Getting your exhaust system right
by Adian Yein Khalid

I might sound show-off-ish by saying this, but a lot of what you hear from our local mechanics is crap. Serious. There is no such thing as an engine requiring a bit of backpressure to make the most power, and certainly no such thing as an Automatic car having different needs to a Manual car.
Often, you should use your own deductions and logic when they mention techniques that sound dubious and illogical. Generally I can assume that we are fairly educated individuals, and therefore expect that you should be able to assimilate the arguments that I bring forth from time to time on car modifications.

Gaining more power

An engine is an engine is an engine, and to gain more power you need it to breathe better and get rid of exhaust gases faster (these increase volumetric efficiency), or make the engine more efficient (lessen internal friction etc.). One can also try and minimise release of thermal energy (heat) but to do so would require severe engineering skills.

A simple example: Stuff a pillow over your mouth and nose and try breathing, in and out. This is similar to what the engine experiences on both its intake (restrictive air filter) and exhaust (restrictive exhaust system). We use straight-flow mufflers and large diameter pipes in order to make the path of the exhaust gases as easy to navigate to freedom as possible, and we use airfilters with better flow to allow more air into the engine.

The very first belief that must be redefined is the "a bit of backpressure is good for the engine" mythology.

Backpressure

What is backpressure?

Pressure can exist in two forms, positive and negative. In it's negative state, we usually call it a vacuum although a vacuum is technically a state of no atmosphere at all. Anyway, for this concept to be understood, we need to think of air (exhaust gases) as a moving object with mass. You see, just like water, a fixed volume of gas moving through a pipe of smaller diameter will have a higher velocity than the same volume of gas moving through a pipe with a larger diameter. Why this is important to exhaust tuners is because of a thing called inertia. Moving objects tend to stay in motion. Therefore, if exhaust gasses are moving at high speeds out of the exhaust pipe, they will tend to keep moving out. If they are moving fast enough (higher velocity), the inertia can be enough to create a negative pressure in the exhaust pipe. This is back pressure. Contrary to popular belief, back pressure is not a positive pressure in the exhaust
system, it's a negative pressure.

Positive backpressure on the other hand is a build-up of gases and pressure that are trying to escape the tight confines of the exhaust pipe. Backpressure means that some of the engine's energy is taken-up just having to "push" these exhaust gases out of the combustion chamber during the exhaust cycle of the 4-stroke internal combustion cycle.

Your next argument will be, if we want the gases to exit the system as fast as possible then why use silencers and pipes at all? Well, simply because an open exhaust would be too bloody noisy, and because there are some beneficial effects that the pipe itself contributes via "shockwave tuning", which is an argument in itself.

Shockwave tuning

Exhaust shockwave tuning, as it is called, only really really makes sense when you run without mufflers and silencers. At the bare minimum, use only a single straight-thru muffler of identical diameter to the existing pipe. The soundwave sees any sudden increase in atmospheric area as a "shock", hence the name, and what happens then is that some of the sound wave then bounces back along the pipe and back into the combustion chamber. An easy assumption to shockwave tuning would be: the shorter the pipe length (or placement of the silencer closer to the engine) relates to a sound wave that take less time to travel down the pipe and back up again. Therefore, generally, you'll get your shockwave tuning HP boost higher up the rev range, as the engine has shorter time intervals between each combustion (ignition), and the fast returning exhaust gases will be able to help "extract" some of the outgoing exhaust and pull-in the incoming air-fuel charge.

This is the real reason why our race cars run what we call "straight-flow" pipes. However, in our road cars we can see these gains slightly by good placement of the centre muffler. Which is why you should pay close attention to where the manufacturer places the first muffler (or "expansion chamber", as it literally that without any glass-fibre sound-deadening). Remember, the manufacturer has spend millions testing this stuff!

Shockwave tuning only works for a particular RPM band and that's it.
Why? Basically, for a given pipe length, the time it will take the shockwave to travel back up the pipe is the same everytime. Therefore, the exact moment when it returns to the combustion chamber to help gas extraction and charge inhalation will be only equal to a certain duration and timing of the exhaust valves and intake valve overlap at an exact point in time, and no other.

F1 and SuperBikes are different.
Why? They rev much higher than our cars do, and the characteristics of their exhaust shockwave a very different to our road engines. My old ZX6R-F2 pulled to 14500rpm. Crazy! And guess what, it ran 4-1 headers. Why?
Numerous tests (I think even Dave Walker once tried) have shown that 4-1 headers work for powerbands above 6000rpm, and their effect isn't just like 2 or 3 HP in a 200 HP engine with race cams and the works, but more like 15-20HP, often with some HP loss at the bottom-end. So, for race cars and engines that rev their bollocks off, 4-1 works.
But what I am trying to demonstrate here is the great difference exhaust length, shockwave tuning, and manifold design has based on different specifications and requirements of an engine.