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Difference between short and long tube headers?


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I know this is probably a simple question to answer, but I can't wrap my head around the difference between long and short tube headers. I'm trying to figure what to buy, but I can't figure out what the difference between the two is. Any help will be greatly appreciated because I don't have even have the slightest clue. :headscratch:

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I pulled this explanation down from another forum. Good explanation of the difference. Hope this helps.

 

Headers improve performance by reducing exhaust backpressure and improving cylinder "scavenging".

 

The advantages of reduced Backpressure should be obvious.

 

Scavenging is a process that takes advantage of overlap between the exhaust and intake valve timings to suck a little more exhaust out of the cylinder before the exhaust valve has to close (both valves are actually open for a little while, at the same time, as the piston crosses TDC at the end of the exhaust stroke to the start of the intake stroke).

 

To help reduce backpressure (aside from the obvious tube diameter) "Equal length" tube Headers are designed so that the time it takes any exhaust pulse to move from it's cylinder to the Collector (the point where individual cylinder's exhaust pipes combine into a single pipe) is such that any one pulse won't "bang in to" one from another cylinder at the "merge" point.

 

Also, whenever a wave (be it in water, gas, or electromagnetic) sees a change in "impedance" (the resistance of the transmission line to the propagation of the wave), the wave generates reflections back toward it's source (it doesn't have to be a low-to-high resistance change, it can be a high-to-low change as well). Those reflections collide with new waves as they propagate from the source, with obvious detrimental affect.

 

The change in diameter of the pipes in an exhaust system can produce this effect. Headers are designed to minimize this.

 

There's another feature, too. Exhaust pulses have a "profile", a characteristic curve that describes the change in gas pressure and velocity that occurs as the exhaust valve opens and the piston progresses through the exhaust stroke. Headers are "tuned" to that profile.

 

Exhaust pulses behave like a "waves". Waves have some unique properties, one of which is that they like to propagate. Once initiated, they can go for a really long time without any additional input of energy (Ocean waves can propagate for many 1000s of miles, lightwaves can propagate nearly for ever, tuning forks can ring for very long times). Friction (water against itself, the ocean floor and the surrounding air, lightwaves hitting dust and gas in space, tuning fork against air, etc...) can reduce the amplitude of the wave, but not the frequency (the profile) of the wave.

 

When a pipe is properly tuned, so that the exhaust pulses resonate (constructively reinforce each other) the pressure gradient (the wave shape) can actually be used to develop a slight vacuum at the exhaust port (think of what happens to you when you're floating in the ocean and a passing wave moves you along). The pipe is tuned so that the low-amplitude phase of the wave occurs at the Port, and that reduced pressure improves scavenging.

 

Shorty Headers don't "tune" as well as long tube Headers because the frequency of the exhaust pulses are low, and low frequencies prefer longer "transmission lines" (tube length). Just like with wind operated musical instruments such as trumpets and tubas, the longer the length of the pipe, the lower the resonant frequency of the notes the instrument will emit.

 

You get better power gains from a long tube Header because they're more optimally tuned for your exhaust pulse "profile".

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