The "backpressure" talk is a myth eh. There's no combustion engine on planet earth that "needs a little backpressure".
See, because your combustion stage in the cycle is a series of controlled "explosions" (let's call them explosions for now) across 3 or more cylinders (in your case, 4 cylinders), there are going to be pulses of exhaust gas flowing through your pipes.
Now, dig this shiznit. Because of the rapid opening and closing of the exhaust valves, there is sometimes cavitation in front of the exhaust pulse and sometimes after. This is a low pressure area, which if comes in front the pulse provides a pressure gradient that allows the exhaust gases to flow out the pipes (let's call this a positive pulse), but if it comes after, it causes the pressure gradient to flow in the opposite direction (let’s call this a negative pulse) and exhaust excavation due to upward piston movement only carries it so far down the pipes.
A proper exhaust system needs to be designed in such a way as to accomodate proper wave propogation to merge these pulses properly for a smooth, consistent exhaust flow (I could carry this back to CXC physics about constructive and destructive interference, but I'd prefer not to). Ideally you’d think that they’d design your exhaust system to cause all the positive pulses to combine and lay the smack down on them roody poo negative pulses, but in real life, positive and negative exhaust pulses travel from your exhaust manifold to the exhaust tip and back in a cycle that, on a factory exhaust, is so friggin smooth, it feels and sounds like a constant, single, uninterrupted flow of gas.
Now, this isn't to say that all factory exhaust systems are designed to fully excavate the exhaust gases from the cylinders. Most times, a portion of the negative wave is desirable to keep some exhaust gas in the combustion chamber.
Here's why this *ahem* exhaust gas recirculation
is so important:
1. Exhaust gas ideally cannot be re-burnt, but it still occupies space in the combustion chamber and can be compressed just like normal air. What this means is that it occupies the space that would more readily be occupied by a fresh air-fuel mixture to make more power, but this is for fuel economy purposes, and with an air pump (yeah, your typical internal combustion engine is quite simply an air pump) as inefficient as what we have under our hoods, there's usually a trade-off between economy and performance.
2. Now, hand in hand with the first reason, is the fact that now that there’s less air-fuel mixture all up in there, thus there’s less fuel to burn, thus there’s a smaller flame front and thus less heat and thus…*dun dun dunnnn* lower exhaust temperatures.
3. If for whatever reason, the burn isn’t as efficient as it’s supposed to be (for any number of reasons like overfueling, poor spark, Satan etc.), it’s helpful to put some of that exhaust back in there with some fresh air for re-burning (HA! You thought I said exhaust can’t be re-burnt ent? I said IDEALLY *puhpses softly*). The end result is *drum roll* LOWER EMISSIONS! *bodow*
Tuned aftermarket, oversized exhaust systems increase exhaust gas excavation because of the volume of gas that can now be removed from the combustion chamber (I not getting into a convo about the decreased speed of the gas with the increase in cross sectional area of the pipe right now nuh..we talking volume here), which, while it’s a good thing for power gains (the extent of which is subjective), throws everything I said about factory exhausts off kilter. So, you get poorer fuel economy because you’re messing with them negative pulses, higher combustion temperatures and higher emissions.
Well that and the fact that a fat aftermarket chrome thingie is fun to braaap so el foot is always on the gas.
*Tiger Woods fist pump*