Photo by Patrick Hill
Before digging into the subject, we need to review some assumptions on which our discussion will be based. This story isn't intended to identify specific gear combinations for equally specific track conditions. Rather, it's meant to establish some lines of thought that will enable you to make informed decisions while utilizing information that relates directly to how you can integrate various factors that help enable sensible gear choices.
In circle track racing, it's problematic that track conditions undergo a constant state of change during a race. Obviously, this can be more of an issue on dirt than asphalt. Some compensation for this can be made by driving technique while other solutions may be based on chassis setup and gearing, the latter of which we'll be reviewing. In addition, there is no such animal as "absolute traction." In fact, in some instances, a racecar will accelerate more quickly during tire spin, based on where in the rpm (torque) range this situation occurs. Then there are times when the engine is not at wide-open-throttle. But even though these conditions tend to dilute the "academic approach" to determining the best gear/track combinations, there are some guidelines we can discuss. There's more to working out proper gear combinations than just selecting gears. Part of the process involves what you'll be providing for their input.
Defining The Problem
Smokey once told me that circle track racing is little more than a "series of drag races, tied together with corners." For as much of an oversimplification as this may be, it does represent a measure of truth. We'd like to have the car accelerate as quickly as possible on those parts of the track that will reduce lap times the greatest, enable passing when the need arises, and provide the greatest level of control. Clearly, there are variables in play that include an ability to integrate available power with chassis setup and reaction. But unless the mechanical link between flywheel power and the driving wheels is properly matched, on-track performance will suffer. At this point, overall gearing enters the picture.
Short track sprint cars benefit from an abundance of torque more so than a full bodied sto
Torque Vs. Horsepower
In some areas of motorsports, there's an axiom stating "torque creates acceleration, horsepower makes speed." As discussed in earlier CIRCLE TRACK technical materials, torque may be defined as a potential twisting force. Acting over a period of time, torque creates horsepower. That's a generic and simplistic perspective. In terms of accelerating a mass (a racecar), what's important is the amount of torque available at the tire-patch. Since many circle track cars operate without a multi-gear transmission, the final-drive ratio becomes particularly critical to this issue.
Also included in the equation is driving tire circumference or roll-out, not to exclude the total weight of the tire/wheel combination. At the end of the day, tire/wheel units become "flywheels" that must be accelerated along with the dynamic mass of the vehicle. In fact, there are instances where a final drive ratio change may be necessary to overcome the rotational inertial resistance of a given tire/wheel package, simply because the package happens to be either required or the best compromise for the particular situation; e.g., track conditions, length, surface, rules, and so on.
Whether you should concentrate on torque or horsepower, the type of racing dictates which is more important. For example, a short track Sprint Car would benefit from an abundance of torque probably more than a Cup car on a superspeedway where the range of rpm variation is narrow and quite high. Consider the fact that camshaft and valvetrain technology (particularly the latter) have enabled Cup engines to run long-distance races at 9,000-plus engine speeds. These are cases where horsepower equates to speed, as previously suggested. For the majority of weekly racing engines, torque can be a saving grace. In this regard, there is some valuable information that can be derived from reading dyno sheets, according to the following section. Bear with us for a few more paragraphs. All this seemingly nonrelated information should then come into focus.
There's little value in trying to conclude final drive gear ratios and an applicable rpm r
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 the most critical track location that relates to a specific
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.
One of the most critical elements to gearing choices is track conditions although you have
Ultimately, you may find that final-drive gear ratios that allow the engine to operate in this "just below peak torque" to "around peak power" range will provide the best corner exit speeds and lowest lap times, respectively. Proper gear choice will help make this happen, somewhat independent of available traction. In fact, this approach does a fairly good job of addressing the traction problem.
A Discussion With Dennis Wells On The Issue Of Gearing
Dennis Wells is well known to the CT readership. Wells Racing Engines has been prominent in both dirt and asphalt circle track racing for more than 25 years, and their exploits having been periodically chronicled on the pages of CT magazine. Relative to how Dennis works with his engine customers on the issue of relating dyno power to on-track performance, the following discussion took place.
CT - Let's assume, for the sake of this conversation, that there are four principle variables affecting on-track conditions over which we have little or no control: absolute traction, constant track conditions, continuous wide-open throttle, and primary environmental conditions. Would you agree these play a role but are not within our control?
Wells - Absolutely. However, even though all of these can affect our gearing choices, we need to first assume they are constant when making overall gear selections, and then adjust the car for how a given set of conditions impacts our choices.
CT - Let's assume I'm one of your customers and you've just handed me a dyno sheet that maps out my particular engine. Maybe I know just enough to be dangerous when it comes to gearing choices, so how would you guide me through the process?
Wells - First of all, you'll probably look at how much power is available and where it's being made. Let's say that's at 7,000 rpm and you figure this where you gear the car to run. However, it's not. My experience has been, especially on a "tacky track," that you want the engine to operate from about 800 to 1,000 rpm above peak power, at maximum rpm. For example, most of my dirt modified engines run best around 7,800 to 8,200 rpm. This means most of these engines, on the dyno, will run peak power around 6,800 to 7,200, depending upon camshaft and cylinder heads. But that's a pretty good estimate.
CT - Do you factor piston displacements into this?
Wells - Well, let's say you have a 400-inch engine with a long stroke. These tend to peak power earlier than the shorter-stroke engines that have about the same cylinder heads and camshafts, pretty much independent of displacement, unless you're talking about significant differences in cubic inches. Then, of course, the larger engines will peak at lower rpm than the considerably smaller ones. What we've found is that the guys who run the longer-stroke engines, without getting into elaborate heads that flow a lot of air, tend to complain about too much bottom end and not enough top end. These include the long-stroke 406-type engines, too.
