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Evaluating Dyno Charts

Cam Basics

Porting: The Black Art

Power Transfer to the Rear


Exhaust Systems

Corrected Compression Ratio

Evaluating Dyno Charts

by John Hildreth

So, you just had your bike dyno'd, and now you're standing there looking at a copy of the computer generated graphs. What does it all mean?

Most people think big horsepower is what they're looking for when they upgrade the performance of their motorcycle. But a better way to evaluate an engine upgrade is through its torque curve. Torque is what you feel when you ride a Harley, and it's what makes riding a cruiser so much fun. An engine's torque is the most accurate way to evaluate its efficiency because torque is what the dyno is actually measuring during a test run. The dyno's computer applies a formula to the torque measurement and then estimates the horsepower an engine is producing.

A torque measurement of energy is stated in "pound feet" or "foot pounds" (the designation usually used in the United States.) Peak torque also indicates that the most efficient burn of the air/fuel mixture in the combustion chamber has been achieved. When comparing charts, be aware that there are five different measurement scales currently used by dyno manufacturers. They are SAE, DIN, EEC, STD, and UNCORRECTED. While any one of them is as accurate as another, make sure that your own "before" and "after" runs are calculated using the same correction factor each time. The difference between scales can show an increase of as much as a 5 pound feet of torque without doing anything except switching to one of the other scales on the computer.

Graph "A" below is an example of a healthy torque curve. Notice that the torque level comes up quickly in the RPM range, carries it's peak number over a wide range, and then falls off slowly. There are three main ways to improve a torque curve: One is to increase the compression ratio up to the point; another is to install heads with more efficient ports to give improved port velocity and air/fuel atomization; and a third way is to achieve the right balance of backpressure in the exhaust. It must be noted, however, that all three of these areas have to work in conjunction with one another. Improving one area will not produce a consistently good curve across the powerband.

On the other hand, Graph "B" shows a bike with a very similar maximum peak torque number. However, it also shows a "dead" or flat spot at the lower end of the RPM curve. This bike will be sluggish and non-responsive at those RPMs. The most common cause of a flat spot like this is due to the exhaust system used. It generally means the system doesn't have enough backpressure, or it is over-scavenging during the exhaust cycle.

One very important point to note when evaluating any dyno chart is the time at which peak torque is obtained. Many times, a peak number comes in an RPM range where you never take your bike. Having a bike with peak numbers at 6,000 is fine, but if you always ride between 2,500 and 4,000 RPM, you'll never be able to enjoy that increased performance. Take a look at the torque in the RPM range where you spend most of your riding time. This is where you should try to improve your bike's performance. Some well-known combinations advertise big peak horsepower numbers at the upper RPM range with nothing at the lower end. If you spend all your time in a powerband that shows up as a flat spot or a dip in the chart, you may even end up with less horsepower and/or torque than a stock bike. Unfortunately, this can be a very expensive lesson to learn. And it can be really embarrassing, too, especially after you've gone around bragging about your big horsepower numbers, only to have a stock bike out-run you in a 5th gear roll-on.

Dynos are great tools for showing "before" and "after" tuning and for helping to evaluate engine problems. By using a dyno graph, you have the ability to see what your performance package is actually doing and to pinpoint strong and weak areas. If you have any questions about reading or evaluating dyno graphs, or other performance questions, you can email me at jhfxr@aol.com.

© 2004 Hildreth Performance

Last Updated on 06/14/2004