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All about dynos

October 21st, 2013

Andy Marcer

A motorcycle mechanic using a dyno in Houston

A dynamometer can only measure actual power at the output location. Actual power produced AND delivered by an vehicle will be highest if measured at the engine’s crankshaft, lower at the transmission output shaft and even lower, but more meaningful, still, at the rear wheel.

The power that you use is the power at the rear wheel.

Dynamometers are used to measure the power output of your motorcycle. The power is expressed in terms of torque and horsepower. In short, torque is the rotational force and horsepower is a specific measure of how much work the rotational force is doing during a given amount of time. The formula is Horsepower = (Torque X RPM) / 5252.

Although manufacturers like to quote horsepower at their engines’ crankshaft because it is the highest number, most shops measure the power from the rear wheel using a chassis dynamometer. The reason is cost. To measure the power at the crankshaft, the engine would need to be removed and special adapters machined to fit the dyno’s input shaft. The downside of using a chassis dynamometer is that transmission, drive train, and wheel/tire weight differences all make a significant impact on the final reading. Throw in the need to adjust for weather conditions such as humidity and air pressure; and you can start to see why comparisons between different motorcycles (even the same make, model, and year) can be misleading.

If that is not enough, the two most popular types of dynamometers used for tuning measure the power output in significantly different ways and generate dramatically different totals. The most popular is the inertial dynamometer, such as the Dynojet brand. The reason this dyno is so popular with many shops is that it is relatively inexpensive and dyno runs only take a few minutes. Using the inertial dyno, the motorcycle’s rear tire spins up a roller drum with a known inertial mass while in top gear at full throttle. A complex version of our basic formula is then used to derive the horsepower and torque the rear wheel should have been delivering to accelerate the roller drum as quickly as it did. Unfortunately, all of the variables used in the formula also introduce the potential for error and the opportunity to fudge the numbers. Incidentally, the typical inertial dyno numbers are inflated by 10% – 15% in the 100 HP range vs. our second type of dynamometer.

The next type of chassis dyno is the brake dynamometer and can actually vary the load or resistance of the roller drum, often using a device similar to an electric motor (eddy current) to precisely apply the load. The brake dynamometer is used for research and development because, rather than implying the horsepower from a complex mathematical formula using acceleration rates, inertial mass and other variable; the power is actually measured via the load being applied by an eddy current brake, as is the case for the Metric Motorcycle’s Pro Factory eddy current dynamometer. This provides a true, repeatable measure of horsepower.

The chief advantage of Metric Motorcycle’s eddy current dyno over cheaper inertial dyno is that we can run a motorcycle at a steady state RPM for an extended period with a known load applied to the rear wheel. Instead of simply sweeping through an RPM range from idle to redline, our eddy current dynamometer allows us to hold a specific RPM and throttle position (partial or full) while we make air/fuel adjustments to maximize horsepower. This also provides the time needed to stabilize the exhaust gases so that an accurate analysis can be made of its composition (CO, CO2, O2, unburnt fuel, etc.) to confirm that we are building a optimal, but safe map. So, when building a custom map for your motorcycle, we can perform a “stepped run”, whereby we stabilize the engine at each adjustment point (throttle position/RPM/gear) of your fuel/air computer.

It is also common for dynamometer companies to add to the power readings by adding transmission and primary gear/chain losses back into the measured power readings. Some companies make a concerted effort try to measure frictional losses and, optionally, add the power to the measured readings. Other companies – some that would surprise you – say that it’s not important and give a blanket, single factor for frictional losses in every engine. That includes some $25,000-$35,000 dynos.

How does this benefit you? For most riders, the vast majority of engine operating time is at less than full throttle. So when we develop a custom map for our customers’ motorcycles, we are able to focus on how the engine is behaving at any combination of throttle setting, in any gear, and at any RPM. The goal, of course, is to deliver useable power in a linear fashion so that when you pick that throttle up at mid corner, you get smooth, predictable power all the way through your motorcycle’s power range.

Given our serious methodical approach to tuning, the Pro Factory eddy current brake dynamometer’s ability to stabilize RPM and exhaust gases at any throttle position and anywhere your engine’s operating range far outweighs the additional cost over the cheaper inertial dynos, such as the Dynojet.

Our dyno room is not a marketing gimmick. It is a serious diagnostic and development tool which, when combined with our knowledge of building racing engines, separates us from the other shops. We have experience with most of the aftermarket computers, such as the TuneBoy, Power Commander, and Bazzaz.

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