^^ that statement is false.
what is true is that exhausts really don't 'make' horsepower. the engine obviously makes ALL the power. but there are a lot of things (like exhausts) that take away horsepower. that's why the difference between bhp (the factory rating of the engine at the flywheel) and whp (the horsepower you are actually putting out) is usually at least 13% loss in a manual. (my gts is rated at 180 bhp from the factory, but i only actually have around 160 whp--actual horsepower...which is a loss of around 20 horsepower).
yada yada yada..the purpose of the exhaust system is to remove waste gasses from the engine. the engine produces these waste exhaust gases in 'pulses' (it's not one continuous stream..think of a heartbeat where your blood is 'pulsed' with every beat of your heart. same with the engine.) to get these exhaust pulses out with MINIMAL gain, they need to be flowing consistently right behind each other...which in effect the previous exhaust pulse 'pulls' the next exhaust pulse along and so on and so forth. when running at a low rpm, your engine pushes out small pulses and produces them relativly slow. at mid rpms, your engine increases the size of the pulses (more air entering the Intake
means more air exiting through the exhaust) and produces them faster. at high rpm's the trend stays the same..the exhaust pulses are even bigger and produced even faster.
this is where the exhaust piping diameter plays a role. toyota realizes that MOST people use their celica as 'daily drivers'. what this means is that most people who buy our cars usually stay in the low-to-mid rpm range while driving. SO, toyota made their exhaust to realize this fact..which means the exhaust piping diameter is rather small to compensate for the relatively small exhaust pulses that aren't produced very fast.
however, if you're a daily driver you probably aren't on this site OR interested in daily driving much. most tuners or motorheads do their driving in the mid-to-high rpm range. because of this higher rpm range, the exhaust pulses are bigger and produced faster...so the small exhaust piping diameter of the stock exhaust becomes restrictive. so what most aftermarket performance manufacturer's do is INCREASE the exhaust piping diameter so the bigger (and faster) exhaust pulses won't be restricted. for our celicas, the best piping diameter size for 'enthusiast driving' happens to be around 2.3" - 2.4"..which is what almost all our aftermarket exhausts are.
for those of you who are interested in high rpm driving (drag, pro-street), you should get an even bigger exhaust than the 'enthusiast driving' exhausts i just mentioned. an exhaust diameter of about 2.5" - 2.6" would give you maxium gains in the high rpm driving range, but you will lose some down low.
why would you lose some down low? because like i was saying earlier at lower rpm's the exhaust pulses are smaller and produced more slowly. what this means is that when you use a big exhaust pipe diameter (such as 2.5"+), these smaller pulses do not 'fill up' the pipe completely and instead 'bounce' around. this creates backpressure because the exhaust pulses are not 'pulling' each other out as they should. and while i'm on backpressure, just let me state:
contrary to popular beleif, BACKPRESSURE IS NOT GOOD ANY WAY YOU SPLICE IT. IT'S NOT GOOD FOR TORQUE, IT'S NOT GOOD FOR HORSEPOWER. IT'S ONLY GOOD FOR MAKING YOU LOSE POWER.
the myth of backpressure was created by people who bolted on a regular 2.3" enthusiast exhaust and saw gains. then they bolted on a 3" exhaust and saw a loss of power. "well, the 3" exhasut obviously has less backpressure than a 2.3" exhaust so the loss in power must mean some backpressure is good". NOT. the reason they lost power is becasue the exhaust pulses were bouncing around in the 3" exhaust CREATING backpressure instead of flowing along smoothly.
so in conclusion: realize what driving style you do mostly and pick the appropriate exhaust. for most of the people on this site that would be 'enthusiast driving' so get an exhaust around 2.3" - 2.4" in that case. if, however you decide all you really want is to go from point A to point B, then just leave the stock exhaust on. add a resonator/muffler if all you want is a sound difference. if you occasionally take your car to a drag strip, then i would suggest just unbolting your exhaust from the axle-back instead of buyinger a bigger diameter exhaust system.
sorry for the long post, but this was needed to educate a lot of the people here. this probably should be stickied cause i dont wanna have to keep saying this, but anyway