^ Benz sounds like a Fn moped with open pipes! BS.
...
Open exhaust as in straight pipe, not recommended! Two words ... "back pressure"
I don't hear the moped, but now I'm not sure if I like that sound. AMG V8 wagon is what excites me the most in that video.
Back pressure... oh boy. This has resulted in innumerable debates and arguments since the first time someone wanted to hotrod an internal combustion engine. It has started many internet flame wars, which is not my intention. I'm going to put in as much information as I can recall and hope for the best. I'll consider any rebuttals that use logical reasoning. This post is to the best of my knowledge and understanding.
Back pressure is for 2-stroke engines, not 4-strokes. Even in 2-strokes, it's more complicated than "need X amount". 2-strokes need X amount
at the exact moment at which there is new unburnt fuel in the cylinder and the exhaust port is open/exposed. Back pressure at any other time only reduces the rate at which exhaust leaves the cylinder, reducing power by leaving more burned fuel and carbon dioxide in the cylinder, occupying precious volume.
Where does the myth of back pressure being needed come from? Sometimes, running open headers seems to roughen up the engine when you get rid of the crossover in a V-configuration engine due to the lack of scavenging between the banks. Scavenging is when the [relative] low pressure zone between one cylinder's exhaust strokes creates a vacuum force during another cylinder's exhaust stroke. Connecting the banks (the LS uses an H-pipe stock) helps one bank scavenge the other. An X-pipe does the same thing, but flows more smoothly.
But the #1 reason why it seems like back pressure is needed is from going too large with replacement exhaust pipe diameters. The common analogy used to describe different exhaust pipe diameters is breathing through different size straws. Normal straw, Capri-Sun straw, and milkshake straw. It seems obvious that the larger the straw, the easier it is to breathe through. However, there' more to it when it comes to an engine than just straight flow rate based on diameter.
For one thing, the engine is pumping out gas much hotter than the surrounding area. Your O2 sensors operates between 600 and 800 degrees Fahrenheit. Yes, they're
heated O2 sensors, but the heating element is only there to heat them up faster, not heat them above exhaust temperature. And yet, most cars' exhaust is only warm at the exhaust tip. You can stick your hand in the stream and feel uncomfortable at most. That is because exhaust gas cools as it passes through the pipes and, as a result, becomes denser. Now that it's denser, it moves slower. The number of molecules of exhaust exiting at the tip is equal to the number of molecules the engine is pumping in. If you look at the exhaust in "slices" (let's say a 1 inch long slice at the manifold for one cylinder), it will have a certain number of molecules. As it cools and condenses, that number of molecules occupies a thinner slice. Using gas law (pressure*volume = Boltzman's constant * number of gas molecules * temperature), if the gas cools 50%, the predefined number of molecules will occupy 50% less volume to compensate, and therefore occupy a 50% thinner "slice" of exhaust pipe at 1/2 inch long. This increases the amount of gas in the pipe and increases back pressure.
So it would seem like increasing the pipe diameter would increase flow, right? Well the answer is "not quite". Having a larger pipe means increasing the volume of gas in the system as well as increasing the surface area of the system. This means a larger pipe gives the exhaust more time and opportunity to cool, condense, and slow down, compounding the engine's problem of pushing gas through this long, constricting pipe. When you stay close to the factory diameter, you don't see much of a difference. However, when you put a 3.5" pipe on a moderate performance car that came with a 2" pipe, you're likely to see reduced power. Larger pipes actually have tremendously turbulent flow compared to smaller pipes for a given air pump (your engine). The turbulent flow causes randomized high pressure sections, low pressure sections, and generally discordant flow that fights movement, further slowing it down. Gas has momentum. That 1" slice moving down the 3.5" pipe isn't going to be as compliant as it was in the 2" pipe when it needs to turn.
"But Frank! Look at this chart from Magnaflow showing how their larger pipe increases power from the factory 02 LS, despite increasing diameter!"
There's more to that system than just diameter. The Magnaflow system uses mufflers with higher flow rates (I'll assume their straight-through design), has an X-pipe instead of an H-pipe, and most importantly, has smoother bends. The factory system (at least the Gen 1) zigzags near the rear axle to move out from the driveshaft tunnel, around the differential but out of the way of the half shafts, past the toe-link nuts, around the spare tire well, and finally turns sharply into the muffler.
Gen 1 rear section (excuse the photo quality, it was unplanned and rushed with a phone)
Now look at the Magnaflow system for the same car
Instead of zigzagging, it just has one straight section going out at an angle, not concerning itself as much for those previously mentioned points of interest. The Magnaflow system is also mandrel-bent, meaning there's no bumps sticking into the exhaust flow at the bends that reduce diameter and increase turbulence.
