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There are two reasons to replace an exhaust with a non-original one: taste / finery and tuning. In the case of taste, you can be pretty cheap with a small gain on throttle response at low revs and slightly better breathing at higher revs. If you go for tuning, pipe lengths and diameters will change. That costs a lot of money, and will only really yield results if you at least do something on the suctioned side. Consider, for example, a more freely flowing air filter and a different nozzle arrangement or carburettors.
If you go even further, you will install a hotter camshaft, the heads flow and the valves increase, or even increase the displacement. But then financially is usually challenging. Because then there will be extras such as cooling and inhibition.
Spaghetti exhaust manifolds
The 'spaghetti' is the most commonly used name for sports exhaust manifolds and owes its name (with V8s and V12s) to the tangle of pipes. The principle is based on the fact that all exhaust pipes have the same length (tuned lengths) for optimal breathing. As standard, with air-cooled VW blocks, you have the exhaust ports at the heads at cylinders 1 and 3 at the rear through the heat exchanger to the rear silencer. The path at cylinders 2 and 4 goes directly to the damper.
The distance here is therefore different between 1,3 and 2,4. By extending the tubes at cylinders 2 and 4 and making them equal to the length of 1 and 3, you get an equal back pressure for all four cylinders. This leads to better and more even ventilation. The so-called “shockwave effect” ensures that the combustion stroke in one cylinder creates an underpressure at the outlet of the cylinder that is just filling.
Spaghetti fills better
This achieves a better degree of filling of the cylinder. Because you have to lose this extra length somewhere under the car, the pipes are braided in all kinds of bends. Because the distances on a boxer engine are relatively much greater than on a V-engine or in-line engine, the boxer therefore benefits from this compared to other types of engines.
There are already virtuous people who think that church bells, children and windmill blades make too much noise. But it is clear: Undamped driving is outdated. So there has to be muffled.
An absorption damper (a perforated pipe surrounded by a layer of sound-absorbing steel wool, basalt wool or glass wool) works excellently at high speed. This damper absorbs the sharp sounds through the absorption material and you can greatly influence the amount of sound that is left by the diameter of the perforated pipe, the thickness of the layer of absorption material and the length of the damper.
Absorption dampers can become louder over time. The sound-absorbing wool is then burnt away. The better absorption damper has a glass fiber filling. This in combination with a good build-up of the core prevents "blowing". But under extreme load, the filling can burn. Then use a racing damper, which is filled with pure stainless steel wool.
A reflection damper makes the sound waves (partly) fall against each other and mainly removes the deep tones. With reflection dampers it is very important to ensure that no sounds escape through the outside. A reflection damper (has damping partitions) gives more back pressure. But has the most effect at low speed.
A combination of absorption and reflection muffler is often used for sports exhausts. Although there are tuners that prefer a reflection silencer for these exhausts. With reflection dampers it is very important to ensure that no sounds escape through the outside. This is usually done by applying several layers around the outside, sometimes even with insulation in between.
Air-cooled boxers and spaghetti
The cheapest option is to connect a bolt-on spaghetti to the existing stove pots with the same diameter as the exhaust pipe in the stove pots. You can then choose a suitable damper for this according to taste / counterpressure. You are ready for a few hundred euros. You can choose a roaring pipe or a street-legal exhaust with a slightly deeper / darker sound. If you go further, you will end up with an exhaust system where the tube diameter changes. This is calculated on the engine adjustments (displacement, cam lift, so camshaft, carburettors). The stove pots must therefore also be adapted to this diameter, or replaced by so-called j-pipes. A j-pipe is a replacement for the stove pot where only the exhaust pipe is placed. You therefore lose the heating. So that is cheaper. And with the climate change quite defensible. And here in the Netherlands, Hotrod and Paruzzi are the names to remember.
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