How to Read a Spec Sheet to Figure Out What Kind of Motorcycle It Is! (Engine Section)

We are very interested in the specs of a vehicle that we like. It is fun to look at the numbers on the specs sheet (specification table) and explore the various aspects of the vehicle.

What kind of power does it have? How is the handling? How does it feel other than the maximum output?

If it is new motorcycle, you can check it out at test-drive events, but if it is a used motorcycle or a discontinued motorcycle, you can only guess from the spec sheet.

However, the spec sheet is a very accurate way to read the characteristics of a motorcycle. Even more so if you are an experienced rider who has ridden many different motorcycles!

WebiQ is here to bring you a little happiness and motorcycle knowledge. We're talking about the trick to deciphering what kind of motorcycle it is from the spec sheet. This time, we'll talk about the engine!

I'm talking about on-road vehicles!

Even if we imagine a motorcycle from a spec sheet, we cannot decipher much about moped type 1 (~50cc) scooters and off-road vehicles.

Basically, there is not much difference between them, and there is very little individuality unique to each vehicle.
It does not mean that there is no difference at all, though.

Scooters and mopeds (50 to 125 cc) have a little more individuality and can be read and deciphered. 

On the other hand, for scooters and off-road vehicles, pictures of the body are more helpful in deciphering what kind of bike it is rather than spec sheets. This is because the body structure, which is not shown in the spec sheet, is much more unique.

Displacement is a different concept.

This is a little off the subject of reading characteristics from a spec sheet, but displacement is super important.
In the case of motorcycles, small-displacement and large-displacement motorcycles are completely different vehicles.

Of course, this is not to say that small-displacement motorcycles are bad. It is not about the common story that large-displacement motorcycles cannot be easily revved to the rev limit at full throttle, but that small-displacement motorcycles can be easily revved and are fun to ride.

A good example is the 85cc motocrosser, which can definitely be called a small displacement bike, and it is impossible for even the most skilled rider to casually rev it up.

It is generally correct to think of displacement as a measure of "leeway".  There are bikes with small displacements that are super fast, but they are usually fast only when they are driven at full throttle all the time. In other words, there is no margin.

This is not a bad thing, but sometimes the purpose is to enjoy that lack of room (small-displacement sports cars are usually like this).

So, what kind of things you want to do, what kind of things you think are fun, and how far you want to pursue them, will determine the displacement you are looking for.

If you want to enjoy long-distance touring with plenty of room to spare, a small-displacement car is not for you, and if you want to enjoy sharp behavior by performing the right maneuvers at the right time, a large-displacement car is not for you.

However, it can be fun to go long-distance touring in a small-displacement car and get into trouble, so it is very important to select the displacement that best suits your purpose.

Maximum power, maximum horsepower, and output characteristics

Once the displacement has been roughly determined, the next question is the maximum output, which everyone loves to be concerned about even if they don't want to.
It is easy to choose the most powerful bike, and it takes courage to buy a bike with a lower maximum output than its rivals.

However, there is a serious pitfall here.
The maximum horsepower listed in the catalog specifications is only the horsepower at full throttle.
The power curve is just a graph showing "what the output is when the throttle is fully open.

The chassis dynamometer graph is the same, it is just a graph of what happens when the throttle is fully opened. The same is true of the chassis dynamometer graph, which only shows what happens when the throttle is fully opened.

Furthermore, if you look closely at the graph, you will see that there is no line in the low RPM range, right?
This is because the engine will stop if the throttle is fully opened below that rpm.

This is not because "I don't care about such low rpm! This is not the reason. All the figures in the catalog specs are at full throttle. This is extremely important.

Of course the maximum output is achieved when the throttle is fully open, but you don't normally start at a very low speed, just before the engine stalls, and then suddenly open the throttle to full throttle and wait for the maximum output to come out, do you? The throttle is not an on/off switch.

So while the maximum output is as shown in the spec sheets and graph figures, in fact, the catalog spec figures are of little or no help when it comes to output in the mid-range.

It turns up to 15,000, but there's a valley at 6,000 rpm. If the engine turns 15,000 rpm, there is no reason to wait for the throttle to fully open from low rpm below 6,000 rpm.

Maximum power is necessary for maximum speed, but only if the throttle is fully open. How often is that the case? If you think about it, you will come to the conclusion that there are almost none.