CT - Specifically, how do you address the issue of "too much bottom end" and "not enough top end"?
Wells - What I generally do is build to either a 3.5-inch or a 3.5625-inch stroke combination so the engine is a little "softer" on the bottom end and doesn't just fall off in the upper rpm. This gives a lot more throttle control, makes gearing selections more effective, and improves lower end traction, plus the car is easier to drive and more predictable.
Scott Bloomquist knows a thing or two about good throttle control and how having it allows
CT - In terms of how you recommend gearing these cars, have you found that what you might call "driver preference" comes into play, too?
Wells - Yes, and sometimes this can complicate the matter. For example, you may have a driver that has really good throttle control with his foot and will want an engine that's really powerful. He'll hunt for the tacky spots on the track. Then you'll get another driver who wants to gear the car higher to where the engine is really "soft" most of the time, so he doesn't spin the tires because he doesn't have the good "foot feel" like the first driver I described. So the bottom line is that no two drivers will want the exact same thing regarding where the engine makes power or how the car is set up or geared.
CT - So is it fair to say that with the number and range of variables that come into play, it's difficult to lay down any hard and fast rules about gearing for all circumstances?
Wells - I think a more accurate way to look at this is to say there are some "guidelines" to be considered, not strict rules.
CT - Let's talk about guidelines.
Wells - Well, first you need to consider that some of the variables over which you have no control, like track conditions, weather, on-track traffic, and other uncontrollable factors, not only change from week to week but even during the course of a race. So what I'll call the "gearing target" is constantly moving. So black and white steps just aren't practical. However, there are places on the track where you'd like more power or torque than others. This is especially true during corner exiting. In our experience, you'd like the engine to be geared where it's slightly below peak torque rpm when you begin to exit the corner so it'll accelerate toward peak power in the straight. Based on track conditions anticipated, this will provide you a starting point from which ratio changes or driving technique can be adjusted accordingly to fit the needs of the track. We've also found that this approach will help compensate for traction problems during corner exit. Of course, much of this is a compromise that's driven by the fact that there are variables like the ones we mentioned, over which you have much less control than when selecting gear combinations.
CT - Let's go back and talk a bit more about where you think engine rpm should be when corner exit begins, if you're running a longer stroke combination. Stroke and rod length, all else being equal, can affect the rpm span between peak torque and peak power.
Wells - That's right. In these cases, where the rpm between peak torque and peak power becomes more narrow, you'd probably want to gear the car to a slightly lower rpm below peak torque so you won't run out of rpm between there and peak power during acceleration onto the straight. Normally, these guys will run a higher gear ratio than those with the shorter-stroke engines, primarily to compensate for how the longer stroke affects low-end torque.
CT - OK, where else on the track do you consider gear selection critical?
Wells - Some of the drivers I work with like to use lower gears to help engine braking during corner entry, rather than rely more heavily on the brake pedal. Of course, like in so many situations, compromise enters the picture. For example, gearing lower for braking into a corner means the car will try to accelerate more quickly out of the turn, but the tendency to break the tires loose is increased. Depending on traction conditions, this can become another problem.
Photo by Todd Ridgeway
CT - Is there anything else you pull from a dyno sheet when reviewing this type of information with your customers?
Wells - I like to see my engines geared to run about 1,000 rpm above peak power at maximum straightaway speeds. Of course, this assumes customers have maintained the valvetrain so it'll sustain 8,000 or so rpm under these conditions. From an overall track performance standpoint, that seems to work best and it'll still provide sufficient margin to make sensible gear choices. I know this isn't high rpm for today's Cup engines, but it's about the limit for the weekly racer who's running stainless steel valves and trying to keep springs in the engine, considering the ramps we run on these type of cams.
CT - Earlier in this story, we mentioned evaluating engines with comparable power where one had a lower BSFC curve (numerically) and quicker on-track acceleration. Have you had any experience with this?
Wells - Yes. In fact, that's not only true but the one with the lower BSFC curve will also be more responsive to the final gear changes, especially when you're able to match corner exit rpm to the rpm range we discussed earlier. These engines just seem to have more throttle response anywhere on the track.
CT - Any other thoughts about relating dyno charts to gearing and on-track performance?
Wells - Overall, I like to see relatively flat torque curves. In fact, I don't like peaky torque or powerbands. The flatter the torque curve, the more driveable the car becomes, and it'll also be more sensitive to gear changes. I compare this with how two-stroke engines come up "on the pipe," so to speak. If the torque hits all at once, the car becomes less driveable. Plus, torque curves that fall off too quickly can cause you to incorrectly gear a car in an effort to make up for torque (through ratio multiplication) that's not there. And this, of course, can cause problems in areas where the engine is actually making decent torque. I've found this to also cause similar problems with powerbands that are too peaky.
CT - Final thoughts?
Wells - Over time, we've seen suspension systems technology make some incredible improvements. As this has happened, we've found that you often need to re-evaluate how a car is geared. In particular, use of four-link systems has led the way for some significant traction improvements in recent years. I never thought I'd see the ability to lock down 650 hp on the size tires (small) we're using today. As this has happened, we've had to rethink how we gear these cars. Where before we might have needed to run higher ratios, improved traction and how this tends to lug engines down into lower rpm ranges has caused higher ratios to be required in order to place the engine in a speed range where it's most torque efficient.
Some Concluding Thoughts
While there are software simulation packages that address the optimization of on-track gearing for circle track cars, an assortment of assumptions prevent results from being absolute, although helpful. Some degree of trial-and-error testing is typically required, although such computer programs are improving as they evolve. Meanwhile, and for racers who do not have access to such programs, information shared in this story can hopefully clarify certain issues while bringing others to the point of consideration. If you've come to the realization that there's more to gear selection than matching peak power with maximum speed, then this particular story stayed on track.