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Motorcycle Exhausts Explained Scientifically

Whether you’re new to buying an aftermarket exhaust, or you’re an old hand seeking to enhance your bike with the very finest in sex-cannons, you’ll agree there’s a lot of high-end science behind the creation of an SC-Project exhaust system. Good thing there’s someone to explain it all to you…


In the words of that great New Mexican philosopher, Jesse Pinkman: “Science, bitches!” It governs every aspect of our lives – and especially the most crucial aspect of those lives, motorcycle exhausts.

But science can be confusing. Few of us are scientists, so that confusion is understandable. Such is the nature of ‘science”. We have lawyers to explain laws to us, so we need scientists to explain science to us.

Terms like “negative wave reflection”, “header”, flange”, “vortex”, and “titanium” can cause eyes to glaze and indigestion to occur in sensitive tummies.

Relax. I got this. You will now be able to shop for your next SC-Project exhaust system with confidence, cloaked in knowledge – and we all know knowledge is power. And power is the best cloak there is.


This is the part of the exhaust system which bolts to your engine. They are named after what happens to you if they fall off, ie. You will do a header over the handlebars and hit the road with your face. Specifically, and coincidentally, the headers bolt to the part of your engine called the “head”. Science is full of such coincidences, which is an effect known as “kismet”.

Intriguingly, you will have as many headers as you have engine cylinders. It’s an easy way to remember how many cylinders you have in case you’re asked. Count the number of headers. Be at ease. You will not ever have to count higher than six. Unless you buy an eight-cylinder motorcycle. Then the number of the counting will be eight.



First discovered in 1791 by William Gregor, all Titanium comes originally from the Titan, which is Saturn’s biggest moon. How it got to earth, and how it is found in all living things, is written in the Bible, specifically in the Book of Titus (which is what the early Christians called Titanium). In Titus 1:15, the purity of Titanium is discussed: “To the pure, all things are pure, like Titanium, which is beautifully pure, and light, and is excellent in motorcycle exhaust systems. Amen.”

Titanium has a magnificent strength-to-weight ratio, like a Ducati Panigale made from carbon fibre. It is shiny, and as resistant to corrosion as both Platinum and Kryptonite. It’s crap at conducting electricity, so they don’t use it in batteries, and it will melt at 1650 Celcius, so they don’t sell it in Darwin.


This is the technical term for the fat, rounded bit where the beautiful noise comes out. It’s commonly agreed that SC-Project makes the sexiest cans. They have been dubbed “sex-cannons” for obvious reasons. The mere sight of them causes arousal in both males and females. That arousal is exponentially heightened when the cans are in use. And, predictably, the higher you rev your bike, the higher the arousal level. It’s like sorcery, but it’s actually science. So be aware and only use this power for good. If you use it for evil, on your head be it, you sinner, you.



NFR occurs naturally in only two places in the universe. The first is when you look at yourself in the mirror after a big night out, and see your face all covered in stripper body-glitter, your wallet is empty, your girlfriend has left you, and you smell like a chimpanzee.

The second is when the exhaust valves (small metallic clicky things) in your engine begin to open and send a pulse of exhaust pressure into that cylinder’s header, and that pulse then travels along the header until it reaches a wider point (usually where the sex-cannon joins the header), and voila! The NFR begins and sends itself back into the engine! It’s very exciting.

And this is where the length of your header becomes very important. I know you were thinking that length is not important. It is. In everything. Jail-time, beard, marriage, beer… everything. So the length of your header is designed to make sure that NFR arrives in the engine just as the valves overlap. It’s that wonderful instant when the exhaust vales have not yet closed, and the intakes have just begun to open like flowers in the sun. Then more science stuff happens, which has no bearing on your enjoyment of the resultant symphony that occurs. I’m only telling you this so you don’t take a hacksaw to your headers because you think they’re too long.


A vortex is a rapidly spinning accumulation of something. Air, water, jelly, cats, and of course, exhaust gasses, which is what we’re dealing with here. Exhaust vortexes occur rarely, but most commonly when you’re pulling a four-gear minger and being showered in female undergarments. The vortex occurs when the exhaust gasses exit your sex-cannon and hit the road instead of the air.

They hit the road because your front wheel is pointing at the sky and your exhaust pipe is pointing at the road. And this is happening when you’re progressing in a forward direction at about 180km/h.

This confluence of vectors (directional things) causes a swirling effect to take place upon the exhaust gasses. If your bike is running too rich, or needs new rings, and blowing smoke, you’ll be able to see this swirling effect. It is very beautiful, so take a minute to enjoy it.


A wondrous metal sculpture that looks a bit like an eye. It is normally used to secure the header to the engine. The very best ones are made from titanium or stainless steel. But I have seen them made from old beer cans, fencing wire, and lead fishing sinkers beaten flat with a hammer. The smart money tends to steer towards the stainless and titanium jobbies, but if your headers have fallen off 200km west of Coober Pedy, you’re gonna do what you’re gonna do.


These are important flat things, which are intended to sit between the exhaust flange and the engine hole where the header enters, like a bull into a cow. Their purpose is to ensure the exhaust gasses continue their exciting journey along the header, rather than leak into the atmosphere where the header joins the head.

These gaskets can be made of steel, or some high-temperature fibre material, like Weet Bix. They can also be made from graphite, but you will need a lot of pencils and some good glue, like Tarzan’s Grip. And, happily, you can make them from ceramic composites, which you can get after you smash your toilet bowl up. Some gaskets are made from a combination of these materials, so you can be inventive.


This is the scientific term for just how much of stuff is coming out of your sex-cannon. The uncultured call that stuff “noise”, the cultured call it “a glorious symphony of orchestral majesty”. The term “decibel” is used to measure how much of that is happening.

In Australia, the legal decibel reading for a bike is 96Db. Which is as loud as the noise your girlfriend makes when you come home smashed off your face. So it’s quite acceptable.

Purists, tattooed men, gunfighters, and most motorcyclists feel that anything under 180Db is soul-crushing, but are forced to comply with state regulations due to threats of savage violence from state organs. Which is worrying on several levels. Clearly, the louder a beautiful sound is, the more beautiful that sound is. Bruce Springsteen sounds best when played through concert speakers six stories high and hooked up to a nuclear reactor. Everyone knows that.

It therefore follows that the more decibels your beautiful-sounding motorcycle makes, the more people will derive joy and happiness from it. If only the government would see this, the world would be a much nicer place.

Author Boris Mihailovic

Author: Boris Mihailovic

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Books published by Hachette: My Mother Warned Me About Blokes Like Me and At The Altar Of The Road Gods.

Book published by Shock & Awe Publishing: The Wisdom Of The Road Gods.

The Wisdom of the Road Gods