One of the biggest and best bangs for you buck. Can be replacing that small size diameter exhaust pipe with a larger mandrel bent free flowing exhaust.
Why? Basically, by lowering the back pressure with a larger diameter exhaust you make it easier for the motor to expel the exhaust gases. Giving you more power and better mpg.
Can you go to large? That depends in a naturally aspirated car, the answer is yes. You need a certain amount of back pressure for the engine ton properly make torque. With out it you get a lot of noise but not much power. And in some cases even worse performance then the restrictive factory exhaust. Now if your talking turbo, then no the less exhaust back pressure the better. The reason is because the needed engine back pressure happens in the exhaust manifolds between turbo and engine. By removing the back pressure from turbo you allow the turbine to spin freely. That means faster boost and quicker response with less lag.
With the said, there are a couple ways to go about upgrading your exhaust. You can go the kit route, many cars have pre-bent pre-made kits from several companies. But what if you need something custom? Well, most exhaust shops can accommodate your needs but most (at least in our area) don't have mandrel benders. What is a mandrel bender? Its a methods of bending pipe that give a smooth rounded turn instead of the crimped bend made by regular benders. The other option is to make it your self like we did and I'll show/tell you how.
We start by ordering our bends, since we don't have the space or money for a mandrel bender. There are several places to order pipe from Jegs, Summit, Ebay, etc. Even companies like Flowmaster offer DIY kits now. We mapped out our exhaust and roughly guessed we'd need 4 u-bends and a j-bend.
To build, you'll need a basic mig or tig welder. A chop saw, cut-off wheel, hack or reciprocating saw (something to make your pipe cuts) a marker (preferably black) and some patience.
Start by bolting your flange on to your header(or turbo in our case) this allows proper fitment to
the flange. Then using your marker mark out the first section of pipe and where to cut. Always tach weld the pieces in place as you go and then seam weld after all are in place. This way if you need to change something you can just cut the small weld instead of the whole pipe. Make sure you are able to remove the section your mocking up. Once you get to a point where its nearly impossible, I recommend using a flange to break the section so it can be removed in the future with out hassle and be easily reinstalled later.