When determining the displacement of any given internal combustion engine aside from rotaries, you’re going to multiple the stroke length, bore, and the number of cylinders that comprise the whole engine. Bore is the diameter of the cylinder and stroke is the movement from bottom dead center to top dead center.
Typically both bore and stroke are measured in millimeters, but for many American engines, you’ll see these measured in inches. So, for example, you were looking at a four-cylinder engine with a 90mm bore and a 90mm stroke, you would simply 90 x pi x (90/2)² x 4. What you end up with is 2,289,060 cubic millimeters or 2,289 cubic centimeters.
So, mathematically bore and stroke is really simple. If you wanted to increase displacement and ultimately increase power, you would just increase bore or stroke. What you have to remember is that the movement of the piston is determined by a crankshaft that’s spinning at thousands of RPMs, which complicates everything.
Measuring Horsepower and Torque
It’s worth noting that the equation for horsepower is as follows (torque x RPM / 5,252) and torque is measured in rotational force. This is why on engine dyno sheets you’ll always see torque and horsepower cross at 5,252rpms. The easiest way to explain torque measurement is to visualize 100lbs on the end of a 1ft wrench. This would equal 100 lb-ft on the center of the axis. If you changed this to a two-foot wrench, now you’d be talking about 200lb-ft.
With this simple visualization, you can see how increasing stroke would ultimately increase torque, since you’re increasing the throw of the crankshaft, which the pivot point, which is the center of the crankshaft, further away from where the piston is pushing down on the crankshaft.
So, what’s the deal then? Can’t you just increase stroke to something huge and call it a day? For an engine with a really low RPM limit, this could potentially work, but for most engines, the RPM limit isn’t very low. With increased stroke comes increased piston acceleration and increase side-to-side piston force, both of which are hard to tame.
A good example of this would be the exact opposite of this would be an F1 style engine, where you’ll see a very small stroke compared to the bore. But why? Because, with a smaller stroke it is much easier to increase the RPM limit of the engine because of piston acceleration, oiling, side-to-side piston force, and some other factors.
Remember, horsepower is measured with two variables: torque and RPM. You can either increase one or the other to increase horsepower. In the real world, increasing RPM is generally easier than increasing torque because of the issues previously mentioned.
What you’ll see with engines found in many production car engines is an average piston speed below 25 m/s. There are some which do exceed this, but the average is typically at or below 25 m/s for the large majority of engines found in road cars.
If were to take an example of an engine with an RPM limit of 5,000, a small stroke engine would see less piston speed than an engine with a big stroke. This effectively means you can increase RPM on small stroke engines at the same maximum piston speed compared to an engine of equal displacement but a larger stroke. Simply put, a smaller stroke means higher RPM limits without excessive piston speeds.
Cylinder Head Flow
Another thing to consider is how bore and stroke affect the cylinder head, and more specifically how valve size is affected. Larger valves, rather obviously, are going to typically flow more total airflow compared to smaller valves, but a small-bore would force you to have small valves. This is a big benefit of engines with large bores, is that the intake and exhaust valves can be much bigger since there is a lot more room inside the cylinder for them to fit.
Simply put, a 2.0L engine with a large bore has greater valve airflow compared to a 2.0L engine with a large stroke, but there isn’t always the case. *zoom in* there are other things to consider when looking at airflow and it’s not uncommon to see larger valves hurts power potential at low RPM due to things like volumetric efficiency, but that’s a topic for another time. At high RPM, however, larger valves are almost always going to provide better airflow than smaller valves.
Then why don’t all cars just have engines that spin to 10,000RPM to take advantage of airflow and make as much horsepower as possible? Well, there are two reasons that road car engines aren’t like this: the power curve and efficiency. If you large bore engine that revved high and made lots of peak power, you’d ultimately be sacrificing low-end torque.
Power Curve and Efficiency
While you and I might find it fun to rev our car’s engine to the rev limiter, the average person just wants their car to accelerate when they hit the throttle. This means road-going cars need to have a flatter power curve so that the vehicle can accelerate decently well at any given RPM, high or low. Of course, there are other ways to do this such as forced induction, but for this article, we’re just looking at bore and stroke.
The next thing to consider would be efficiency and more specifically thermal efficiency. Most of this comes down to heat and the amount of surface area inside the cylinder that heat can escape to. All that heat from combustion is what drives the piston down, so the more heat we can retain for pushing the piston downwards, the better. This means that less surface area inside the cylinder is going to lead to better thermal efficiency.
Without getting into too much of the math, an engine with a 2.0l displacement and a large bore is ultimately going to be less thermally efficient near the top dead center when combustion is occurring compared to a 2.0l engine with a large stroke.
Yet another thing you need to consider for a gasoline engine is fuel burn. Since the combustion event is kicked off by the spark plug which is located in the center of the bore at the top of the cylinder, the combustion moves from the spark outwards. To simplify this, if you had a narrow bore, the combustion would reach the cylinder walls quicker and then direct all force downwards, compared to an engine with a large-bore where it would take longer for the combustion to reach the cylinder walls and then force everything downwards.
That explanation on fuel burn is extremely simplified, but the idea is basically that a smaller bore engine is going to have a shorter burn duration than an engine with a large bore. This again just translates to more efficiency. You’d also have to factor ignition timing into this to better understand how fuel burn changes depending on cylinder bore, but that’s not something we’re going to get into with this article.
Which One is Better?
So with all this info, we’re back to square one. There are benefits to having a large bore and there are benefits to having a large stroke. All other things not considered, which one is better? The answer leads to more questions than answers.
If you were for example engineering an engine designed for race use, it would more sense to use a large bore and short stroke so you could more easily increase the RPM limit to increase power.
If you were engineering an engine designed for a truck, it might make more sense to use a large stroke to increase power while having a low RPM limit, since you wouldn’t want a truck to be revving to the moon while towing.
If you were engineering an engine for a simple commuter, somewhere in between would make the most sense, since you would have acceptable power at low and high RPM. This is what’s known as a square engine, which is what you’ll see with many small engines such as 2.0L four-cylinder where 86mm bore and 86mm stroke is common.
If you were to consider other factors into this, then it gets even more confusing. For example, an engine with a small turbocharger that produces peak boost at a low RPM could use a larger bore, since low-end power is now being added to by the turbocharger.
The method of cooling changes everything as well. For example, Harley Davidson V-Twin engines are almost all air-cooled. With an air-cooled engine you want a tall stroke to increase the amount of air cooling fins you can place on the outside of the cylinder, otherwise overheating would become a common occurrence.