Engine RPM

This channel monitors the speed of the engine, and is usually tapped into the tachometer signal from the ECU. Most loggers provide a dedicated input for this channel that senses a typical tach signal, so you don’t have to use one of the system’s additional channels, nor will you (in most cases) have to provide any special configuration information. The wire to use into can be found by checking the service manual or wiring diagram for your bike; use a tap-in squeeze connector in the harness as close to your logger box as possible, to avoid running a long, unshielded wire that can be susceptible to interference.

Speed and rpm

A typical rpm graph (red) has a sawtooth appearance as revs change with upshifts and downshifts. The blue circle shows rpm and speed (black trace) on the fastest part of the course. The bike should be at or near maximum rpm and in top gear at this point for optimum overall gearing.

The engine rpm channel is used to both check how the rider is using the engine as well as provide guidance for internal and final gearing selection. The overall graph of rpm versus distance or time will show at a glance if the rider is hitting the rev limiter at any time, or allowing the rpm to drop too low in a particular corner. Make sure the rider is using maximum rpm in each gear along each straight. To check gearing, check the rpm at the fastest point on the track – it should be at or close to redline for optimum final gearing.

RPM histogram plot

This rpm histogram shows how much time the engine spends in each rpm range. Ideally the shape would resemble the power curve for the motorcycle. This histogram is from AimSport’s Race Studio Analysis software package, with rpm on the vertical axis and percentage on the horizontal axis; some other programs will show the opposite axis. In this case, the histogram turned on its side should represent the engine’s power curve. Any differences, such as more time spent at higher or lower rpm, indicates a change in final gearing may be beneficial.

There are additional ways to view the rpm trace to show different aspects of the data. The histogram – a bar graph that shows the distribution of rpm – will show how much time the engine is spending at each rpm. This should mimic the power curve for the engine if the rider is utilizing the rev range correctly and the gearing is correct. The engine should spend the most time in the rev range where maximum rpm is produced. If the gearing is too short, the distribution will be biased toward the upper range; likewise, with gearing too high, more time will be spent at lower rpm. Note that the rider can also affect how the histogram appears and this should be taken into account also. For example, the rider may be short-shifting in an increasing-radius corner, skewing the histogram. Before blindly changing gearing based on the data alone, the rider’s thoughts on the subject should also be considered.

RPM trace showing engine at maximum rpm

Time spent on the rev limiter is indicated by the rpm trace staying constant at maximum rpm, as shown here in the blue circle. Other channels, such as the lambda sensor and throttle position, can be used to corroborate this, or conversations with the rider. In this situation, the rider and tuner decided to leave the gearing unchanged so as not to affect other areas of the racetrack. Another alternative is to have the rider make the upshift/downshift; the best option can be found by experimenting and using sector times in the analysis.

Finally, don’t forget to consider the effects of tire circumference on rpm. When the motorcycle is leaned over, the tire is effectively smaller, shortening the gearing. In turn, rpm rises as the bike is leaned over, even though the bike is going the same speed or maybe even slower. If the rider uses a different line in a particular situation the rpm trace will reflect this. For example, when crossing from one side of the track to the other on a straight to set up for the entrance of the next turn; if the rider leans the bike over significantly and crosses over quickly, rpm will rise momentarily. If the rider takes the length of the straight to cross and leans very little, rpm will be affected very little. Note the subtleties here: leaning the bike over effectively shortens the gearing and may provide more acceleration in certain situations.

External Links:

Using Data Acquisition: RPM Analysis (Sport Rider)