Before digging into the subject, we need to review some assumptions on which our discussion will be based. This story isn't intended to identify specific gear combinations for equally specific track conditions. Rather, it's meant to establish some lines of thought that will enable you to make informed decisions while utilizing information that relates directly to how you can integrate various factors that help enable sensible gear choices.
In circle track racing, it's problematic that track conditions undergo a constant state of change during a race. Obviously, this can be more of an issue on dirt than asphalt. Some compensation for this can be made by driving technique while other solutions may be based on chassis setup and gearing, the latter of which we'll be reviewing. In addition, there is no such animal as "absolute traction." In fact, in some instances, a racecar will accelerate more quickly during tire spin, based on where in the rpm (torque) range this situation occurs. Then there are times when the engine is not at wide-open-throttle. But even though these conditions tend to dilute the "academic approach" to determining the best gear/track combinations, there are some guidelines we can discuss. There's more to working out proper gear combinations than just selecting gears. Part of the process involves what you'll be providing for their input.
Defining The Problem
Smokey once told me that circle track racing is little more than a "series of drag races, tied together with corners." For as much of an oversimplification as this may be, it does represent a measure of truth. We'd like to have the car accelerate as quickly as possible on those parts of the track that will reduce lap times the greatest, enable passing when the need arises, and provide the greatest level of control. Clearly, there are variables in play that include an ability to integrate available power with chassis setup and reaction. But unless the mechanical link between flywheel power and the driving wheels is properly matched, on-track performance will suffer. At this point, overall gearing enters the picture.
Short track sprint cars benefit...
Short track sprint cars benefit from an abundance of torque more so than a full bodied stock car on a superspeedway where the range of rpm variation is narrow and quite high. Photo by John Meirhofer
Torque Vs. Horsepower
In some areas of motorsports, there's an axiom stating "torque creates acceleration, horsepower makes speed." As discussed in earlier CIRCLE TRACK technical materials, torque may be defined as a potential twisting force. Acting over a period of time, torque creates horsepower. That's a generic and simplistic perspective. In terms of accelerating a mass (a racecar), what's important is the amount of torque available at the tire-patch. Since many circle track cars operate without a multi-gear transmission, the final-drive ratio becomes particularly critical to this issue.
Also included in the equation is driving tire circumference or roll-out, not to exclude the total weight of the tire/wheel combination. At the end of the day, tire/wheel units become "flywheels" that must be accelerated along with the dynamic mass of the vehicle. In fact, there are instances where a final drive ratio change may be necessary to overcome the rotational inertial resistance of a given tire/wheel package, simply because the package happens to be either required or the best compromise for the particular situation; e.g., track conditions, length, surface, rules, and so on.
Whether you should concentrate on torque or horsepower, the type of racing dictates which is more important. For example, a short track Sprint Car would benefit from an abundance of torque probably more than a Cup car on a superspeedway where the range of rpm variation is narrow and quite high. Consider the fact that camshaft and valvetrain technology (particularly the latter) have enabled Cup engines to run long-distance races at 9,000-plus engine speeds. These are cases where horsepower equates to speed, as previously suggested. For the majority of weekly racing engines, torque can be a saving grace. In this regard, there is some valuable information that can be derived from reading dyno sheets, according to the following section. Bear with us for a few more paragraphs. All this seemingly nonrelated information should then come into focus.