Shift Points

The best way to understand how the shift point affects the run is to plot the various factors (so sue me, I'm an engineer so I like to see things on paper). Each gear in your transmission is a factor in the final drive ratio in that gear.  For example, a stock T5 with a 3.35 first gear coupled with 4.10 rear end gears yields a final drive ratio of 13.735 which is great for 'getting out of the hole', but quickly runs out of steam. Alternatively, a T5Z has a 2.95 first gear so the final drive is reduced to 12.095. This lower first gear ratio has effectively reduced the 4.10:1 gears to about 3.60:1. This may not be so great for that hole shot, but then it'll provide more speed at the same RPM. To figure MPH at any RPM, the following equation can be used:

MPH = [0.00297 x Tire Diameter x Engine RPM]/[Rear Gear x Trans Gear]

So at 2000 RPM in first gear, with a final drive of 13.735 and a 26.0 inch tire, the car is moving at 11.26 MPH.

Ratios for some of the popular transmissions are:
 

The Cobra T5 gear characteristics can be shown graphically,

So now the vehicle speed can be found for any engine RPM.  But how does this affect the shift point and ET figures? We still need to have an idea of where the car makes its power.  Quite some time ago, our car was dyno'ed after installing  GT-40 cast iron heads, GT-40 intake and E303 cam to produce the curves shown here. Now the real interesting stuff comes when the two graphs are superimposed upon each other. 

So at any shift point, e.g.. 5800 RPM, the resulting RPM loss can be found for each gear by drawing horizontal intersection lines from the shift line back to the next gear curve.  The first gear shift is the worst, resulting in a loss of 2300 RPM in this situation.  This loss of RPM is unavoidable (unless you change to a closer ratio transmission), however, what can be controlled is where that bottom RPM point lands at the end of the shift. For example, if the shift point were set at 5000 RPM instead of 5800 RPM, the 1-2 shift would drop the engine all the way to about 3000 RPM resulting in a loss of about 40 horsepower and 25 ft-lbs of torque when compared to the 5800 RPM shift. This would require the car to 'spool up' an additional 500 RPM before reaching the same horsepower band.  The time spent coming back into the heart of the power band results a loss in top end mile-per-hour and ET.  Conversely, if the shift point is moved say up to 6200 RPM, a lot of time is spent reaching that shift point while both horsepower and torque are dropping off rapidly.  This will cost ET and MPH just as easily as under shifting the car.  There's a fine balance of selecting the shift point where the area under the curves and between the shift lines is maximized.  If you can maximize these areas, you'll optimize your potential ET and MPH.

This graph can also be very useful in selecting an optimum launch RPM.  If you've got an idea how much you're RPM falls off when your tires hook (called the stall point), you can easily determine how much HP and torque you're losing.  Ideally, you'd like to launch at the point that gives maximum torque to the wheels, but this may not be feasible due to track conditions, the weight of your car, and other factors. Its a fine line again between bogging and spinning when you dump the clutch. However, the graph can be used to determine if you're past the point of 'diminishing returns'; meaning that just because you can keep increasing your launch RPM without spinning the tires doesn't mean that its actually improving your 60' times.  The idea would again be to maximize the area under the torque curve, maximizing your average torque and thus hopefully minimizing your short times.

Update...

Since we switched from the T5 to a Lentech AOD in our car, we had to re-evaluate the shift points because of parameters such as stall converter setting, converter slip, and converter lock-up.  The basic principles are the same, but the curves take on a little different look, mainly because of the slip of the converter.  With the transmission out of lock-up mode, the HP & TQ curves are very broad and flat compared to those for the manual transmission set up and the RPM-vs-MPH curve is less steep since some engine RPM is lost in converter slip. This all complicates quite a bit, but the time and effort are well spent in optimizing the performance of the car.  Another item that we have added to the car to help this optimization is an Auto Meter Ultimate II two-channel tachometer.  This tachometer lets us record both engine and driveshaft data for the entire run, plus it gives us the ability to set up to four separate shift points. These features help us to be even smarter racers.