that must balance as the tuner chooses carburetor jets and engine timing
is set. Additionally, since practically prevents turning a car during a
race, getting it correct before the race starts is vital.A typical passenger car runs with a nominal air fuel mixture of 14.1 parts air to 1 part fuel, or 14.1:1 air /fuel ratio, which produces the best mileage to drivability compromise for a street car. Also, the tuning of a passenger car is maintained many times per second with the advent of computer controlled engine management, which gives the production engineers ways to cope with the wide range of parameters that occur with the general public’s varied driving styles.
A NASCAR classed engine, however, is a totally different beast as the air-to-fuel ratios running the gambit from much leaner than a production car to much richer — all dependant on the track. Of course no engine management systems are used in NASCAR racing to make adjustments to the air/fuel ratio. Therefore, the engine tuner’s role on Sunday morning is a vital part of the pre-race decision making process.
Air/fuel ratios, in large part, control how much power a car makes. With that said, the tuners will tune for all-out power or a touchless power and better fuel mileage. While the tune options might sound simple and straightforward they’re certainly not, because items including weather come into play.
Len Wood, chief engine man for the Wood Brothers No. 21 Motorcraft Ford, says that race morning weather is carefully studied before the final adjustments are made.
“When you get up in the morning and the barometer goes down and the air is dry you can fatten the jets up,” Wood said regarding choosing carburetor jets. “Or if you get up and the barometer is down and the moisture is up you can lean it up.”
Wood then added, “We’re constantly trying to balance [the jets] all weekend.”
Fuel, besides providing the energy for combustion, offers cooling to the
combustion chamber. Too little fuel leads to too lean an engine, which in
turn can lead to burnt pistons. Too much fuel and the cylinder pressures
decrease, because the fuel does not light off as quickly compared to a
perfect air/fuel mixture in the combustion chamber.The air/fuel ratios at a restrictor-plate tracks (Daytona International Speedway and Talladega Superspeedway) typically tend toward what would be considered very lean with a 15.1:1 - 15.5:1 ratio the norm for these venues. The tuner has the luxury of the motor running in a very narrow revolutions per minute range and the worries about throttle response are close to non-existent.
At all other tracks the tuner, however, has to deal with RPM ranges that can swing from 6,500 to 9,200 rpm, several times on each lap. This RPM swing can be a challenging factor to address since the car must maintain crisp throttle response because the driver needs his car to lunge out of a corner, not fill the combustion chamber with fuel, when the throttle is applied.
In the normal configuration, a finely tuned Winston Cup motor will have an air/fuel ratio of about 13.5:1 at the exit of a corner and will lean down to approximately 14.5:1 at the entrance of the next corner. This difference is covered, as best as possible, with a carburetor, and a good deal of engineering.
With all the considerations that face an engine tuner the word compromise figures highly into the equation. Therefore, it is left to the tuner to prevail with his experience. He must find the sweet spot. Otherwise, all the long hours go up in smoke prior to the race conclusion.
