What is a dyno (dynamometer) and how does it work?
There are 3 basic types, an engine dyno, a chassis dyno and a hub dyno. We will refer to a chassis dyno in this article.
A dyno is a tool used to simulate stresses which a vehicle/engine will encounter when on the road/race track. It is a tuning tool designed to speed up the tuning process as well as give the tuner the ability to calibrate the engine under simulated conditions and all possible loads without the variables and dangers associated with actual operation.
How does it work?
A Dyno uses the measured speed to calculate the acceleration of the rollers which then converts this into a torque figure. The torque is then converted into a power figure using a simple mathematical equation. Correction factors can then be applied after the above calculation. (See here for formulas)
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Of these 3 types, each of these use the same principle to calculate the torque and power output of an engine, they just have different ways of measuring acceleration.
To best simulate real work stresses that an engine will undergo, one needs to be able to put resistance onto the engine. This resistance can be in the form of a water brake, hydraulic, eddy current or A/C motor.
Keep in mind, a dyno uses mathematical calculation to get to a power and torque figure. This means that these figures can be manipulated to read whatever the dyno controller wants it to read and thus give false or unrealistic power readings.
What should you be looking at?
It is common for most car enthusiast to just look at peak power and torque values to judge performance increases of the engine or how fast the car is. They are not incorrect by doing so but there is more to a dyno graph than just peak power.
When looking at a dyno graph, one can see how power is delivered over the RPM and how long it has that power for. With any motor, power can be delivered differently (more aggressive or later/sooner in the RPM) and so a crucial thing to keep in mind is how quickly the engine will rev (cover distance) and how well it will deliver the power.
Say for instance you have 2 vehicles with the same motors that are indentical in every form other than where (RPM range) the power is delivered. Both engines rev up to 7000RPM. Engine 1 makes 500HP between 5500RPM and 6500RPM. Between 2500RPM and 4500RPM the power only climbs gradually to 300HP and at 4500RPM it rapidly starts climbing to its peak power of 500HP. This means that the car makes its peak power for 1000RPM (between 5500RPM and 6500RPM) but anything below that, the car is very sluggish.
Engine 2 produces the same peak figure of 500HP but this engine produces this power between 4500RPM and 6500RPM. Which would you say would be the faster vehicle on the road?
The second engine of course as it makes the power for a longer period.
What we’re getting at is, the shape of the graph is important. How quickly power is delivered will relate to if your tires will b able to maintain traction on the road or not. How wide the power band is means you can proel the weight/mass of the car forward (distance) in a shorted time relative to the above illustration.
A second thing to look at is the percentage gain throughout the rev range. Using the hypothetical dyno graph of the engine comparison above, have a look at the area under the curve. It can be an important indication to how effective a modification is. Say for instance an intake is installed to an otherwise stock car. There is a 25% increase of the area under the curve but only a peak gain of 4Kw/5.3HP. You’ll be far quicker on the road compared to a car without the intake fitted even though it’s just a 4Kw peak power gain.
Every dyno will read differently so one cannot look at actual numbers when comparing one’s power runs on different dynos. Always use the same dyno under the same conditions when comparing your power gains. This will allow you to get the most accurate comparison available. Remember, the dyno is a tool which can measure power increases throughout the RPM range.