There are a few key factors that result in the average motorcycle having such a high rev limit compared to an average automobile. It is important to understand these aspects to choose the correct motorcycle for your needs and wants as a motorcyclist.
The reason why a motorcycle engine can rev so high is that the engine is typically oversquare with a short stroke. For every full revolution of the crankshaft, the piston has less distance to travel resulting in higher RPMs. Motorcycles weigh significantly less than cars and therefore require less torque.
Follow along below to get more information about motorcycles and their engine characteristics.
Before you read any further, this is an applicable video to put you in the right mood.
How Motorcycle Engines Rev So High (Technical Deep Dive)
A typical motorcycle engine, similar to a car engine, consists of (at minimum) a crankshaft, piston(s), and connecting rod(s) to bridge the gap. Some engines only have one cylinder (piston / connecting rod pairing). The piston mounts to the crankshaft using the connecting rod. When the engine is running, the crankshaft is rotating and the pistons are moving up and down.
The stroke of the engine is calculated by subtracting the height of the piston at top dead center from the height of the piston at bottom dead center. The connecting rod attaches the piston to the crankshaft at two fixed points, so as the crankshaft makes one full revolution, the piston will travel to a high point (TDC) and a low point (BDC).
For a standard four-cycle internal combustion engine, the engine first intakes air then compresses the air-fuel mixture in the combustion chamber as the piston rises toward the cylinder head, then the spark plug ignites the air-fuel mixture, then power is produced when the explosion forces the piston back down the cylinder.
In a motorcycle engine, typically the pistons are smaller and the stroke is shorter than a car engine. As a result, the piston has a shorter physical distance to travel vertically inside the cylinder and therefore the cycle time is reduced resulting in higher RPMs.
Motorcycles are typically equipped with short-stroke or oversquare engines which means the bore of the cylinder is larger than the length of the stroke.
Why Motorcycles Typically Rev Higher Than Cars (But Not Always)
The purpose of a high-revving engine in motorcycles goes far beyond making a high-pitched noise. Before I ever owned a motorcycle, I always wondered why they sounded so much different than cars when they sped down the road.
The average motorcycle is far lighter than the average car and therefore does not require as much torque to get up to speed. Instead, in exchange for torque, a motorcycle engine can produce much more horsepower for its weight.
Motorcycles have a significantly higher power-to-weight ratio, meaning they make more horsepower for every pound of weight. For example:
|2021 Ducati Superleggera V4||2021 Lamborghini Huracan STO|
|224 HP||630 HP|
|350 pounds||3,172 pounds|
Based on these figures, the motorcycle power-to-weight ratio is .64 compared to .20 for the car, which is over 3 times greater.
The tradeoff, however, arises when measuring the torque figures. Car engines typically have smaller pistons with smaller bore diameters than motorcycles and can make more torque at lower RPMs because of the longer stroke length. As a comparison, if using the same vehicles in the paragraph above as examples, the motorcycle only makes 85.6 ft/lbs of torque versus the car at 416 ft/lbs.
Ultimately, both vehicles are extremely fast, just in their own ways.
As with most ideas, there are exceptions to the rule. Not all motorcycles are made to deliver big horsepower for crowd-pleasing rev bounces. The popular Honda Gold Wing motorcycle makes an astounding 109 ft/lbs of torque at only 1,210 RPM and 125 HP at 5,500 RPM.
This bike can achieve these numbers due to the “square” nature of the engine, meaning the bore diameter and stroke length are equal. The reason for this change from the standard sportbike engine is the increased weight of a big cruiser motorcycle like the Gold Wing. Its curb weight is 836 lbs which is over double the race bike listed previously.
However, this weight increase does serve a purpose in achieving more stability when riding down the highway. The increased torque also allows for a passenger and some cargo to come along on long road trips where comfort is more important than speed.
Consequences Of Revving A Motorcycle Too High
One fact underpinning this idea of high-revving motorcycle engines is that as the RPMs increase, more power is produced. On a standard motorcycle spec sheet, peak horsepower will be listed with an RPM figure attached which is typically near if not in the redline zone.
See above for the 2022 BMW S1000RR. Peak output of 205 HP is achieved at 13,000 RPM with the rev limiter set at 14,600 from the factory. This limiter is installed by design and allows for a margin of error to ensure the longevity of the machine while still accessing plenty of power.
There is a dichotomy when discussing motorcycle operation at higher-end rev ranges for prolonged periods of time. Redline is where the power is and naturally where fun is to be had. Simultaneously,
However, it must be noted that motorcycle manufacturers are aware that patrons will be partaking in aggressive riding techniques and have crafted these machines with parts that can withstand quite a beating. The materials used inside modern engines can handle excessive heat and strain which allows for plenty of irresponsible joy rides.
It is important to understand that while the limiter is in place to protect the engine, it is not advised to relentlessly bang an unloaded engine off the rev limiter or wind out gears beyond redline. When the rev limiter engages, either fuel or spark are cut off resulting in a loss of power. Therefore, it is advised to shift within the redline to access maximum power while avoiding a sudden cutoff.
Over-revving a motorcycle either at idle or while riding can result in two negative consequences. Sudden loss of fuel from engaging the rev limiter can cause the engine to run hot from a lean running condition. Cutting the spark will allow some raw fuel to enter the exhaust which can damage the catalytic converter
When And Where To Rev Up A Motorcycle
Motorcycle owners and enthusiasts enjoy their machines in different ways. Some folks are more laid back and like to cruise, while others like to feel the need for speed and want to rev their engines to the moon. There is an appropriate time and place for the latter type of behavior.
The best place to rev up a motorcycle and push the limits is on a closed course. The best time to ride aggressively and hit the rev limiter is when there is good visibility during a clear day.
If you cannot resist the urge to find the limit and bounce off of the rev limiter, sign up for a track day where you will have access to a properly prepared closed course where you can safely enjoy the full capabilities of your motorcycle.
In this setting, you will be surrounded by like-minded riders and not have to worry about the dangers of other vehicles or outside influences typically encountered on public roads. There is also zero chance of being pulled over by a police officer where you might be charged with a related infraction.
It is not advised to partake in aggressive motorcycle riding in the high rev ranges on public roads. Performing crowd-pleasing maneuvers like riding at high speeds or doing burnouts can result in costly tickets or crashing.
Choosing The Best Motorcycle Engine For Your Riding Style
By this point, you have an above-average understanding of engine operation and the ideas that contribute to the design of different vehicles. This will allow you to make an educated decision when selecting the right motorcycle for your needs and wants.
An oversquare engine allows for access to five-figure RPM ranges to ensure maximum horsepower and speed. A motorcycle with this engine configuration will achieve high top speeds but compromise torque at lower RPM’s. This would not be ideal for carrying a passenger or carrying any cargo.
A longer stroke will reduce the peak horsepower that can be achieved but will allow for more low-end torque to accommodate for increased weight in the form of a passenger or luggage. A motorcycle with this engine type would be a good choice for long road trips.