NASCAR is a stickler for tradition. Since its birth in the 1940s, while safety regulations have changed, many of the stylings of the original racing stock car have not. When NASCAR first began, the cars were mostly unmodified aside from the interior, and straight from the factory, with the popular model among drivers being Ford's design. The 1950s Ford had a four-speed transmission as is used today in races. Another popular model from early racing days includes the Muncie four-speed transmission. The Chevrolet muscle cars they powered are affectionately called "Rock Crushers." Debuting in 1968, but with roots in the 1950s, the transmission is high-endurance and heavy duty. The original H-design for the four-speed transmission can be traced back to as early as the 1930s with the Borg-Warner transmission. While modern day street cars are mostly automatic now, NASCAR requires that cars use a four-speed manual transmission.
The manual transmissions mean the driver must be very in tune with the car they are driving. One may notice while watching NASCAR that the drivers do not use the clutch to change gears. Rather than using a clutch to change gears, drivers use a technique called "floating" to shift. Floating is done by careful acceleration of the vehicle until one reaches the desired RPM (revolutions per minute) and then "floating" the shift into gear. This is possible with non-synchronous cars, which means the driver must manually synchronize the car's inner and outer force, also referred to as the engine RPM and driveshaft speeds. This means floating, if done wrong, can cause serious damage to the vehicle.
Unlike manual transmissions for a street car, NASCAR stock cars use straight cut gears and no synchros. Straight cut and helical gears are the most commonly used in cars. The advantages of a straight cut gear while racing is that unlike a helical gear, they produce no axial load, which means there is no restriction on the amount of torque used. Due to there being no axial load, the transmission does not produce any heat, which is very important for racing cars, as precautions must be taken against overheating. No axial loads also mean a smaller and lighter engine, as axial loads require bearings and output shafts. Straight cut gears are also cheaper, which is cost-productive as gears must be changed in the transmission after every race. While helical gears provide far less noise and an easier ride, straight cut gears have the advantage of speed, as long as a driver can handle the noise.
The four-speed transmission hasn't changed for a variety of reasons, one being that the more gears added, the heavier the transmission becomes. Keeping the car light and without unnecessary equipment weighing it down is important in NASCAR. For example, their stock cars do not have an air conditioning system, as an AC unit is on average around 50 pounds, and can range from 30 to 100 pounds. At four gears, the transmission weighs in at about 80 pounds.
Part of what makes NASCAR so different from other forms of auto racing is the strict guidelines drivers, teams, and mechanics must adhere to. These regulations don't mean a mechanic can rest easy. The guidelines may be very strict, but a good mechanic knows that adhering to the rules does not mean a day off for them. The best NASCAR mechanics know that the devil is in the details, and will find the best gains for the vehicle which still adheres to the NASCAR rules and standards. While the four-speed transmission is a simple design, mechanics know that it must withstand the test of extreme durability. Poor shifting and improper clutch release lead to degradation of the gears and transmission. This is part of why mechanics must replace gears, face plates, bearings, sliders, and shafts after every race.
While the four-speed transmissions are a fixture of NASCAR, 2021 may change this reality as we know it. In development for official NASCAR racers is the introduction of a six speed transmission. This is a radical change for NASCAR to take, and tests have already been performed at three tracks according to a January 2020 article. Testing began in Richmond on a short track with low speeds, then to Phoenix, where the track it was tested on is designed for cars with high speeds and heavier loads. The third track in Miami used an amalgamation of these designs, and the transmission has received positive feedback from test drivers. If all goes to plan, these transmissions and next gen modifications will begin to be introduced in 2021 into the race circuit.
Manual transmissions have been used in stock cars ever since NASCAR began. There is a lot of disparity between drivers who prefer manual versus automatic transmissions. The argument for the superiority of the manual design is that it offers more control to the driver, and is able to reach higher speeds. It is also more fuel efficient and has lower service costs to have a manual transmission. It requires far more practice and requires an experienced driver. Many drivers of manual cars comment on how they enjoy the sense of control they have over their vehicle versus an automatic.
The team and pit crew is responsible for bringing backups for all elements of vehicle repair, including the transmission. Teams make sure to come prepared with at least three transmissions, including the primary transmission already installed in the race car. The other two are backups brought to the track in case of any hiccups the team may face during a race.
Because the track is usually oval in shape, there is actually a minimal amount of shifting that happens in NASCAR. This is part of why the four-speed transmission has been used for so long; it is cost effective and not necessary on the track to have more than four gears. A four-speed is also far more reliable than a transmission with more speeds. The introduction of six speed transmissions in NASCAR could change this, as they require more shifting and will require practice for driver's who have known nothing but the H-design of the four-speed transmission in their racing cars.
While NASCAR is a popular sport among Americans, other car racing such as drifting or street racing do not adhere to the strict regulations and rules which NASCAR requires of its teams. Four-speed transmissions are virtually unheard of in these cars, as racers have ramped up their gears to six and even ten speed transmissions.