What's in an exhaust?
A look into how technical a piece of pipe can be.
When viewing any car forum, you can put a safe bet on finding a thread or twenty on what exhaust system design is the best for any suited application.
Sometimes there are many conflicting idea's and theories put behind an individuals chosen answer.
So what actually goes into the design of an exhaust system?
What is the aim of an aftermarket exhaust system?
What should you avoid and why?
What is the main objective of an exhaust system?
To start with, lets look at the most basic fundamentals of what the exhaust system does.
When people talk about the exhaust system on a car, they generally only refer to the pipe work running from the motor it's self. However to fully understand what's best for your application you need to look at where it all begins.
After the engine has done its job, and the combustion mixture has been spent it needs somewhere to go, the cam shaft opens the exhaust valve and the spent charge leaves the combustion chamber, from here there is a number of things it will do. Primarily it will cool, rapidly. But secondly it will come into contact with objects and obstacle it must over come to exit the car and out into the atmosphere.
After leaving the combustion chamber the exhaust charge is pulled through the exhaust port and into the exhaust manifold.
These are the exhaust ports on the right hand head.
This is the Subaru single turbo exhaust manifold. Obviously excluding the up pipe.
Once the exhaust gasses have entered the exhaust manifold they are guided towards the turbine wheel of the turbo charger. Here the hot exhaust gas enters the exhaust housing of the turbo and turns the turbine wheel, this then turns the compressor wheel. The exhaust then leaves the turbo via the dump pipe and is then piped out to the rear of the car.
It all seems pretty straight forward yeah?
Well in theory it is. But there are many factors which will determine how efficiently any given exhaust system will do it job.
A good exhaust system will allow for better boost control and lower engine temperatures, while giving you the ability to safely run a higher boost pressure.
In the same note a good exhaust system will give your car the ability to build boost quicker and give you more mid range torque, while still gaining top end power.
One of the most popular upgrades to the exhaust system made by performance enthusiasts is the an aftermarket Dump pipe.
Dump pipes come in many shapes and sizes and many different designs. Lets explore these some more.
The standard dump pipe consists of a flat flange and a 2.25" pipe. Depending on the year and vehicle these some times contain the catalytic converter in them.
This is a standard Subaru single turbo dump pipe.
Standard dump pipes are poorly designed. This can lead to a number of issues when looking to extract performance from your Subaru.
The internal waste gate:
The internal waste gate is a flap pre-turbine that vents the exhaust gasses straight into the exhaust before it can reach the turbine wheel. This is how your car controls the amount of boost pressure that is fed into the engine. When the waste gate actuator opens the waste gate flap to allow exhaust gases to escape, it does so straight into the main flow of exhaust gas leaving the turbine housing. This foreign flow of gas causes turbulence and slows down the exhaust flow from the turbine. The turbulence effectively creates a traffic jam of exhaust gas leading back to the exhaust manifold and right back up to the exhaust valve its self.
Having back pressure on the exhaust valve can cause a number of issues. If the exhaust isn't being removed from the combustion chamber quick enough it can stifle the following combustion strokes and cause undue heat. Heat is the killer of power. It can cause your car to ping (Pre detonation) and oval bores, neither of which are particularly good for your engine.
One simple way to get around this issue, is to use a open mouthed dump pipe.
This is a open mouthed dump pipe.
This style of dump pipe allows the waste gate to vent much more efficiently and the larger diameter of the pipe allows for quicker exhaust flow, the fact that the waste gate still opens into the main flow means that it will still cause turbulence, and there for slow the exhaust flow. This style will still work more efficient than the standard setup but obviously has its flaws.
To counter this issue, you can use the same style dump pipe but have a waste gate splitter fitted.
This is an open mouth dump fitted with a waste gate splitter
The objective of the splitter is to allow the main flow enough time to cool and move far enough away from the turbine housing to allow the entry of the waste gate flow and not cause turbulence and damaging back pressure. With splitter fitted dump pipes it is critical to make sure that the splitter is the correct size.
It needs to create a firm seal on the turbo, but not travel to far into the dump pipe its self.
These designs work great with standard factory turbo's. But once you start dabbling in the dark art of aftermarket turbo chargers, you've entered a whole other world.
I had the choice of making it seperate for the wastegate, but what for.. they're only lil turbines.
