Dispelling the backpressure myth
Myth: our engines need back-pressure otherwise we lose low-end torque.
Not quite. It's true for larger exhaust size we lose low-end power, but it's not because the engine needs the back-pressure; far from it. Otto-cycle 4-stroke internal combustion engines do not need back-pressure--what they need is good scavenging--the ability for the exhaust system to suck the gasses out of the combustion chamber, and to do that, you need good exhaust velocity. Therein lies the problem. A lot of new systems on cars--like VVT (Variable Valve Timing), and CVT (Continuously Variable Transmission) are designed to be adaptable for driving in ALL conditions. your exhaust system is pretty static:
For example, using the two most common sizes of J-body engine (2.2 and 2.4), the difference in the amount of exhaust gases the engine pumps out varies considerably the RPM range of the engine. Since I'm not an expert in matters, I'll keep it simple, assuming that the engine will expell as much gas as it's displacement--2.2 and 2.4l.
Taking that into consideration, at a 800rpm idle, a 2000rpm "cruise" and at 6000rpm engine speed, the engines are expelling this much gas:
2.2l / 2.4l
31.08 / 33.90 <--800
77.69 / 84.76 <--2000
233.08 / 254.27 <--6000
(all numbers are in CFM--Cubic Feet/minute. Numbers figured out by Displacement multiplied1/2 of the RPM's (since exhaust stroke is every other revolution), and converted from litres/min to CFM).
The cross sectional areas of 2", 2.25", and 2.5" are 3.14, 3.98, and 4.91 square inches, respectively. Now, as you can see, each of those pipes has to be able to flow the aforementioned CFM of exhaust gasses per minute. Now, I'm I'm no expert on that math, but it would be near impossible to design a pipe that can meet idle, cruise, and at 6000rpm's. The smaller pipes give better low-end performance, because they can keep the exhaust speeds up at lower rpm's, but in the upper ends, they choke because the pipe pressurizess itself and can't flow enough, and you get back-pressure.
The bigger pipes flow well at higher RPM's, but, at the lower ones, there's too much area in the pipe, the exhaust loses pressure filling the pipe, and thus loses energy (it's got to push all the gasses in the pipe ahead of it), the exhaust speed slows down, it's not going to "draw" as much of the exhaust gases out the the combustion chamber--either some "stay" in the combustion chamber, and thus less air/fuel can get in, or restrictive elements in the exhaust *will* cause back-pressure, and the engine will be working against itself.
The best way to demonstrate this is to get a bunch of straws. Blow through one causing the air to flow out of it as fast as you can make it. Then try two, three, and so on. There will come a point where there's no more added resistance to you blowing into one more straw, but the speed of the air slows down. This is what happens, except your car reaches that point *somewhere* in the RPM range--unless you're using a ridiculously small--or ridiculously large pipe.
So, as you see, even though yes, a large pipe will kill your low-end power, it's not back-pressure.
So, what size pipe to choose? Well, first off, you have to think about your displacement, any charging of the incoming air (turbo/super/nitrous), and what your use is. Most stock cars have a restricted exhaust, because they are good to go around cruising speed--where the manufacture expects the engine will be most of the time. For those going N/A and drag racing, you spend little time at the lower ends of the RPM range, and as such, larger pipes will work for you, giving you better flow characteristics at the higher ends. AutoX and road racing will most likely want something that flows well in the mid-range RPMs to handle changes in engine speed.
As such, this size you choose for your exhaust will depend on the engine you have and the use of it. So, if you're driving a 2.4l, and you're dragging her and she's N/A. there are dyno results out there that prove that 2.5 is *not* too big. However, because of the lowered displacement of the Eco this may not apply to them.
So, please, DON'T SAY OUR ENGINES NEED BACK-PRESSURE It's erroneous at best. At worst, it shows you have little knowledge of fluid dynamics. A small exhaust pipe doesn't provide back-pressure at low RPM's, it provides high exhaust velocity at low RPM's. Unfortunately, were stuck here until someone can design a pipe for a car that changes it's cross-sectional area based upon the output of the engines, we're stuck trying to pick a size that will provide a "catch all" result for whatever we use our cars for the most.
Some links:
http://magnaflow.com/05news/magazine/05sportc.asp
http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/
Thanks to Keeper of the Light and Event for the info :)
