Quick-change and "not-so-quick-change"...
Quick-change and "not-so-quick-change" rearends both need the correct gear ratio in order to have maximum performance.
Finding the right gear ratio for a particular track in a certain class of circle track racing may be as easy as asking your competitors. Most racers who regularly run the same racetrack each week will settle on the same gear and run it the entire season. But is the gear you are using producing the fastest lap?
Differences in the track, tire sizes, class rules, and other factors can cause you to rethink your selection of gears, possibly from week to week and even from the beginning to the end of an event (in the case of dirt track racing). The reasons for this may become more apparent as we study the whole concept of gear selection.
Basic Gear Primer
There are two basic rearends that are used in circle track racing. There is the OE ring-and-pinion in a pumpkin case, where gear changes are rather difficult and racers are less likely to make week-to-week changes. The other is the quick-change rearend that is designed for much easier access to the drive gears and therefore facilitates quick changes to your gear ratio. The reasons for making these changes are varied, but can be necessary in order to maintain performance levels.
To say the quick-change is superior would not be entirely accurate. It can be less durable than the OE rearend, but holds up fine for most short-track racing with regular maintenance. For larger cars that run long and fast racetracks and have high-horsepower engines that run at high rpm, the Ford 9-inch rearend is almost mandatory.
For our discussion, we will use the quick-change rearend. Some of the discussion will also relate to the OE rearends as we talk about reasons for gear change, not necessarily the process of making those changes as involved with each type.
In this exaggerated example,...
In this exaggerated example, we can see where the use of a lower gear (5.61) might improve acceleration while showing the same rpm at the end of the straightaway. Looking at 6,800 rpm, we see where we reached that speed approximately 0.8 seconds sooner with the 5.61 gear. The real gains will be smaller, but still significant. Notice how the 5.61 speed gain trails off toward the end of the straightaway but is still higher than the 5.48 gear gain at 7,200 rpm.
Choosing the Correct Gear Ratio
We should always consider the highest and lowest rpm in our powerband when choosing the gear for our cars. You need to know where the engine powerband starts and ends for your motor. Then, look at the engine rpm for each point around the track for the gear you are currently running. You don't want to begin accelerating off a turn below the rpm where the power starts to build.
Likewise, it may not be the best thing to run out of the powerband somewhere down the straightaway. A bigger mistake might be to begin to hit peak horsepower just as you are getting ready to brake into the corner. There is a compromise that may produce a faster lap.
There may be two or more gear ratios that will produce the same rpm at the lift point at the end of the straightaway. Each will begin at the same rpm off the turns and end at the same rpm at the lift point going into the next turn, but one will be faster. How so? Here's how.
If we choose a gear that will pull from 6,000 rpm just past mid-turn to 7,100 rpm at the end of the straightaway, we might feel we have the right gear if the horsepower curve goals are met. That means our engine horsepower curve comes on at 6,000 rpm and peaks at 7,100 rpm. It sounds like we have the right gear, but maybe not.
On some tracks where acceleration off the corners is critical for passing, a lower gear might help us get off the turns better while not necessarily hurting us at the other end. The lower gear will accelerate the car quicker, and if we don't lose traction with the switch, we will be better off (at least to the flag stand).
For the last half of the straightaway, if we have gained half a car length, our speed will be mostly peaked while our competitor will still be accelerating. We will still be pulling away from the other car because the other car has not yet reached top speed/rpm.
Our turn entry will be much smoother because we will have slowed our acceleration before we lift to brake, while the other car is still accelerating when the transition to braking occurs. It is very disruptive to make the change from hard acceleration to hard braking. It is much smoother to transition from steady speed to hard braking. Just ask any high-performance driving instructor and they will agree.
It may be better to install a lower gear, beat our competition off the corner, and have a better corner entry all by doing a little experimentation. All it takes is a little effort and testing with a stopwatch, and the results can be evaluated. Most teams don't know what areas to test at a test session. This is one of those areas that may improve your performance.