How-To read a Dyno-Graph

[Justin@VMP]

Important information about the vehicles performance is displayed on the dyno-graph.

• Maximum recorded power and torque are displayed on the top left. Spikes in the graph can skew the numbers artificially high. Spikes can result from a torque converter locking/unlocking or hitting a rev limiter.
• Horsepower and torque are displayed as light and dark blue lines on the above graph. The Y axis is Power on the left and Torque on the right.
• The X axis is most commonly RPM, but can also be wheel speed in MPH or time in seconds. If a good RPM signal can not be acquired sometimes wheel speed will be used.
• Boost pressure is graphed for engines with forced induction (supercharger or turbocharger).
• The air to fuel ratio is graphed, ideal ratio is between 12:1 and 13:1 depending on type of engine. There is a red dashed reference line at 13:1
• Current weather conditions are displayed on the bottom left, showing humidity, barometric pressure, and temperature.
• Based on conditions a correction factor is applied to the power numbers, the SAE correction factor is the most common (shown at top right). A 1.02 correction factor would add 2% to the power numbers due to less than ideal conditions, a .98 correction would take 2% away from the power numbers due to very good conditions. If an STD correction is used the power numbers will usually be higher. The numbers can also be displayed as Uncorrected.

What is ideal?

• What is an ideal graph look like? Different engines have different power curve characteristics, so it is difficult to say. The power and torque curves should be smooth with no sudden changes or drop offs. If the air to fuel ratio and ignition timing is properly tuned you are much more likely to have a smooth graph. The very beginning and very end of the graph can be largely ignored, things like not completely flooring the car initially, and hitting the rev limiter at the end, can sometimes make the graph look odd but are not actual problems.
• What does an ideal air to fuel ratio look like? This is different for each combination. An engine with a centrifugal blower does not make much boost until 3000rpm, so they can be run 13:1 below that point to make more torque, and 12-11.5:1 once in boost. A roots blower makes boost very quickly, so I usually tune them to run 11.5-12:1 at 1000-2000rpm. A naturally aspirated engine can be run between 12.5-13:1 to make the best power. The initial few hundred RPM of air to fuel ratio information the graph is usually erraneous, and so is the last 100-200rpm.
• The boost graph can be used to troubleshoot slipping belts on supercharged engines and wastegate problems on turbocharged engines. Not all dynos can log and display boost, but I have found it to be an invaluable tool.