<TABLE BORDER=0 ALIGN=CENTER WIDTH=85%>[TR][TD]Quote:
On 2002-09-28 03:05, Dustin wrote:
If you want to get right down to it. Ideally you shift at peak horsepower
No, that is incorrect:Lingenfelter Performance Engineering, Inc.
LPE Drag Racing Tech Tips:
BFG #6 Proper Shift Points
As previously promised, we will try to explain proper shift points. Whether you drive an automatic or a manual transmission equipped vehicle, shift points are an important factor in obtaining the best performance at the drag strip. Even in an automatic equipped vehicle, you can have varying amounts of control over shift points. Many automatic transmissions will allow you to manually shift the transmission.
To determine proper shift point, it is important to know the gear ratios of the transmission and the power curve of your engine. Some owner´s manuals will provide the transmission gear ratios and may even have an engine dynamometer power curve of the engine. If possible, a chassis dynamometer test of the vehicle in each gear is the most helpful. Because the transmission ratios are rarely evenly spaced, your optimal shift points may not be the same for each gear change. Also, because shifting actually takes time and full power is not applied to the wheels during the shift, it can often be advantageous not to shift if the shift is going to occur just before the lights.
In choosing the proper shift point, you are trying to maximize the power delivered to the ground. In terms of the engine power curve, you are trying to maximize the area under the curve. As an example, in a stock 1995 manual transmission Camaro Z28 you have the following:
Ratio Shift RPM @ MPH Drop to RPM
1st gear = 2.66 5500 45.6 3680 25.60 = tire diameter (in)
2nd gear = 1.78 5500 68.2 4017 3.45 = Rear ratio
3rd gear = 1.30 5500 93.4 4231
4th gear = 1.00 5500 121.4 4070
5th gear = 0.74 5500 164.1 3716
6th gear = 0.50 NA NA NA
As you can see, despite the 5600 rpm shift point in each gear, the different gear spreads result in different rpms that the engine drops to when shifted into the next gear. This engine makes its peak horsepower of roughly 255 hp (at the rear wheels) at 4900 rpm. At 5500 rpm the engine is still producing 247 horsepower at the rear wheels. The engine also makes 247 horsepower at and above 4500 rpm. At 3700 rpm the engine is only producing 215 hp so if you shifted at 5800 rpm out of first instead, you would drop to 3880 rpm and the engine would be producing 225 hp at that rpm. If you had an engine that lost power more quickly after it reached its peak it might not be advantageous to shift so far past the power peak.
When determining proper shift points it is also important to consider the safe operating speeds for your engine. Consult your owners manual or your engine builder when selecting shift points. Shifting after the redline or past the safe operating speed of the engine can result in premature engine wear or damage.