Motorcycle Dyno Tuning

There are several misconceptions out there about dyno tuning, what it does, and its importance. We would like to clear these up.

When you make changes to the way an engine breathes there needs to be compensation in fuel whether air in or out has been restricted or improved there needs to be a corresponding change in fuel delivery. Yes, there are some minor changes that can be made that the ECM is prepared to compensate for but that change usually amounts to an air filter only. The ECM has tables that work within a set window of parameters that are appropriately calibrated to the stock parts that come from the factory. Most often (but not always) the motorcycle runs excellent from the factory with few issues. However, OEM manufacturers are constantly updating calibrations for many reasons including but not limited to performance, engine temp control, emissions, knock retard protocols, etc.

The ECM is *NOT prepared to compensate for cams, heads, throttle bodies and certainly not exhaust changes. These are things we have learned over the last decade of experience dealing with everything from stage ones to 150hp pump gas builds.

Here are some Frequently Asked Questions we get about dyno tuning and the best answers we can give:


Why do I need my bike tuned for just a muffler change?
Harleys (and many other motorcycles) do not operate the same as cars do in terms of airflow. They do not have the advantage of intake tuning due to space constraints for both practical and aesthetic reasons. If you were to have a properly "tuned" intake on your v-twin it would be around 18 inches long! So much of the pulse tuning to hold the incoming air/fuel charge is done through the exhaust. This is why proper baffle size for the application is crucial to the best combination of torque, horsepower and run-ability. The factory exhaust is incredibly restrictive and the ECM has been calibrated for it. Even small changes to the exhaust are enough to throw it out of tune.


I made some changes and my bike runs fine, why tune it?
There is a 99% chance that although your bike runs "fine" that it is far out of tune. Just because it is not bucking and snorting does not mean that the air/fuel ratios are properly calibrated. Many modified and untuned motorcycles are running around out there burning as rich as 11:1 or as lean as 16:1 in areas and the rider may never know it! That is a great way to cause cylinder washing due to overfueling (not to mention costly mpg's) or incredibly high cylinder head temperatures that can definitely lead to premature failure of parts.


I had my bike tuned and it seems to run much smoother. What happened?
Vibration is something that is perceived differently by each individual. What one person may find annoying and intolerable another may ride cross country and never notice. In fact, many studies have been done that even found differences between how genders detect vibration differently (get your minds outta the gutter guys!). When tuning, many cells in the many tables need to be calibrated to a specific air/fuel ratio. When you twist the throttle the ECM makes decisions in milliseconds based on each cell it moves through as RPM and engine load and throttle position change. If these cells each have different air/fuel ratios that go from lean-rich-lean-rich, etc then that will be perceived by the rider as a vibration due to the varying amounts of fuel that need to be burned through by the engine. Take that problem and multiply it by the number of cylinders out of tune and one can easily see how the problem can get out of hand quickly.


I had my bike tuned but it made less power on the dyno at bike week. What's up with that?
Dynamometers are just tools. Big, expensive tools. At the end of the day the only thing it does is create a safe, controlled environment to duplicate riding conditions so that problems can be easier detected. They do measure torque and calculate horsepower from there which is what most dynos are used for. Having said that, dyno numbers can vary wildly from one dyno to the other. Other factors such as correction factor, ambient air temperature, dyno calibration, wide open throttle gear, etc can all cause numbers to vary from one dyno to the other. We have seen huge differences between back to back dyno runs with other very well known names in the industry. Does that mean one or the other is inflating results? Not necessarily...


I have a runability problem. Can the dyno fix that?
A dyno is only as good as it's operator. And run-ability is only as good as the combination of parts used. There are some parts out there that no matter how much time is spent tuning, they simply do not work well. Some of these parts are VERY well marketed by some well known names in the industry. We are quite aware of which parts work and which do not. If we don't know or haven't tested it we will tell you. Rarely there is a well thought out combination of parts that simply will not play well together. We have seen cam and exhaust interactions that cause reversion that is so bad it cannot be tuned with traditional methods. It may be made to run well on the street and show well on a dyno graph but the correct course of action would be to try different parts. A dyno cannot fix mechanical issues such as leaky fuel injectors, low fuel pressure, bad coils, bad grounds, etc etc. If your motorcycle began running poorly there is a good chance it has a mechanical or electrical issue.


What model dyno does Vex Cycles have?
We use a Dynojet 250 with eddy current brake and the latest Winpep 8 software from dynojet.