Thus, what is important for a motorcycle is not the maximum power output at full throttle, but the power characteristic in the intermediate range, but this important part cannot be read from the maximum power or maximum torque figures, nor from the power curve or torque curve graph.

From the output characteristic graphs, we can read, "Is it a high-revving, high-power type? or "Is it a low-speed torque-oriented type? or "Low-speed torque-oriented type?

Therefore, we honestly do not recommend you to be overly concerned about the maximum output, maximum torque, or the power curve.

Number of cylinders and engine characteristics

I wrote that you cannot read the true engine characteristics from maximum output, maximum torque, or power curve graphs.

To review, the reason why they are not readable is that they are all at full throttle. But that doesn't mean that the mid-range engine characteristics are completely unreadable.

The most influential engine characteristic in the mid-range is neither maximum power nor maximum torque, but the number of cylinders in the engine.

Huh?" If you are thinking "What?", your motorcycle may have been a 4-cylinder car for a long time. Each engine, even a four-cylinder engine, has different output characteristics, but these characteristics are minor compared to the difference in the number of cylinders.

To make the story easier to understand, imagine a scene where an engine accelerates and starts up from a fully closed position where the engine speed has dropped to about half of its original speed on a certain corner.

First of all, the Japanese love the parallel four-cylinder engine, which was originally designed for high-revving, high-output engines.

It has matured so much that it now produces plenty of torque from very low speeds (in fact, it is the easiest to make a U-turn with), but it still retains traces of its high-revving roots. This is most noticeable in the mid-range of rpm.

The 4-cylinder responds sensitively to throttle operation (said to have good pickup), but the engine only responds and does not actually produce much torque or power. The power and torque values are high to begin with, so even though we say "not much" is being produced, it is quite a bit, but despite the numbers, it has a gentle feel.

As a result, it starts accelerating smoothly, and if you continue to open the throttle, it will show intense power that is typical of a 4-cylinder. It is such a characteristic.
The result is that the engine starts to accelerate smoothly, and if you continue to open the throttle, you get the strong power that is typical of a four-cylinder.

On the other hand, a single-cylinder engine does not respond immediately when the throttle is opened, partly because the engine rpm is not as high as that of a four-cylinder engine.

However, this is only when compared to a four-cylinder engine, and it is not to the extent that the engine response is sluggish when you are riding.

Once the engine starts to react, the amount of torque and power that is generated by opening the throttle comes out at once. It is quite violent, and the acceleration like being kicked off with a bang is completely different from the gentle acceleration of a 4-cylinder.

It is like "the moment you open the throttle, as much power and torque as you want comes out with a bang.

Two-cylinder and three-cylinder engines are in between the two-cylinder and three-cylinder engines.

The basic characteristics of the number of cylinders, which may vary in degree, are as follows. Whether it is a high-revving, extreme single-cylinder or a four-cylinder with an emphasis on low to medium speeds, the basic characteristics remain the same no matter how far you go.
The four-cylinder feels as if a hand is gently placed on its back from behind and then it is pushed hard and continuously.

A single cylinder is like being kicked from behind, but the kicking foot gradually becomes out of reach.

People who are supremely concerned about the power of four-cylinder engines get angry when I say that it has a gentle output characteristic, but it is true, so there is nothing I can do about it.

This is the reason why single-cylinder and two-cylinder motorcycles, which obviously have lower maximum power than four-cylinder motorcycles, are so fast in situations where there is a lot of mid-range speed (which is usually on public roads, including mountain passes).

Relationship between bore and stroke

Even with the same displacement and the same number of cylinders, the piston diameter (= cylinder bore diameter) and the amount of piston stroke differ from engine to engine.

The "bore diameter x stroke" is a combination of these two items. The combination of these items allows us to read the characteristics of the engine to some extent.

First, for the same displacement and the same number of cylinders, the relationship between bore and stroke is inversely proportional.

The larger the bore, the less the stroke, and the smaller the bore, the more the stroke. Displacement is, in essence, the volume of the cylinder in which the piston moves, so if you imagine how to find the volume of a cylinder, it is easy to understand.

I won't go into all the details because it would be full of numbers, but it can be determined that the smaller the stroke volume (the larger the bore diameter), the higher the engine is designed for high rpm and high power output.

There is a physical reason for this: there is a limit to the speed at which the piston can move inside the cylinder.
The longer the stroke, the lower the engine rpm, the faster the piston speed reaches its limit, and thus the engine cannot be a high-rpm, high-output type.

