Is displacement the most important factor in determining a motorcycle engine's overall performance?

Question

This is a simple question to follow up the question from the thread below:

Engineering formula to relate engine displacement and power

Motorcycle manufacturers tend to show the displacement value in mass production motorcycles incorporated in their names such as Honda Wave 125i, Suzuki Raider 150, Kawasaki Z1000, etc. Most of these are below 200cc displacement. (Exceptions are Yamaha YZF-R1M, Kawasaki Ninja H2, etc).

The question is: If power is not determined by the displacement alone based on answers and comments on the thread above, does this imply that the displacement is the most important factor in determining the engine's overall performance?

Answer 1

Me again. It really depends what you mean by performance.

As you increase displacement, you're going to be looking at an increase in mass (bigger cylinders, larger crank and case). Silly example - you could throw a locomotive engine at a bike and it wouldn't get you anywhere useful.

Power will be proportional to displacement per second, so the engine speed is going to be a factor as much as bore squared x stroke.

It depends what you want to do. Green lanes over the moors? Good bit of low down torque. Drag strip? Sheer power. A track or road with lots of corners? Nice progressive delivery.

Answer 2

Assuming by performance you mean the total power or torque output of the engine, the displacement is certainly a major factor, but it cant be analysed individually. Other factors such as number of cylinders, number of strokes, Compression ratio, mode of aspiration, bore/stroke ratio, cam profiles/ timing, head geometry, redline,etc etc etc... when considered for a given displacement reveal the overall picture and are equally important.

A single cylinder 1000cc engine will have a entirely different performance figure than a 4 cyl, and a 1000 cc 2 stroke will behave highly unlike a 4 stroke, and with a different powerband.

And if the engine size, weight and NVH is also considered towards overall performance, a single cylinder will require a much heavier flywheel and thus a heavier overall engine. Also a multicylinder engine of the same capacity allows for reciprocating load balance and better vibration characteristics.

Along the same lines, a litre class bike engine has a horsepower figure of around 190 HP, whereas a 1.2 L car engine could peak at sub 100 HP. What makes the difference in this case is a higher redline and a different bore/stroke ratio.

Displacement also doesnt reveal a correct picture when forced induction comes into the picture. Small displacement turbocharged engines make much more power than NA higher capacity engines; current 1.6L F1 engines are the smallest ever displacement wise, but generate the highest power ever, all thanks to turbocharging.

And then race spec versions of road vehicles having the same engine with the same displacement churn out entirely different numbers. ECU fuel and ignition maps matter too. A lot.

In all, displacement alone isnt enough to predict the engine performance, knowing at least the 3 other factors described above as well can help get a clearer idea. And in the end it always comes down to what application the engine is designed for.

Cheers!

Answer 3

If you take "performance" as a mix of how fast the bike will accelerate and travel along its intended terrain, you can roughly approximate it as a ratio of thrust to mass.

As mentioned by others, thrust can be increased by having a larger capacity engine with a corresponding mass penalty. Depending on the terrain, this penalty may be more or less important - there are plenty of 1000cc street bikes but very few off road bikes.

There are other reasons for numbers in the name though, one being marketing (I don't remember Mazda being big on publishing the engine capacity of their rotary cars) the other being licensing. In the UK, I can ride a 125cc bike on my current license. Having numbers in the name makes it incredibly clear what I need to be looking at.