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Here we have two engines. This one is oversquare it's bore is twice as large as it's stroke giving is a bore to stroke ratio of 2. The other engine is undersquare it's stroke is twice as large as it's bore giving a bore to stroke ratio of 0.5. And in this video we will answer the question of which one of these makes more torque and why and because horsepower is essentially torque times rpm we will also answer the question of which one makes more power. And we will also apply the lessons we learn onto real engine examples to see if theory and practice match up.

Simple physics tells us that these two engines make the same torque. Torque after all, is a product of the force and the leverage applied. If we imagine a hand turning a wrench our leverage is the length of the wrench and our force is how hard we push on the lever. If I push twice as hard on half the lever length the torque will be the same as if we pushed half as hard on twice the lever length. So with this logic in mind, if these two engines have the same displacement, they make the same torque? No, they do not. Here’s something that’s often overlooked when it comes to bore and stroke. An increase in stroke is a guaranteed increase in torque. It’s guaranteed because the connection between the rod, crank and piston is a fixed, constant, mechanical connection.

But an increase in bore is NOT a guaranteed equivalent increase in torque because doubling the bore does not necessarily double the force acting on the piston and that’s because combustion is a variable that constantly changes with RPM and engine load.

To create combustion we need air and fuel. We need somewhere between 11ish to 14ish parts air to just one part fuel. So the struggle is to bring air into the engine. There are many factors that determine how much air comes in and at which rpm. Throttle body diameter, intake manifold size and shape, intake manifold runner diameter and runner length, intake port shape, length and diameter, the number of intake valves, the angle of the valves against the centerline of the engine, intake valve size, and then we have camshaft duration and lift which determines how much and how long the valve opens.

Now some of these we can continuously control throughout the rpm range and partially compensate for different engine breathing requirements at different rpm, some have 2 or maybe 3 different settings, but many of these are fixed. The fixed nature of some engine parts and the partial control of others means that they can only be truly optimized for a certain rpm range.

Now let’s see how bore and stroke impact air quantity and air velocity. If we observe our two engines side by side at the same rpm we can see that the undersquare piston travels much faster. It travels faster because it must cover the much greater stroke distance within the same period of time. This results in greater piston velocity which means that we can have decent air and fuel mixing even at low rpm.

But here’s something else that works in favor of increased air velocity in undersquare engines and that is that the reduced bore diameter forces the engine to have smaller valve diameters. These small valve sizes than dictate the rest of the orifices of the engine because the intake ports, runners, and throttle bodies must all match. If we have vastly different airflow capacities at any one point in the system we create airflow inefficiencies, in other words air struggles to get past such points and engine performance suffers noticeably.

So the small valve diameters result in reduced diameters throughout the system which further improves air velocity. But as we know this reduces maximum air quantity which means that maximum power potential at high rpm suffers and torque starts falling off as the engine struggles to breathe through the small orifices.

But an undersquare engine isn’t particularly concerned with high rpm performance and that’s because it can’t even reach very high rpm anyway. It can’t do it because of the increased piston speeds. To achieve high speeds we need high acceleration and high acceleration produces high force, which means that at high rpm the piston forces in an undersquare engine can actually damage which means that we must reduce our redline in order to preserve the engine.

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#d4a #bore #stroke

00:00 Force and Leverage
02:38 Air Quantity vs Air Velocity
07:14 Piston Velocity
10:37 Rod Ratio
14:45 K20, LS, S65, KLR, HD, R18
24:00 2 pieces of a puzzle

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