Transmission, Fuel and Tuning
Unlike the engine, the transmission of a Champ Car is something that the team is intimately familiar with. The team rebuilds the transmission and can change the gear ratios depending on the track.
The Motorola team's transmission is supplied by Reynard along with the chassis. The transmission is built by Xtrac, a very well-respected transmission company in many areas of automobile racing.
A Champ Car uses a six-speed sequential transmission that is more similar to a motorcycle's transmission than to a typical automobile's. Shifting is done by the driver using a small lever to the right of the steering wheel:
Another feature of a Champ Car's engine and transmission is called Shift Without Lift, or SWOL. The engine control unit allows the transmission to shift gears without the driver lifting off the accelerator. The driver can therefore upshift without using the clutch or letting off the gas, and this maximizes acceleration. The SWOL feature is also available during downshifting, but the driver must match engine rpm with the gear choice during the downshift.
Champ Cars burn methanol fuel. Methanol is a form of alcohol and has several advantages over gasoline in an engine:
- Methanol can run at much higher compression ratios, meaning that you can get more power from the engine on each piston stroke.
- Methanol provides significant cooling when it evaporates in the cylinder, helping to keep the high-revving, high-compression engine from overheating.
- Methanol, unlike gasoline, can be extinguished with water if there is a fire. This provides a nice safety feature.
- The ignition temperature for methanol (the temperature at which it starts burning) is much higher than it is for gasoline, so the risk of an accidental fire is lower.
The only significant problem with methanol is that it burns with an invisible flame -- you cannot see a methanol fire. People don't know that they are near a methanol fire until they feel the heat. This includes the driver, who in a crash may have methanol spilled on his suit. The driver will therefore move flagrantly once he has detected a fire to let other people know that there is a problem.
The car carries 35 gallons (142 liters) of fuel in a fuel cell located behind the driver. This cell is made of a flexible Kevlar and polymer material -- it is more like a bag than a tank. Inside the bag is a sponge-like substance that gives the bag its shape. The bag is designed to withstand a crash without rupturing -- rather than rupturing, it flexes and changes its shape. The idea behind the sponge is to hold the fuel so that, in a severe crash, it does not spray over the driver, other cars or the track.
The engine burns methanol at a rate of approximately 2 miles per gallon, meaning that the car must make a pit stop for fuel approximately every 70 miles or so. During a pit stop, the fuel pours into the cell through a large filler mounted just behind the driver. Thirty-five gallons of fuel can flow into the cell in just a few seconds!
CART rules allow each team to use fuel at a rate of up to 1.8 miles per gallon. That is all the fuel that the team gets, so each team must manage its fuel consumption to work within that limitation.
Once the team receives the chassis from Reynard and the engines from Mercedes, the team assembles the car. The team and the driver then begin the season-long process of tuning the car. The team has intimate control over many different aspects of the car's setup, including:
- Tire toe-in/toe-out, camber and caster
- Air pressure in the tires
- The height of the car off the track, as well as the height of each of the four wheels
- The stiffness of the suspension
- The adjustment of the aerodynamic wings as well as the overall downforce on the car
- The driver's position
- The gear ratios in the transmission
- Brake bias (the relationship of braking force between the front and back wheels)
- Weight distribution on all four tires (known as balance)
- Anti-roll bar settings
- Various engine settings
- Length of the wheelbase (by changing the wishbones, the team can make the wheelbase longer or shorter)
The goal is to adjust all of these variables in concert with one another to create the perfect setup. Obviously this is not easy because all of the variables have interrelationships with one another. Getting the car tuned and keeping it in a state of perfection are two of the team's most important tasks during the season.
A big component in the tuning process is the data gathered by the car's telemetry system. The team can adjust things and then look for changes in the car's performance in the data that the car's sensors gather.