Why Do you See Lower Power On The Dyno

There was a recent viral video of the 2020 Kawasaki Ninja ZX-25R being dyno’ed.

The stock bike returned a peak of 42 bhp at 15,350 RPM, while the exhaust upped it to 43.7 bhp at 15,450 RPM. At this point, there were musings from netizens that the power is actually lower or at least equal to other current 250cc sportbikes.

Image source: Akrapoviç

Truthfully, the figure on the spec sheet seems disappointing. For example, the Honda CBR250RR’s twin-cylinder engine produces some 40 hp; so, surely the ZX-25R’s 17,000 RPM inline-Four should produce more than 42 hp.

The confusion arose due to the figures that came off the rear wheel. We should provide some explanation.

What is a “dyno”?

Let us first understand what a “dyno” is (short for dynamometer).

In the automotive industry, a dyno is a device utilized for measuring an engine’s power and torque. The readings obtained from the dyno are used to calibrate the engine’s characteristics and to achieve optimal numbers for marketing purposes i.e. printed on the specification sheet.

We will skip all the fine technical details and focus on two types of dynos: the Engine and Rear-Wheel Dyno. There is another called Chassis Dyno, but we’ll focus on these two.

Engine Dyno

The engine dyno is used at the factory. A fully operational engine is placed on a test stand without the transmission. A shaft is then connected from the crankshaft’s output shaft to the dyno’s input shaft.

The engine is then started and spun all the way to its maximum RPM. Multiple runs are made and the results (maximum torque, power and RPM) are either averaged or the best figures noted for marketing purposes. The numbers obtained are called “crankshaft torque” and “crankshaft horsepower”.

Rear-Wheel Dyno

The rear-wheel dyno can be found in the open market.

A complete bike is strapped down, with the rear wheel placed on a rotating drum. Power is transmitted through the entire drivetrain which includes the transmission, final drive (chain/belt/shaft) and rear wheel.

Since the engine is now equipped with a gearbox, the operator must “pull” the engine through all the gears. This is because gearing alters the power delivery of the engine.

Engine versus Rear-Wheel Dyno
Truth is, factories use both engine and rear-wheel dynos. The latter is for final quality control stage—it is used to check if the powertrain and drivetrain are working correctly. Therefore, it is not used to obtain the maximum numbers for publication. The reason simply being that engine dyno eliminates the many variables that a rear-wheel dyno has to take into account. These variables include, but not limited to, the gearbox, chain condition, cush-drive condition, tyre condition, tyre inflation pressure, etc. Besides those, the dyno itself may wear out parts that are out of calibration.

The confusion arises because virtually all manufacturers do not publish whether those power figures were from the crankshaft or rear wheel. However, it is safe to assume that they are from the crankshaft, unless stated otherwise.

Therefore, you can conclude that the engine dyno looks at the engine’s unadulterated performance, while the rear-wheel dyno shows us the real-world performance. This is why the latter dyno should only be used for tuning.

Power losses

So, let’s come back to the ZX-25R.

Since power was transmitted through the gearbox, to the chain, to the cush-drive, and finally the tyre/roller interface, some of that power was lost along the way. Depending on the state of each variable, a motorcycle engine loses between 5% to 10% of its performance. (Car engines will lose up to 20%.) This is why you will see lower power figures if you put your bike on a dyno.

Working backwards, the ZX-25R’s engine should produce between 44 hp (5% loss) and 46.7 hp (5% loss). The manufacturer may claim 48 hp on paper. On the other hand, CBR250RR’s rear-wheel rating should be between 36 bhp (10% loss) and 38 bhp (5% loss), compared to the ZX-25R’s 42.

Hope this clears things up.

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