8 Cylinder Engines – Do They Produce Lower RPM?
Most people consider that the number of cylinders determines the RPM rate. However, it is not true; the cylinders do not act as a major factor for RPM functioning. Other aspects contribute to deciding the RPM of a vehicle or a car. Today, we are going to clear if 8 cylinder engines produce lower RPM.
But first, let’s take a look on the eight cylinder engines!
Eight Cylinder Engines
Every vehicle has cylinders, and the engine size is generally denoted by how many cylinders a vehicle has. A 4 cylinder has 4, a V6 has 6, a V8 has 8 and so on. Inside each cylinder is a piston, which moves up and down. Gasoline and air combine inside the cylinder and a spark creates combustion. The combustion then pushes the cylinder down, which creates motion that is transferred to the driveshaft, propelling the vehicle. This is why vehicle motors are referred to as internal combustion.
For more information, the eight cylinder engines can be timed for inherent primary and secondary balance, with no unbalanced primary or secondary forces or moments. However, crankshaft torsional vibration, present to some degree in all engines, is sufficient to require the use of a harmonic damper at the accessory end of the crankshaft.
The 8 cylinder engines are not directly proportional to the maximum RPM a vehicle can produce. The primary factor is the engine’s piston mass and the gathering system of the rod. The second factor that contributes is the mass of each valve. Similarly, the third one can be the length of the piston’s stroke following the piston’s speed.
The Number Of Cylinders Doesn’t determine RPM Rate of Cars
The maximum rpm of any engine is limited, not by its number of cylinders, but primarily by:
1) the inertial mass of the engine’s piston and rod assemblies,
2) the mass of each valve and by the design of the valve actuating mechanism, and
3) the length of the piston stroke and the resultant piston speed in the bore.
1. The Inertial Mass Of The Engine’s Piston And Rod Assemblies
The maximum rpm of any engine is dependent on the mass of each piston. For in a reciprocating engine each piston must be accelerated first to top dead center and bottom dead center (the bottom of the piston’s travel in the bore), stopped and reversed on each intake, compression, power, and exhaust stroke.
So a larger piston with more mass is burdened with more inertia, and that necessarily limits the rotational speeds of the crankshaft. Therefore, the engine with more cylinders, in this case 8 cylinder engines instead of 4, will have a higher potential maximum rpm as each of its individual pistons will weigh less and have less inertial mass.
2. The Length Of Piston Stroke And The Resultant Piston Speed In The Bore
The length of the piston stroke also affects the production of rpm. An over-square engine (one whose bore is larger than its stroke) will be able to operate at a potentially higher maximum rpm than an engine of identical displacement that is under-square. The reason behind this is: on the long stroke engine each piston travels a farther distance in its bore for every rotation of the crankshaft.
For example, an engine can produce 455 of horsepower with 6,000 rpm where the maximum torque remains 455. The calculations for horsepower and torque can remain the same with a much lower RPM such as 2000. It is possible when you drive the vehicle in top gear. This way, regardless of the cylinders, you can produce different RPM each time.
The massive torque can produce massive RMP too, and there is nothing related to the cylinders of the engine as we mentioned above. The cylinders do not limit a driver to take advantage of the lower or higher RPM; it all depends on the need of the moment.
The Truth Behind Do 8 Cylinder Engines Produce Lower RPM than 4 Cylinder Engines
If talking in depth, the mass of each piston that is in the system of your car determines the RPM. Each piston has to fulfill its task, such as acceleration to the top dead center and then to the bottom dead center. It has to be stopped, reversed, and compressed. This way, the larger the piston is, the more inertia it faces, and this limits the RPM. Thus, to determine the RPM in a vehicle, you should go for the pistons’ mass rather than the cylinders.
Similarly, another factor is the length of the piston’s stroke. A piston having a larger bore than the stroke operates at higher RPM than a piston whose stroke is larger than its bore.
Four cylinder engines seem to operate at generally higher rpm’s because they are generally small displacement engines that have small pistons with low inertial mass. Moreover, lacking the displacement to generate much torque, these small four cylinder engines must compensate by using high rpm to generate power. On the other hand, eight cylinder engines produce relatively higher RPMs.
As per the discussion, you can notice, it has nothing to do with the number of cylinders. It is the other factors such as the piston and the length of the piston’s stroke that decide if it is going to be a higher RPM or a lower one. Modifying these aspects in a vehicle can result in altered RPMs regardless of the 8 cylinder engines.
However, those who want to focus on the smoothness of the ride and power of the engine should focus on the number of cylinders. An eight cylinder engines works wonders as a universal concept when you are confused among six, ten, or twelve cylinders.