Racepak also offers a display-only dash for use on the V-net. It can play 21 channels in real time and has turn indicators and other functions necessary for street machines.
Reading the Data
Engine and Driveshaft RPM
Graph A (page 72) illustrates a near perfect run from a Pro Stock five-speed car. A slower street machine would create similar curves, just not as steep! Note that the driveshaft rpm (blue) follows the engine rpm (red). As each shift occurs, there is a small rpm spike before the two curves converge at the end of the run. Pro Stock racers monitor this area carefully to avoid bogging the engine between shifts and to adjust tire pressure and suspension for the launch and 60-foot time. A spike in driveshaft rpm can only mean tire spin or component failure. Since the graphs are time based, it is easy to determine where and when tire spin is occurring.
This graph is from a five-speed car on a run. The upper red line shows engine rpm in relation to driveshaft rpm in blue and the difference between the two in orange. Note the tire spin on the blue line just after the shift. As the car reaches 1:1, the red and blue should meet.
Another use for this information is transmission analysis. Since the best torque converter in the world keeps you close to peak horsepower for the greatest amount of time, knowing how much it's slipping can be used to increase acceleration. If you are trying to optimize your pass, you want to go through the lights at peak hp with the converter putting all the power on the ground. The program automatically compares engine versus driveshaft rpm and determines the percentage of torque converter slippage. A high engine rpm and low relative driveshaft rpm (high percentage of slippage) for example, would tell you to use a tighter or lockup converter.
If the car suddenly slows down, eliminating variables like fuel pressure and battery voltage will help to diagnose the problem. Carbureted cars using nitrous or superchargers are vulnerable to fuel pressure and voltage drops.
Finally, this info can be referenced to engine rpm to determine if the clutch is slipping in a manual trans, and abnormal spikes in rpm at the shift points will indicate an auto transmission that is slipping between shifts on its way to failure.
Battery Voltage and Fuel Pressure
Graph B shows the load that is placed on the fuel and electrical systems during a run. It might be a bit of a surprise to learn that the Racepak guys encountered a drop in voltage as one of the main reasons racers fail to realize the full potential of their vehicles. A typical example is the racer who moves the battery to the back of the vehicle or disconnects the alternator for more speed thus increasing the resistance between the battery and the charging system and reducing available voltage from the battery. Laying the curves over the rpm curve, any drops in rpm that correspond to a drop in voltage is an indicator of a problem. A downward curve could indicate that the ignition is starved for electrical power. The overall voltage curve is important especially when using high-end ignitions that can draw upwards of 35 amps that require a minimum voltage to operate. On a nitrous car, for example, the nitrous solenoid draws more amperage than the fuel solenoid. If the alternator fails on a run, the fuel solenoid will bog the engine with fuel and possibly backfire.
What Does It All Mean to You?
If you are serious about what you are doing, there is a basic system that will help you analyze tire spin and transmission, voltage, and fuel pressure. As the car gets faster, the system is easily upgradeable to add items like a g-meter or air-fuel ratio monitor. Getting the results you want is a matter of going to the racetrack, building a database, and learning your way around the data. It will add a whole new dimension to the Friday night drags and give you a real advantage. CC
The g-meter produces a graph that is best utilized by comparing it with other data from the run. If there is an abnormal drop in g-force, it will correspond to tire spin and driveshaft rpm or point to another problem.
The g-Meter
The next logical step is actually a step away from the basics. A g-meter (or accelerometer) helps you attain the goal of accelerating as fast as possible for as long as possible by graphing the rate of acceleration throughout the run. This graph illustrates a two-speed car on a run. As you can see, the g's drop dramatically after the car shifts to high gear. As soon as the line on the graph dips, when it spins the tires for example, you'll know that the car is not accelerating as hard as it should. You can lay the g-meter graph over the other data and determine the cause. It is also possible analyze two runs at the same time after a change is made to the car.
Where Is the O2?
Most systems will record air/fuel ratios, but generally it is something that should be handled on an engine or chassis dyno, or during a test and tune night with adjustments for altitude and temperature calculated before race day. Once you build a database of air/fuel using a meter like the one from Innovate (see it online at carcraft.com), or with output to the Racepak, you should have a grasp of where the power peaks are for the engine and what the air/fuel ratio is. You should already have this data before you get into other methods of data acquisition, but if you want to take another step, the air/fuel data could be used on track to put a finer point on the overall package.