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.