Optimum shift points

[QSHIPQ at aol.com]

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If you wander thru the Scar list archives, optimum shift point posts was
discussed only a few weeks ago, here's my post.

Can be used for any car, but you do need some sort of HP/torque graph.

HTH

SJ

Subj:   Shift point formulas 101 for bench racers
Date:   3/31/02
To:  <A HREF="mailto:s-car-list at yahoogroups.com">s-car-list at yahoogroups.com</A>



Ok, you bench quarter dudes are starting to make Rod's wedgie hurt, maybe
requiring a "classic Operation Research problem", more likely, just an "old
fart" classic operation.  Once again, this list is trying to reinvent well
known wheels.  Quarter milers use pretty simple formulas for calculating
shift points.  Rod, I'm flaber-gas-ted at you for not "streaking" in
earlier....  Ok, enough brown-nosing...

RPM After shift = (ratio shift into/raito shift from) x rpm before shift
and
Driveshaft torque = flywheel torque x transmission ratio (NOT final drive).
For you "awd is different" advocates, just think of this as Input Shaft
Torque

Let's do the RS2 6 speed, and see if we can get an optimal shift point from
2nd to 3rd.  I'll use all of Audis published data:
6spd trans gears: 1.32/1.89/1.32/1.03/.86/.71

Peak torque = 302 at 3000rpm (but do note you have 295 from 2500-4600)

So

1.32/1.89 x 6000rpm = 4190rpm in third gear with a 6000rpm shift

Now we go over to the hp chart listed for the RS2
6000rpm = 266lb/ft torque
4190rpm = 295lb/ft

266x1.89 = 502lb/ft driveshaft torque @ 6000rpm (2nd)
295x1.32 = 389lb/ft driveshaft torqure @ 4190rpm (3rd)
so,
loss = 113ft/lbs

try 6600 (243lb/ft)
243x1.89 = 459lb/ft at 6600
295x1.32 = 389lb/ft at 4600
Loss =  70lb/ft

Let's try 3rd to 4th shifting at 6000
1.03/1.32 x 6000rpm = 4700rpm
266x1.32 = 351lb//ft @ 6000rpm
295x1.03 = 304 at 4700rpm
Loss = 47 lb/ft

try shifting at 6600rpm (243lb/ft)
243x1.32 = 320lb/ft @ 6600rpm
280x1.03 = 288lb/ft @ 5150rpm
Loss = 32lb/ft

6600 looks good to me.  Though I must say, given the torque in this motor,
missing shifts by up to 500rpm isn't going to really make all that much
difference in the big picture.  Go to to the track and find out.

Some easter candy for the bench racers....  As Rod might point out in the
context of this post, YSMMV (Your skid marks may vary)

Scott Justusson
QSHIPQ Performance Tuning