Some Header Theory

[DirtyLude]

Some Header Theory

I labeled this as 'Some' theory, because, I don't understand all the theory and fluid dynamics that goes into exhausts. It's very complex and not for the likes of amateurs like me to understand. I do know more stuff, like anti reversion, inertial sloshing... but this is going to be a freakin' novel all ready. I'm avoiding all math in this, but there is alot of math here, and I hate math.



Finite-Amplitude Pressure Waves
Pressure waves are generated both in the exhaust and to a lesser extent in the intake plenum and runners. These pressure waves are called finite-amplitude waves and they are much more powerful than sound waves. Unlike sound waves these waves change, not just by losing energy the farther they go, they are also effected by other pressure waves that will change their shape, and they will change shape without even being effected by other waves. In fact modeling how these waves will move in anything more than the most basic sence is extremely complex. Even the most advanced modeling software is not 100% accurate and is usually backed up with multiple sample products to test. These amplitude waves move through the exhaust gas at the speed of sound, rather than along with it. Just like sound waves travel through the air.

Old theory
There's an old theory that some people might have heard where exhaust gasses move in high pressure low pressure packets down the exhaust. The low pressure area behind the last exhaust burst would help pull the next exhaust burst out of the cylinder. This theory is incorrect, and was proven incorrect a long time ago.

A positive compression wave is generated by the exhaust gas comeing into the header tube. It travels down the header primary tube until it reaches an opening. A compression wave will help push the gas particles along with it, towards the back of the car. Once it hits an opening a negative expansion wave is generated moving back towards the exhaust valve. Expansion waves pull the exhaust gas, also helping it move towards the back of the car.

The trick is to time the returning negative pressure wave with the valve overlap point where both intake and exhaust valves are open. The negative pressure wave will deliver a low pressure point that will help the intake charge move in. This is called scavenging.

Tuning The Header

There's 2 main things that effect the tuning of pressure waves. The header tubing diameter and the length.

Diameter: Pressure waves are directly related to the energy in the force of the exhaust. If you increase the header diameter, you decrease the speed of the exhaust, the compressed energy, and the strength of the pressure wave and it's scavinging effect. Diameter is a compromise between generating strong pressure waves and decreasing restriction.

Length: The length of the header tubing directly effects when the negative pressure wave will return and hit the exhaust valve. Remember that the pressure waves move at the speed of sound, so they will generally return in a fixed length of time, but the engine speed changes. So, there will be a peak RPM point where the pressure wave returns at exactly the right time, and at engine speeds above and below that, the timing will not be optimal. The longer the tube, the longer it will take for the returning wave, and the optimal RPM will be lower.

The size of the collector opening will determine the strength and duration of the returning pressure wave. I'm not going to give you the math, but for example if you had a 4 into 1 header, the opening from one of the header tube into the collector would be hitting a 1:4 increase in diameter. This would return a reflected negative pressure wave with 0.6x the intensity of the initial wave, and the compression wave would still continue on at 0.8x it's initial intensity.

4-1 Header vs Tri-Y 4-2-1 Headers
A 4 into 1 header is usually considered a peaky race header because, it's tuned to a single optimal RPM range. The expansion wave hits the one and only collector and a negative wave returns, taking a fixed amount of time, and creating only a single optimal RPM point.

4-2-1 headers are more for flatter power. The waves come back with less intesity, because this time there are 2 collectors that the exhaust has to pass through (There's 3 collectors in total, but exhaust gas from 1 header tube only passes through 2 of those.) That means that there are 2 returning negative pressure waves, 1 generated by each collector, and the each hit the exhaust port at different times. This gives less peak power, because the returning waves are less intense, but better range, since there are now 2 spaced out waves.

This get much more complex when you realize that the returning negative pressure wave, when it hits the exhaust valve will generate, yet another wave going back out the exhaust, and back and forth, causing general chaos, but to a lesser extent than the main returning wave.

Well that's it. Hopefully someone managed to follow it, or get something out of it. I think next will be a general ECU overview, since I'm currently working on my own, trying to fix the damn thing.

[getbig]

Sweet post

[Arjun]

Great stuff. Some one sticky this post.

[Overdrive]

Well done Mark......That was great

[Silent_Sam]

Originally Posted by Overdrive

Well done Mark......That was great

You ... stop whoring and get to bed !

[Overdrive]

lol.......who me?

It was a good read 3 wheel

[prelude_vtec99]

is there a coles notes version in everyday english?

[Kri$han]

Coles notes:

- Exhaust gases move kind of like pressure waves through the header.
- During the exhaust stroke, a positive pressure is created at the valve, and thus during the other three a negative pressure results.
- The positive pressure wave will reflect backwards (back to the engine) when it reaches the collector, which also has an effect onthe strength of the negative pressure at the valve.
- This negative pressure is needed to create suction to draw air in for the intake, known as scavenging.
- Lower the pressure (higher suction) = More power.
- Having a 4-1 header creates a larger suction, but since there's only 1 collector, peak power is achieved at one RPM based on the collector's location.
- A 4-2-1 header will have two reflection points which can be tuned for two local peaks, and power can be achieved at two points in the RPM band.

sorry that was still pretty long