Here are some thoughts on how to use GPL Setup Manager's Torque and Horsepower versus MPH chart based on research I've done over the years. I'm sure our aliens may have different ideas though. The goal in setting the gears is to produce the most torque at the driving wheels throughout the speed range for the track in use. This produces the maximum possible average acceleration throughout the track.
The transmission and differential gears act as multipliers for the engine torque. For example, if an engine produces 100 pounds feet of torque, the gear ratio is 2.00, and the differential ratio is 4.00, then the torque at the driving wheels is (neglecting friction losses):
100 X 2.00 X 4.00 = 800 pounds feet of torque for accelerating the car.
The most important engine characteristic that governs torque output is its capacity. This is one case where bigger is actually better.
Obviously, engines do not produce the same amount of torque throughout their rpm range and there is always a maximum torque rpm. Good racing engines produce a broad torque range where the torque does not deviate much from maximum over a wide range of rpm. If the engine torque band is wide, then fewer gears are needed and the engine is easier to drive. However, many racing engines do not have a broad torque range which requires the use of several gears to generate decent driving torque over a wide speed range.
A good example is in the 1967 Sports Car Mod. The Chaparral 2F engine produced gobs of torque across a wide rpm range. In fact, these cars only needed a 3 gear transmission! On the other hand, the 1967 BRM P115 F1 engine had very little torque to start with and the torque range was narrow. This required the use of a 6 gear transmission which spawned the joke that you rowed the BRM more than you drove it.
Horsepower is a measure of torque applied over time. The formula for converting torque to horsepower is:
Horsepower = Torque X RPM / 5252
So if our engine above produces 100 pounds feet of torque at 5,000 rpm, then it is also producing:
100 * 5000 / 5252 = 95.2 horsepower.
If either torque or rpm increases, horsepower increases proportionally as well. At 5,252 rpm, torque and horsepower are the same number for any engine (although in different units).
Horsepower is important for setting the top speed of the car as maximum speed is only obtained at maximum horsepower rpm. The horsepower required to overcome aero drag is proportional to the cube of the car's speed. Therefore, to double a car's speed, you need eight times as much horsepower (neglecting friction losses). So if we want to increase the car's top speed, we either need to produce more torque at the same rpm or the same torque at a higher rpm. In racing, where engine capacity is limited by the rules, the road to higher horsepower and top speed is usually through higher rpm.
Now for some rules of thumb:
1. Select a mid range differential gear. This comes in handy later for adjusting the car to different tracks by making it possible to tailor the overall gearing by changing only the differential setting.
2. Select the highest 1st gear ratio that does not cause wheelspin when accelerating out of the slowest turn on the track when using maximum throttle. Wheelspin is wasted torque and can cause loss of control. So the idea is to set the 1st gear ratio just high enough so that you can aggressively get on the throttle without spinning the tires. If you are an alien or an accomplished driver, then you have learned how to squeeze the throttle to control wheelspin and can get away with higher 1st gear ratios.
3. Select the top gear ratio that results in the maximum horsepower rpm being reached at the end of the longest straight. This will result in maximum acceleration along the straight while also producing maximum speed at the end.
The reason for this rule is simple. Consider the two extremes of top gear ratios:
A. The gear is so low that that you never reach maximum horsepower rpm prior to the end of the straight. The acceleration will very slow.
B. The gear is so high that you accelerate very quickly to maximum horsepower rpm, but are maximum engine rpm limited for the remainder of the straight. Neither of these extremes is the quickest way down the straight. Rather, setting the top gear so that you achieve maximum horsepower rpm just at the end of the straight results in maximum acceleration to the maximum possible speed for the length of the straight.
Now there are exceptions to this rule for tracks with very long straights such as the Mulsanne at LeMans. This straight is so long that a very low gear ratio would be needed if we followed this rule to the letter and acceleration could be very slow. So use this rule of thumb with some discretion.
4. Select the intermediate gears using either of the following rules:
A. Equally spaced between the lowest and highest gear. When plotted, the maximum rpm points will lie in a straight line. This is the easiest method and works just fine, but is not the best.
B. Space the gears so that there is a lower drop in rpm with the higher gears than the lower gears. In other words, you want less difference in ratios between the top gears than the lower gears. This shows a slight convex curve when you plot the maximum rpm points and results in optimum acceleration. The reason we need closer spacing between the higher gears is that aero drag is increasing at a rate that is proportional to the square of the car's speed. To keep the torque at the wheels versus drag similar between gear shifts, the gear ratios should approximate a square curve relationship as well.
For some tracks, you may be stuck between two gear choices when exiting an important turn onto a long straight. The second Lesmo at Monza is a good example. You may have to alter one of the two gear ratios to get the best acceleration out of that turn.
5. When to shift:
In a nutshell, always shift at maximum engine rpm. The reason for this is that you want the maximum possible torque at the driving wheels (except when exiting the slowest turn on the track). When you upshift, to maintain the same car speed and tire rpm with the lower gear ratio, the engine rpm must decrease. This places the engine at a lower point on its torque curve. Normally this results in more engine torque, but the gear ratio decrease invariably more than offsets the engine torque increase which causes the driving wheel torque to decrease.
Now you may say that engine torque at maximum engine rpm is less than the engine torque at maxmimum horsepower rpm. Why not shift at maximum horsepower rpm instead? This is true, but what is more important is the torque at the driving wheels which we already know is engine torque multiplied by the transmission and differential gear ratios. There may be exceptions for some engines such as diesels which produce gobs of torque well below their maximum engine rpm, but for normal racing engines where maximum horsepower rpm is very close to maximum engine rpm, the least amount of torque loss to the driving wheels occurs when the shift is made at maximum engine rpm.