7. This Dynojet chart is...
7. This Dynojet chart is printed in an optional format in which the air/fuel ratios are listed along with the horsepower graphs. Sometimes the air/fuel ratios are located below the power graphs. Ideally, you want the engine's air/fuel ratio relatively flat. Maximum power will be produced when the A/F is near 13:1 (13 parts air to 1 part fuel) while best fuel economy is typically slightly above 14:1.
Examining Power Curves
Chart 4 illustrates that peak torque occurs at 3,000 rpm and then hits the valley of fatigue. If it is assumed a stock ratio Big Twin transmission is up-shifted at 6,000 rpm, engine speed would drop down to roughly 4,500 rpm after each shift. For racing, this means the 4,500 to 6,000 rpm range would be most important. However, since torque peaks at 3,000 rpm, torque occurs at too low an rpm for maximum performance, although it would be acceptable for a general street engine.
Chart 5 indicates that by improving the engine's breathing the torque peak is moved to a higher rpm and into the engine's working rpm range. Notice that the amount of torque is not increased, but only the rpm at which it occurs. In fact, the torque curve in Chart 5 is shaped identical to the curve shown in Chart 4, except that the curve is moved horizontally 1,500 rpm higher. Yet, look at how much horsepower jumped. This is because horsepower is a function of torque multiplied by rpm. Again noting Chart 5, since horsepower peaks at about 6,400 rpm, better performance could be achieved by up-shifting at an rpm higher than 6,000. This would result in a higher average horsepower over the rpm band.
Because horsepower peaks at 6,400 rpm in Chart 5, 7,000 rpm would be a reasonable up-shift rpm to maximize average horsepower. Additionally, since we have already established there is a 1,500-rpm drop after an up-shift; the bottom of the shift would now be 5,500 rpm. But notice that peak torque is at about 4,500 rpm. For pleasant street riding with a Big Twin engine, peak torque rpm should be no higher than 4,500 to 4,800 rpm (maybe as high as 5,000 rpm). However, for maximum acceleration, peak torque rpm should be equivalent to or slightly lower than the bottom of the shift rpm. Since the bottom of the shift rpm is 5,500 and peak torque rpm is 4,500, the engine combination should be changed to move peak torque rpm closer to 5,500 rpm. Once peak torque rpm is near 5,500 rpm, any further component changes should always move peak torque and peak horsepower rpm in unison.
Chart 6 provides an example of improving power throughout the rpm range, thereby moving it vertically up the graph. Increasing compression generally increases power vertically over the entire rpm band but particularly in the low and mid ranges. Adding displacement also raises power vertically throughout the rpm band as long as breathing keeps pace with the airflow requirements.
Summary
Remember that the optimum power curve depends on whether your priority is dyno shoot-outs, racing, or street performance. But regardless of the dyno curve's shape, first and foremost, you want an accurate dyno chart. And for that, you need an honest dyno operator who is knowledgeable in tuning your engine combination, whether it is carbureted or EFI. For EFI engine, that means your tuner must be experienced using your fuel/ignition software program or any add-on EFI module. When done properly, dyno testing can save you time and money and has become a vital tool for optimizing EFI fuel and ignition maps. [Editor's note: The numbers and graphs used were for demonstration purposes.]