It is a bad habit of Japanese people, who love catalog figures, to think that a larger bore and smaller stroke is better, but this is not true.

Of course, it is easier to achieve maximum output with a high rpm, high output type engine with a large bore, but such engines tend to be uninteresting engines that simply spin at the low and medium rpm ranges frequently used for city driving.

If you compare the bore x stroke ratio, you can see the characteristics of the two engines.

As a side note, the bore x stroke ratio is called "Boost", which sounds like a "know-it-all", so give it a try.

Compression Ratio and Engine Characteristics

After displacement, number of cylinders, and bore ratio, the next step is compression ratio. To be honest, very few people care about compression ratios, but they can significantly affect the character of an engine.

The high compression ratio is a sign of a high performance engine, and as a result, it requires high octane gasoline. *The story of compression ratios and high-octane gasoline is below.

What is the difference between high octane and regular?
October 20, 2020
High-octane = high-grade gasoline? High-octane gasoline is a luxury gasoline with higher octane than regular gasoline. It burns with a bang and seems to give you more power, right? And it's expensive, too! But! High-octane gasoline does not burn more powerfully than regular gasoline. So why go to the trouble of putting in overpriced gasoline.

The engine is not continuously producing power, but rather it is producing power in small bursts, one combustion cycle at a time.

In between these combustions, the engine is doing power-hungry things such as exhaust, mixture intake, and compression, so there are big waves in the power output.

The size of the wave (the height of the wave) is proportional to the engine output, so the higher the output engine, the larger the wave.

But that's not the same as the characteristics depending on the compression ratio, because that's also what happens with differences in displacement.

Rather, the higher the compression ratio, the closer the angle of the wave becomes to a right angle.
Conversely, at low compression ratios, the angles of the wave become rounded, and at very low compression ratios, the wave has a mellow, "wave" like feel.

For example, a small-displacement engine with low power but high compression and gruffness, or a large-displacement engine with high power but low compression and mildness, the difference in feeling goes beyond mere power difference.

So, what compression ratio gives the engine the most gruff and extreme feeling? You may be wondering, but there is no clear delineation. The reason for this is that it is very much related to the "actual compression ratio," which is not written on the spec sheet and is intertwined with the compression ratio and valve timing, and it is difficult to make a blanket statement.

However, an engine with a compression ratio of over 12:1 is definitely a high compression engine, so it tends to have a more aggressive and extreme feeling than an ordinary engine.

For the same displacement, the higher the number of cylinders, the higher the number of waves and the lower the height of the waves, so the engine is less affected by the waves.

So, even though they are the same "high compression ratio engine," four-cylinder engines do not have such an extreme feeling, whereas single-cylinder engines tend to have an extreme feeling like, "Amateurs, go home! In contrast, a single-cylinder engine tends to have an extreme feeling like, "Amateurs go home!

The "large-displacement single-cylinder racing single engine with a high compression ratio" is the worst, with less than half the power of a high-performance 4-cylinder engine with 200 horsepower or more, but the feeling is so extreme that it is impossible for ordinary people to ride.
In many cases, veterans who have experienced various motorcycles end up tuning Yamaha SRs, not because they are nostalgic, but because they are the most extreme.

In addition, off-road competition vehicles are usually single-cylinder with high compression and extremely high rpm boost ratio, and they are the most extreme in terms of engine feel.

There's more, but it's more challenging.

As mentioned above, there are many other factors that affect engine characteristics, such as cam timing, valve count, and cooling system.

Since many of these factors are written in the spec sheet (specification table), it is possible to make a rough estimation by combining various conditions as you become accustomed to them.

However, it is very difficult to imagine the factors other than "displacement," "maximum output/maximum torque," "number of cylinders," "boost ratio," and "compression ratio," which are described in the spec sheets on.

Unfortunately, the number of experience is the only thing that matters in this world, as it is difficult to make a general statement.

Lubrication system is irrelevant

I don't know who started it, but there has long been a common belief that dry sumps produce more power because the crankshaft does not scrape up oil, thereby reducing oil agitation resistance.

However, this is false, at least in the case of recent bikes.
Because even with a regular wet sump, the crankshaft is not immersed in oil. It was a long, long time ago that the crankshaft of a wetsump was actually immersed in oil and stirred up by the oil.
Therefore, there is no difference in engine feel due to differences in lubrication systems. You don't have to worry about the lubrication system.