Info about Dynos

[bd-850]

OK so this was an article in a Motorbike magazine, (but it can still relate to cars)


What is a dyno?

In Theory, a Dyno measures the amount of work done by an engine over a period of time and converts this into a power figure. In practice, there are several ways of doing this. Early Dynos connected the engine to a shaft that turned a rotor inside a tank of water. The tank of water was suspended by a spring balance (or similar) that measured the turning force (torque) produced by the rotor pushingagainst the resistance of the water. This torque figure, together with the rotational speed of the engine, let you work out the power being produced.
With the advent of compact, reliable electronics, this “water brake” Dyno arrangement has been largely replaced by “eddy current” electric brake dynos. With this type, the tank of water is replaced by a rotor and stator with magnets and coils – not unlike a bike’s a;ternator. As the engine turns the rotor, electromagnetic forces provide the braking effect, this force is measured and used with the engine RPM to produce a power figure.
These “brake” dynos are the more traditional design, but many lower-end dynos are “inertia” designs. On these, there’s no brake. Rather, the engine turns a heavy (300-750 pounds) drum, accelerating it as the revs rise. Sensors on the drum measure its speed, and with this info a computer works out how quickly its accelerating, and ultimately how much power the engine is making.
The downside here is theres no way of isolating a running problem or dip in power at certain engine speeds – you can only measure the engines output in a state of acceleration.
Nowadays though, many dyno shops have a combined ineria/eddy current dyno. This lets you run a simple full-throttle acceleration run for a basic power figure, and then run the engine at given RPM intervals, measureing the steady-state power output at each point.


Fueling

The main use fir Dynos is to adjust the fueling on the bike to suit a new exhaust or air filter. To do this, the dyno incorporates a gas analyzer that “sniffs” the exhaust gasses coming out of the running engine. Measuring the amount of oxygen left in these gasses can tell the operator whether the engine is running rich (too much fuel) or lean (not enough fuel), or just right. So if an operator has an eddy current dyno (with approptiate software) he can run a bike at 2,000RPM, 2,500RPM, 3,000RPM, etc… all the way up to redline. At each 500rpm point, the gas analyzer will tell him if the engine is rich or lean, and he can then use an aftermarket fueling unot to adjust the amount of fuel squited in by the injectors at each point. He will then repeat this job at different throttle positions. This means that at all revs and throttle positions he’s measured the power output and wheather the engine is running with the correct fueling. The result should be a smooth-running engine makes as much power as possible in that physical state.


Accuracy counts

Its important to remember that a dyno is just a measuring tool, albeit quite a complex one. And like all measuring tools, it cant be 100 percent accurate. If you gave schoolkids each a yardstick and asked them to measure the length of the school gymnasium, chances are you’d get 50 slightly different answers. That’s why the same bike can make very different peak power outputs on different dynos.
There are certain factors that affect the accuracy of the dyno too. The temperature of the engine, the temperature of the air going into the engine, and the air pressure inside the dyno room will affect what power it makes. Losses from drive chains can vary widely, and different tyres (and pressures) give very different outputs too.
Dyno designers overcome these inaccuracies with correction factors, and most dynos have a small weather station that measures temperature and pressure, using that data to adjust the power figure accordingly.
What this all boils down to is that a dyno can only be a comparative tool for before and after testing. The figures themselves are meaningless on their own, without a comparison. If your bike makes 150HP on a dyno then you fit a pipe and it makes 160HP on the same dyno, theres a 10HP gain. But if the bike made 150HP on a dyno ‘A’ last winter with a brand new chain, plugs and oil, there’s no point fitting a pipe and then running it on dyno ‘B’ with a worn chain and old oil in the heat of summer. You are not comparing like with like, and any changes aren’t to be trusted! Consistency is the key.


Dyno dupes

Dy nos are just a tool and they can be used by unscrupulous operators to give optimistic readings. On the most basic level, an operator can exploit a customer’s ignorance about different ways of measuring power. Most dynos give power figures at the wheel, and also at the crankshaft; the crankshaft being around 10 percent higher. But there are more dastardly methods too. During a dyno run, the operator will perform a ‘coast down’ run, where he pulls the clutch in and lets the rear wheel run free on the drum. This measures the friction in the drive train, which the dyno then adds onto the power figure to give an adjusted figure. If the operator grazes the rear brake during this coast down, the dyno ‘thinks’ there’s more friction in the drive line, and adjusts the power figure up accordingly.
Temperatures play a role in dyno runs too. Hot air gives less power, so the dyno adjusts readings accordingly to the temperature in the dyno cell. Make the dyno think it’s a bit hotter, then it is by fiddling with the sensor and you get more power. Another trick is to run the bike a couple of times to get the oil good and hot, then stop to let the top end of the engine and the inlets cool down. Then make a fast run the cooler air going into the engine increases output, while the hot oil is thinner, meaning lower friction losses, and a little more power.
The majority of the dyno operators are honest souls, and the rip off merchants are the exception – but they’re out there. Don’t be afraid to ask questions about your run if you’re not sure what is happening or if the numbers seem unusual!

[JaCe]

Interesting article- very true that the real usefulness of a dyno is in comparability for before/after tuning as opposed to "my car has more power than yours" arguments.