Once I looked out over the mouth of San Francisco Bay with the eyes of a surfer. As the tide fell millions of gallons of water drained into the sea at the narrows below the Golden Gate. Yet, against this tide huge ocean waves rolled in from the deep against the flow of water.
My wife spreads the white top sheet over our bed. Standing at the foot she grasps end and with a deft snap sends a wave of motion down the length of fabric. The uncooperative end of the sheet whips itself into alignment and together we spread the coverlet.
There are moments on a motorcycle when an engine seems to come alive. It acts with an uncanny precision and willingness, it seems to almost sing. There is, of course, a reason for this. We speak of tuning an engine and imagine that this speak is figurative, but it often isn't. It's often literal. It turns out that calaculators used to design pipe organs work very well for creating good working four stroke exhaust systems.
Connected to an engine there is an exhaust and an intake. These are not just tubes routing fuel and gasses to and from the engine, they can perform useful work.A four stroke engine turning 5000 RPM opens its exhaust valve 41 times per second. Every time this happens a "slug" of air moves out of the engine and into the exhaust header. Once the valve is closed the "slug" of exhaust gasses keeps moving but since it's in a sealed pipe, the exhaust header, it creates a partial vacuum behind it.
In addition to the physical movement of gas described above pressure waves move up and down the exhaust pipe much, much faster than the physical flow of gasses in the header. When we lit firecrackers as children we felt that pressure wave. It wasn't wind, like the flow of gasses. Never the less, it could do "work" like remove a finger.
In my original example of waves traveling into San Fransisco bay against the tide I attempted to demonstrate that waves of energy can flow against current in the media that carries them. In the case of San Francisco bay both forces are powerful and move in opposite directions largely irrespective of one another. In the case of Priscilla spreading out fresh linen I wanted to show that a wave can move though media and release it's energy (flipping the end of the sheet into the desired location) once it "crashes" on the shore.
Exhausts are tuned by changing the length and diameter of exhaust systems and how they join together. The powerful pressure wave that escapes the exhaust port when it opens races down the pipe and is reflected back as a negative wave everywhere the pipe expands.
If things are timed right the negative pressure wave arrives just as the port is opening and sucks the spent gasses right out of the combustion chamber. If it's at the magical moment when the intake valves are open it can also suck clean charge in from the air box as well. Conversely there must always be a positive wave as well. That means that at some other RPM or throttle position the opposite is happening. A positive wave is arriving that is forcing dirty exhaust into the combustion chamber.
There are people that think that when you add an aftermarket exhaust to an engine that you are "uncorking" it. That, in some sense, is true. When we add a "slip on" exhaust or debaffle a stock muffler we are simply increasing the amplitude of the wave, making it more powerful. In a sense we are making the effect of exhaust tuning "more good" or "more bad". When the positive wave arrives forcing dirty air into the combustion chamber a slip on muffler this happen in a way that makes things much worse instead of simply worse. When the negative waves arrives pulling exhaust helpfully out of the combustion chamber we are making things much better instead of just better.
A well designed exhaust system works with an engine to so that its positive tuning effects happen in often used parts of the rev range or at places where the engine is weak and could use the help. Conversely it "hides" the negative effects behind engine strengths or in seldom used location.
I can describe the characteristics of exhaust tuning with some accuracy but so what? It can all seem quite clinical or logical or exploitable; like we can own it in some way. I feel a divide between knowing and wonder that I want to break down. I want joy and wonder to be part of the equation and support my world view with equal shoulders, even in reason's inner sanctum. It may be weird to understand exhaust scavenging as a wonder, but it is.
When I started to find the answers in bedsheets and ocean waves I began understand this: Something isn't beautiful because it works, it works because it's beautiful.
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