Got Dyno Fever? Don't Be Fooled!

Take one look at the current V-Twin performance market and several things quickly become evident: almost everyone is searching for big horsepower numbers, electronic fuel injection requires more sophisticated tuning procedures than carburetion, and emissions standards are becoming tougher. As such, it's lucky for Harley aficionados that during the past 15 or so years, dynos have become commonplace in motorcycle shops because the dyno has proved it's an invaluable tool for engine tuning and can save the engine builder, racer, or street rider time and money when optimizing power or exhaust emissions. But to get the most for your dyno-tuning dollars requires an honest and knowledgeable dyno operator, along with a fundamental understanding of what to look for in the world of dyno tuning. Following is a compendium of things to pay attention to when plunking down your hard-earned cash on dyno tuning.

What is a Dyno?
Dynos have been used for quite awhile in the automotive world, but during the '90s proliferated in the motorcycle community due to the introduction of the rear wheel dyno and its relative low cost and ease of use. A dyno or dynamometer is a tool that loads an engine as if it were being run in a vehicle. It measures (some dynos calculate) an engine's torque output and transmits the results to a software program that controls the dyno. The software program calculates horsepower and supplies a wealth of other information to the operator that includes detailed insights into the operation and efficiency of the engine. Dynos are commonly used for engine development, performance testing, tuning, and as a diagnostic tool.

Types of Dynamometers
In one fashion or another, all dynos measure the power output of an engine, but how they measure output can vary. Today, most dynos fall into two categories: (1) absorption or (2) inertia. With an absorption dyno, the mechanical energy produced by an engine is converted by some medium into heat energy. In most cases, the medium is either water or electricity. Since absorption dynos are usually connected directly to the engine's crankshaft (or indirectly through a jackshaft), they are normally called an engine dyno. In contrast, an inertia dyno records the acceleration of a known mass to determine the power output of an engine. Rear wheel dynos, the kind typically found in motorcycle dealerships, are an inertia-type of dyno.

A rear wheel or inertia dyno consists of a heavy roller driven by the motorcycle's rear wheel. The advantage of a rear wheel dyno is that a motorcycle can be easily and quickly rolled up onto the dyno and tested without having to remove the rear wheel or the engine from the chassis. This makes it perfect for the shop that services engines or for the racer who has only a few days between races and needs to do some quick testing. It also makes it possible to test the efficiency of the bike's drivetrain. In the automotive world, rear wheel dynos are used mostly for final tune-ups instead of R&D testing, but in the motorcycle environment, both types of testing are prevalent. Rear wheel dynos are relatively low cost, convenient and designed for motorcycles, automobiles, go-karts and other types of vehicles. Dynojet was the first to introduce a rear wheel dyno for motorcycles, and it is the most extensively used dyno in the motorcycle marketplace today.

In its most basic form, a rear wheel dyno is a fixed inertia-only type device in that it does not actually measure an engine's torque output using an absorption unit. Instead, the motorcycle's rear wheel accelerates a heavy roll of known mass and inertia. The dyno measures the time and rate of acceleration to a given engine speed at wide-open throttle (WOT) conditions. Torque and horsepower are then calculated by software from the time and acceleration rate. The more rapidly the heavy steel drum is accelerated to a given rpm, the greater the engine's horsepower.

Repeatable power measurements are only possible under WOT conditions with an inertia-only dyno. If partial throttle testing under a load is required, then an optional eddy current power absorption unit is required. The absorption unit applies a load to the roller, reproducing conditions similar to what a motorcycle encounters on the road. This allows partial throttle testing under a sustained load and steady speed driveability tests to be performed, which are helpful with fuel injected engines or where engine problems are encountered at a specific mph or rpm.

The advantage of a rear wheel dyno is that engine performance can be tested as installed. There is no need to remove the engine from the chassis or remove the rear wheel. This design results in quick and cost effective testing while factoring in drivetrain power losses. However, accurate and repeatable testing is more difficult to achieve than with an engine dyno because several factors such as drivetrain losses, along with tire temperature, wear, and traction influence the results. With the appropriate options and computer software, a rear wheel dyno can be made as sophisticated as an engine dyno, but many are not. Nevertheless, the rear wheel dyno has become the de facto standard for dyno testing motorcycles due to relatively low cost, ease of use and high availability.

It should be noted that horsepower and torque would be higher when measured directly at the crankshaft than the rear wheel or a jackshaft. When power is measured at the rear wheel, it is reduced because some power is lost through the vehicle's drivetrain, which includes the primary and secondary drives (clutch, transmission, chains or belts, and rear wheel). The exact power loss is dependent on the efficiency of the drivetrain, but in most cases, you can assume 10 to 18 percent lower than crankshaft power. Be aware that most factory power ratings are taken at the engine's crankshaft, as are some aftermarket power figures. When comparing dyno charts, always verify that each dyno pull measures power at the same location-at either the crankshaft or rear wheel.