One of the most critical elements...
One of the most critical elements to gearing choices is track conditions although you have no control over them. Photo by Jeff Huneycutt
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.