what they do, are fairly ineffecient devices,
and turn a substantial percentage of the power they're transmitting into
heat.  This means that the amount of power coming out of a gearbox
(especially a typical, helical gearbox, like the 164's) is substantially
lower than that being delivered by the engine.  I don't have any data on
the 164's gearbox.  If John or anyone else has any efficiency data on it,
please send it on, and we'll rerun the next set of calculations using it. 
In the meantime, allow me to illustrate why it's important to keep all of
your Alfa Owner magazines, regardless of how much the rest of your family
complains about the mess.  In the March 1998 Alfa Owner, Fred Di Matteo had
a lovely article on his 2.8 liter Milano conversion.  On the dyno, his car
produced 139 HP at the rear wheels and 6,000 engine RPM.  My simulation
says that the engine was probably producing about 185 HP under these
conditions.  Note that John says:

>According to the previously cited "Product Bulletin" PA60492039 published
by
>Alfa Lancia S.p.A. the 164 and 164 L maximum power (SAE net) is 183 at
5600
>rpm and the maximum torque is 191 ft.lbs (259 Nm) at 4400 rpm

So, we're seeing a 2.8 liter engine producing about the same power as a 3
liter engine, but at a higher RPM.  This makes me think that my simulation
is probably correct.  That makes the gearbox on the Milano about 75%
efficient.  A 25% loss seems a bit large to me, but I'll wager it's in the
ballpark.  Let's assume that the gearbox on the 164 is about as efficient,
and work with that figure.  The power required at the 164's crankshaft
would then be:

Speed (mph)	Power (hp)
======================
120		102
125		116
130		130
135		146
140		164
145		181
150		201
155		222

183 HP falls at 145.25 MPH which, gearing aside, should be the absolute
terminal speed of the base and L models.  200 HP falls at 149.6 MPH which,
gearing aside, should be the absolute terminal speed of the S model. 
'Course, gearing isn't just an aside, and we have to work with what's in
our cars.  According to John's figures, 5th gear in these cars (with the
stock tires) gives the following RPM/speed relationship:

Speed (mph)	RPM (base, L)	RPM (S)
===============================
120		4,683		5,211
125		4,878		5,428
130		5,073		5,645
135		5,269		5,862
140		5,464		6,079
145		5,659		6,297
150		5,854		6,514
155		6,049		6,731

And this is where John comes back into play.  John has a set of "power
available" curves for these two engines.  The data I've generated here
establishes a set of "power required" curves.  John can now plot, on the
same engine curves, the power required for each of these cars to achieve a
given speed.  In other words, I've defined a set of "gearbox input" curves.
 For example, for the base and L models, gearbox input at 4,683 RPM is 102
HP.  Input at 4,878 RPM is 116 HP.  For the S model, input at 5,211 RPM is
102 HP.  Input at 5,428 RPM is 116 HP.  As long as the power available
curve is above the power required curve, the car can reach that speed. 
Where these two curves cross, is the actual terminal speed of the car,
given the gearing.  John, let us know where those points fall.

Ideally, you want the curves to cross at the peak of the engine's power
curve.  Let's see how the Alfa engineers did.


> After rereading a couple of other posts I wonder if Rich would comment on
how
> much one gains by raising the rev limiter when the power curve is already
> heading south like a rock-

None in acceleration or terminal speed.  However, if you are constrained to
a fixed number of gears (five for example), running up past the power peak
can get you farther into the workable RPM range of the next gear.  If the
final gear is very tall, that can be important.  So, if you've got a 1.5
liter engine that produces 235 HP over a <very> narrow power band, and the
car needs to hit 200 MPH using only 5 gears, it can come in handy. 
However, for a car with a terminal speed of 140 MPH and a power band
3,000-4,000 RPM wide, it's not any help at all.


Rich Wagner
Montrose, CO
'82 GTV6 Balocco

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End of alfa-digest V7 #468
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