Gears are expensive and hard to get to, so we are all motivated to get the right transmission and final drive gear ratios in the car the first time. Unfortunately, there is a lot that goes into determining the best gear ratios for your car and your next race track, and you won’t know some of those things in advance. Chasing the optimum ratios can be an expensive quest, but a little bit of careful analysis can give you answers that are really close. Also, if the rules for your racing class or the reality of your budget limit you to the gears that are in your car now, there are still techniques that you can use to improve the acceleration performance of your car.
Acceleration is stronger with a shorter gear ratio because the engine has more leverage over the tires. When your acceleration is not traction limited, it pays to use the shortest ratio that is practical for each gear.
The conventional procedure for selecting gear ratios is to pick ratios that run the engine rpm to redline at the end of the medium and longest straights, and pick the lower gear ratios to minimize the rev drop at each shift. This method minimizes the number of shifts per lap. However, there is a different approach to gearing that can make your car faster.
The optimum gear ratio for each corner will run the engine to the optimum shift rpm just after the track-out point of that corner. That maximizes corner-exit acceleration without upsetting the corner exit balance by requiring you to shift while cornering. Of course it is not possible to optimize the gear ratio for the exit of every corner, so just focus on the most important corners. The track-out speeds depend on the track and how fast your car is today. So, the best gear ratios to use depends on several factors that change regularly. If you have the ability to change the gear ratios in your car, you are going to get a lot of practice doing that unless you run at the same track at the same pace all the time, or if you are willing to sacrifice performance. The track-out gearing approach is more beneficial to your performance than other considerations like minimizing the number of shifts per lap or minimizing the rev drops between gears.
For the top two or three gears, the conventional method of reducing the rev drop with each shift is usually close to optimum. However, if you have a larger rev range between your peak power rpm and shift rpm, the optimum rev drops for the top few gears are closer to equal.
If your engine does not have a rev limiter, add one before your next event. Optimizing your straight-line performance will probably mean running your engine up to or against the limiter regularly. In fact, there may be some straights where the optimum driveline management means holding a gear against the rev limiter for the last few seconds on a straight, even on the longest straight in top gear. If your dyno graph shows a steep power drop-off as the revs continue up past the power peak, that drop will act somewhat like a rev limiter when you are in top gear. Accelerating up to a lower top speed faster by using a shorter gear is quicker than accelerating more slowly to a higher top speed.
On the other hand, if your car benefits significantly from drafting, and your run group is always crowded, it is best to select the top two gears to benefit from the extra speed that a draft will give you.
Upshifting quicker can take a shocking amount of time off of a lap. It’s not just the time that is spent not accelerating that matters. Air drag and rolling drag slow the car down during the shift, and the car has to regain that lost speed using the weak end of the power band. Also, it has to regain that lost speed with less leverage over the tires because it is in a higher gear. These factors magnify the effects of the shift time. Upshifting faster reduces the speed lost during the shift, which shortens the lap time. Of course shifting quicker requires driving techniques that are less gentle on the driveline and engine, and your first sign of trouble might be expensive shrapnel and a non-finish weekend. So, take small steps toward shortening your shift time with a more aggressive driving technique.
Transmissions designed specifically for racing have a dog-drive gear engagement system with no synchromesh device to match the input and output shaft speeds to the next gear ratio before it engages. Dog boxes enable faster shifts, and shifts without using the clutch, but they are expensive and the dogs wear, requiring regular fluid and part replacements. On the other hand, racing with a road car transmission requires clutching both upshifts and downshifts. Even with that, shifting faster wears the synchros faster, so plan on occasional maintenance. If your transmission is a mass production unit, that maintenance might be as simple as swapping a worn out transmission with another one from a junk yard. Of course that is unlikely if your car has a racing transmission.
In addition to the availability of many interchangeable gear ratios that a racing transmission offers, there is another major difference. A racing box will last longer, and you are less likely to miss a shift, if your shift lever movements are really fast and really firm. A dog box reacts badly to limp-wristed treatment because the drive dogs will bounce off of each other instead of engaging. The last major difference that a racing box offers is professional preparation and guidance. There are several businesses that specialize in fine tuning racing boxes for even better performance and easier servicing. These professionals are also available to answer the hard questions you will have.
If the engine and clutch are up front and the transmission is out back, the additional rotary inertia of the drive shaft and couplings slows the shifts and wears the synchros faster, so that configuration is less than optimum for racing. Of course the additional yaw inertia of that driveline layout is another negative for racing.
The faster your engine accelerates, the more that minimizing the rotational inertia of the propulsion system matters. The power required to accelerate the flywheel, crankshaft, and everything else subtracts from the power available to accelerate the car. This can be a really big deal. Of course a lighter flywheel and clutch will help, but there is more to it than that. Because the alternator is usually driven considerably faster than the engine, the effect of its rotary inertia is magnified. A larger alternator pulley can have a surprising effect on the engine assembly rotary inertia and the acceleration rate of the car.
There are several inexpensive programs and mobile apps that you can use to analyze or optimize race car gearing. The good ones allow you to enter the dyno data for your specific engine to improve the accuracy of your results. CarTest 2000 is particularly useful because it includes an iterative gear ratio optimization routine. It varies all of the ratios that you allow it to change to achieve the goal that you define. It tests each combination of ratios, tracks the best result, and resets it when a better combination of ratios is tested. The end result is the combination of gear ratios that produces the best possible acceleration between the two speeds or the distance that you define. Of course the practical limitations of transmissions, gears, and final drive ratios that you can buy will be different, but the optimum ratios will guide your gear or transmission purchase decisions. Compare the optimum ratios to the ones that are available for your car, transmission, or ring and pinion, and select the ones that are closest to the calculated optimum combination.
Shift RPM by Veracity Racing Data is one of the better mobile device apps, even if changing your gear ratios is not an option. This app will calculate the optimum RPM for each shift to maximize your straight-line performance. It might come as a surprise that the optimum shift point is different for every gear, and this app will show you why.
When your competitiveness reaches the point that you are less than a second from the pole, you are close enough that optimizing your gearing is worth the time and expense. This is one of the invisible tweaks that separate the contenders from the also-rans. The moving-target nature of gearing optimization makes it expensive and annoying, but there is performance to be had in optimizing the connection between your engine and tires.
We found this video on YouTube, and it explains how a manual transmission operates.