How Do Exhaust Headers Work to Improve Performance?
When you’re new to racing, you’ll look for performance boosts anywhere you can get them, and you can usually find them pretty quickly. One place to start to improve your sprint car performance is adding exhaust headers.
Engine Mechanics Basics
An engine ignites gasoline to create a controlled explosion, which then powers the car into motion. The gas is then injected into a small, enclosed space and ignited, and the energy created is released in the form of expanding gas. The ignition of gasoline is why a car engine is known as an internal combustion engine.
Gasoline engines use a four-stroke combustion cycle to convert gasoline into motion. The four strokes include:
- Intake stroke, wherein the air and gasoline are put together into a mixture and injected into the piston.
- Compression stroke, wherein the gasoline and air mixture is under pressure. Compressing the mixture makes the explosion more powerful.
- Combustion stroke, wherein the spark plug ignites the compressed fuel and air mixture, explodes, and forces the piston down.
- At the bottom of the stroke, the exhaust valve opens to expel exhaust through the tailpipe. Then the engine is ready for another intake.
The piston strokes move up and down in a linear fashion. A crankshaft turns that linear motion into a circular motion, which then transfers that rotational motion to the wheels.
An exhaust header is a bolt-on accessory that makes it easier to push exhaust gases out of the cylinders, improving the engine’s performance. The power stroke is the only stroke that generates power (the other strokes drain energy), so an improvement in their performance can make the engine more efficient.
During the exhaust stroke, an engine can lose power through backpressure. While the piston pushes the exhaust fumes, it wastes energy if it faces any resistance in this motion. An additional exhaust valve can increase the area in which exhaust gasses exit and improve the flow.
After exiting the cylinder, exhaust gasses end up in the exhaust manifold. A four or eight-cylinder engine will have four cylinders using the same manifold. If exhaust gasses from a single-cylinder build up in a manifold, it can affect other cylinders using the same manifold; this is called manifold backpressure. Exhaust headers eliminate the manifold’s back pressure giving each cylinder its exhaust pipe rather than sharing.
Backpressure is an opposing force that acts on a gas, attempting to move through a confined space. Backpressure generated in an exhaust system drains the engine’s power and reduces its efficiency.
Ideally, the exhaust gasses flow out at optimum volumetric efficiency with minimum combustion products remaining in the cylinder. Your exhaust system needs to be at maximum pressure differential and minimal backpressure to gain optimal volumetric efficiency.
High backpressure can cause poor scavenging, the release of the exhaust from the cylinder while it takes in a fresh intake fuel mixture. A lower volumetric efficiency can reduce fuel and air intake, which would result in less power. Additionally, leaving some products from combustion can negatively affect subsequent combustions.
Backpressure can vary based on the size of the exhaust pipe. If the exhaust pipe is small, then the exhaust gases will have a narrow path out, forcing pressure to build up within the exhaust system, causing exhaust flow to exit at high velocity. The narrow pathway can also cause backpressure as the exhaust will have nowhere to go; after the path forward is at capacity, the exhaust will attempt to occupy space behind it.
On the other hand, a large exhaust pipe will give the exhaust gasses too much leeway and reduce the pressure they need to exit, leading to low exit velocity exhaust flow. Remember, what is ideal is maximum pressure differential and minimal backpressure to gain optimal volumetric efficiency.
So, what you need is a balance between a small and large exhaust pipe. You want it to be large enough to create an efficient high-velocity exhaust flow without causing backpressure. You can also use other exhaust system elements, like exhaust pipe diameters, to create negative pressure waves that travel back to the emptying cylinder.
Exhaust heads are a simple and straightforward method to increase your car’s power and efficiency. There is also a more subtle art to balancing your backpressure to improve your exhaust flow efficiency after achieving the basics. Experiment with the size and diameter of your exhaust pipes to squeeze out more power.