There's little value in trying...
There's little value in trying to conclude final drive gear ratios and an applicable rpm range without first understanding your engine's power range. Some time on the dyno will tell you that story. Photo by Jeff Huneycutt
Reading The Dyno Sheet
Traditionally, translating dyno-measured power information to on-track performance can be difficult. Unless the dynamometer you're using actually accelerates a mass of vehicle-equivalent weight (inertial resistance), an engine doesn't experience the type of loads found in the vehicle. Even so-called dyno "acceleration" tests that incorporate a controlled unloading of the power-absorber doesn't duplicate what an engine sees on the track. So what information can you use from an engine dyno that will, at a minimum, help you anticipate on-track performance? Let's talk about brake specific fuel consumption data.
We know that BSFC is essentially a measure of how efficiently an engine (through the combustion process) is converting fuel into heat or power. The process involves observed horsepower and fuel flow, in units of time. So, in a sense, it's the time-rate measurement by which fuel is converted into horsepower. What can this tell us about track performance? As an example, let's assume you have two engines of essentially the same power characteristics. This might even be one engine with back-to-back, comparatively equal power levels that differ primarily in BSFC performance.
In either of these two cases, the BSFC differences outweigh the power differences. And, you could say BSFC includes a time-based perspective; the combination of parts (or tune) that produces the lower BSFC performance will tend to perform better on the track, particularly regarding acceleration (or throttle response), all else being equal. The kicker is this will also be the combination that responds best to gear changes you make. Thought we'd never get there, right? You'll also discover that on-track fuel economy will be the best with this same lower BSFC combination.
If there is a consensus about...
If there is a consensus about the most critical track location that relates to a specific power range, it's the point of corner exit. Photo by Kevin Thorne
Where's The Power Being Produced?
Before we discuss applying available power to the track, it's necessary to determine what's available. Overall, you need to gear to the rpm range linked to the most power available. In other words, there's little value in trying to conclude final-drive gear ratios and an applicable rpm range without first making certain the engine's power range matches the gears you'll be using, or the opposite.
Once again, even though the ability to achieve optimal traction may not be possible, using "absolute" traction for the purpose of determining initial gear selection will eliminate one variable that's unavoidable. Whether through experience or after experiencing a given set of track surface conditions, you can make subsequent gear ratio changes to compensate for differences between absolute and those of the track on which you're racing. In any case, you'll need to begin somewhere and matching engine power range to on-track rpm (gearing) is a starting point.
Critical "Gearing Points" On The Track
If there is a consensus about the most critical track location that relates to a specific power range, it's the point of corner exit. In fact, you will find that you'd like the engine to be (at wide-open throttle) roughly 200 rpm below its peak torque value when the car reaches and begins to leave the corner exit point. This will encourage maximum acceleration up to peak power as the car travels the back straight (or that portion of a more circular track) or heads for the flag stand.
This optimization of corner exit acceleration, based on what you know about the engine dyno data, can be achieved by selecting a final-drive gear ratio that places the engine in this rpm range at that point on the track we've identified. In the end, you'll likely determine that corner exit speed is more critical than cornering speed, thereby placing particular importance on selecting a final-drive gear combination that allows the engine to begin this acceleration process just below its peak torque value where throttle response and its ability to rapidly increase in rpm (don't forget the lower BSFC notion) past peak power.