Gearing and SLR - Drag Racing Applications
Today we are going to discuss how to correctly gear a drag racing combination to achieve optimum torque converter function as well maximize overall performance potential from the entire vehicle.
Keep in mind that this discussion is focused around maximizing performance in a drag racing environment. Street driven combinations as well as budget-oriented builds will frequently have compromises in gearing to either achieve a specific set of drivability characteristics and/or fit a particular budget level. What we will share here still applies to these scenarios with respect to achieving optimized drag strip function & performance, of course, but if you're compromising ratio selection based on drivability or budget then ultimately your end results are going to reflect said compromises.
In order to correctly gear a vehicle for optimum performance in a drag racing environment we have to start with the final drive ratio, i.e., axle ratio. Before a torque converter, transmission, or transmission ratios are ever selected the vehicle needs to have a final drive ratio selected with high gear & the finish line in mind.
Unfortunately many racers & enthusiasts take the completely opposite approach by selecting all of the other vehicle components including their transmission before any real thought or consideration is given to gearing, or they select a final drive ratio that "sounds" right but has no foundation in reality of actually being optimized for the parts combination or goals.
For optimum drag racing performance the final drive ratio should always be selected FIRST based on the following parameters:
1.) Desired finish line MPH
2.) Desired finish line engine RPM
3.) Desired finish line torque converter slip percentage
4.) Drive tire height
5.) Transmission ratio at finish line
You can then plug all of those parameters into the following equation to arrive at your mathematically ideal final drive ratio so that you can select a ring & pinion that most closely matches said mathematically ideal ratio:
(RPM * Tire Height) / (Rearend Ratio * 336.136 * Trans Ratio * (1 + (Conv Slip% / 100)))
There are also numerous online calculators available that allow you to simply fill in the data fields for these parameters & then will give you the mathematically ideal final drive ratio as a result.
Once you've selected the appropriate ring & pinion based on this formula you can then move on to selecting your transmission & transmission ratios in order to establish an ideal Starting Line Ratio (SLR) for your combination. If optimum drag racing performance is your end goal for your combo then transmission ratio should always be adjusted accordingly to tune SLR right from the beginning of your build using the approach outlined here.
Let's take a look at a practical example of this approach to help provide a visual. We'll utilize an 1/8-mile drag raced combination with a MPH goal of 150 MPH, an RPM ceiling of 8,000 RPM at the finish line, a tire height of 28-inches, & a high gear ratio of 1.00:1. We'll include several torque converter slip percentages in this scenario in order to help better illustrate a range of mathematically ideal final drive ratio options for this specific set of parameters:
~ 150 MPH / 8,000 RPM / 28" tire / 1:1 / 1% slip = 4.399:1
~ 150 MPH / 8,000 RPM / 28" tire / 1:1 / 5% slip = 4.231:1
~ 150 MPH / 8,000 RPM / 28" tire / 1:1 / 10% slip = 4.039:1
~ 150 MPH / 8,000 RPM / 28" tire / 1:1 / 15% slip = 3.863:1
If a racer is targeting single digit converter slip percentage then you can see in the above visual that the combination in question is going to require a final drive ratio in the 4.30:1 range based on commonly available ring & pinion ratios for most popular axle assemblies utilized in drag racing.
Also worth noting is that the math doesn't care if the combination is naturally aspirated, nitrous assisted, supercharged, or turbocharged. The math doesn't lie & will ultimately be correct for any of these options. You may be thinking, "Turbo cars don't use 4.30 gears."
This is where you'd be mistaken, & where most people fall into the trap of selecting a ratio that "sounds" right based on either a fundamental misunderstanding of how ratio selection works and/or basing a decision on the loads of misinformation out there online as well as in various circles of racing.
You may then be thinking, "Well how am I supposed to use the correct rear gear when I have a TH400 with stock ratios? The 2.48:1 low gear will be too violent at the starting line."
The answer in this type of scenario is simple; you don't use stock ratios in the transmission when the available stock ratios are incorrect for the desired outcome. This is why it's so important to take the time to clearly define your budget, your goals, & sort out your parts selection BEFORE you start spending money or just blindly selecting a particular model of transmission because it sounds like the right choice based on what everybody else is doing or was price shopped.
Ultimately if you compromise the final drive ratio in order to manage your SLR then you've immediately sacrificed the maximum performance potential of your combination. This approach results in having insufficient final drive ratio in high gear to allow the vehicle to continue accelerating in a manner that is effectively utilizing all of the work being produced by the engine. This insufficient final drive ratio dramatically reduces the torque converters ability to couple all of that work efficiently.
Further, it doesn't matter who builds a custom torque converter for you if the vehicle isn't geared correctly to begin with. A torque converter will **not** overcome a deficit in ratio. If you find yourself currently dealing with a situation of excess torque converter slippage then the first place to start your diagnosis is your ratio selections in the transmission & axle.
Utilizing this type of systems approach with a no-compromise mentality in as many areas of the vehicle as possible combined with appropriate tuning front-to-rear will always yield the most favorable results & help you achieve your goals with your max effort drag car or truck.