A straight pipe will not hurt anything. It is a myth that a straight pipe, or an exhaust with no muffler, hurts the engine’s performance. However, if your pipes are laid out poorly, it could decrease performance or even cause physical damage to components on your motorcycle. This is typically caused by people who try to cut corners and go cheap when building their exhaust systems.
The answer to this question depends on your system design. I will outline the different types of systems and how they work. There are two main categories when dealing with exhaust systems: positive pressure and negative pressure.
A positive pressure system creates that same type of airflow you would get if you stuck your head out of the window of a moving car or truck (this would be considered “positive” because there is more air coming in than going out). An example of this type of system would be stock OEM exhausts, which use tuned headers to create low-pressure zones at specific points along the pipe, reducing back pressure while increasing vectoring of flow.
Back Pressure
If you do not have enough bends in your system, then there is nothing for the sound waves to dissipate, causing “back pressure,” which restricts how efficiently your motor can suck in air and expel gasses.
The more back pressure created by running larger diameter piping has also been found to reduce horsepower because there is less room for the exhaust gas to expand and move, which slows down the cycle of each expansion stroke.
Turbulence
Another issue with straight pipes is that the open environment creates mass turbulence in their immediate path. This robs your engine of kinetic energy, which is directly responsible for powering your bike forward.
It also increases the amount of heat generated by forcing your motor to work harder to push through all that extra air and gas resistance. While this doesn’t harm components like a broken rod, it will decrease performance and create more strain on some parts than others, depending on where you set up shop on your rev range or throttle position.
For instance: A right-hand bend coming off the header pipe works by using the pressure differential between inside and outside of the turn (Bernoulli Effect) to actually “pull” the bike forward, giving it a good boost of speed and acceleration through that one spot. That same area might also be used as a “stall point” (no pressure differential between inside and outside of the bend), where the engine is stopped for up to 1-2 seconds before momentum takes over and moves on down the track.
On the contrary: A left-hand bend will require more energy from your motor because there is no positive pressure differential on either side pushing you forward. More work equals more strain, which could equate to component failure if continued consistently over a long period.
Negative pressure system “rings.”
A positive wave consists of all high-pressure with no low-pressure area to go with it. Hence, as it moves from one end of the line to the other, nothing is pulling along the exhaust gas behind it, and this causes turbulence which will reduce overall power gains. If you have ever heard people talk about how good their “pulse” technology is on their pipes, they refer to this effect.
Positive pressure waves move very quickly (much higher velocity) throughout much of the pipe’s length due to high-pressure zones in them. Still, around your engine’s rpm range, they can become unstable because low-pressure zones will start to appear in them with a cheerful wave, which can cause noise and even damage if not tuned correctly.
Harmful pressure waves tend to lose velocity the further they travel from the collector (decreased overall pressure). Still, their design maintains consistent low-pressure throughout their length with no turbulence or backflow from high-pressure areas behind it.
They also have much lower velocities than positive pressure waves, so there is less chance of them becoming unstable. Harmful waves are what you want in a design for a performance pipe, and this is why most tuners who know what they are doing will use a negative pressure system when designing an exhaust system.
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