Many people are just too careless in exhaust modification. They love the cheap loud exhaust, which gave them additional 20 psychological horsepower. For the extractor, everybody seemed to be suggesting 4-2-1 extractor, which could be just right for their street cars but the problem is they chose a custom made extractor made by some non-reputable company or by normal workshop. Those extractors are no more than factory standard, or might be worse. Making an extractor is not as simple as wielding steel pipes together, or bending the pipes as you wish, but the calculation of the length and the curve of each pipe which are the most important. That is why tuning house or performance part companies produce tuned extractors at expensive cost. Cheap ones are waste of money.
1. 4-2-1 type extractor
This setup gives power through out the rpm range. Drive ability at lower rpm is quite good, making it the most suitable setup for street usage. Expect more top speed compare to 4-1 type.
Pro : Good power all across the rpm range, more top speed, good for cruising
Cons : Weak top end power, torque ends earlier
2. 4-1 type extractor
This setup gives more top end power. The low end power is weaker compare to 4-2-1 setup, but usually the power gain is significant from the midrange up to high rpm. This causes the torque range to come later, thus creating more horsepower at high range rpm compare to 4-2-1 setup. 4-1 is best for high revving motors like VTEC and MIVEC.
Pro : More midrange and top end power, higher hp, good for acceleration
Cons : Poor at lows, lower top speed compare to 4-2-1
3. I drive a Proton/Perodua.... which extractor should I get?
For Proton with 4G13/4G15, one of the best extractor by far in the local market is the PowerZone 4-2-1. It is made for better street performance and developed under proper R&D. HotBits also make extractors for 4G13/4G15 but a few reviews mentioned that it tends to get cracked after awhile. I would say, stay away from custom made extractors if you are looking for some performance gain, or else just get any brand/custom made if you just want to replace your old rusty ones. For those with 4G92/4G93, the OEM 4G92 MIVEC extractor should be good.
As for Perodua, there is not much choice for normal aspirated application. But if I am not mistaken, PowerZone do have tuned length extractor for Kancil.
Other than that, HotBits probably have extractors for various Proton/Perodua models.
4. Exhaust piping
Exhaust piping diameter is a crucial part in the exhaust system. For those with 1600cc engines and below, stick with anything less than 2.0" except if you had extensive jobs on the engine such as higher compression pistons or performance camshaft upgrades. For stock VTEC or MIVEC 1600cc and above, around 2.0" to 2.25" is acceptable. Some people even use 2.5" on stock VTEC (like myself) but the result is poor at lower rpm because the exhaust gas have less velocity to travel. Apart from the diameter, the piping route is also important. Straight pipes are not really advisable for street because they tend to get in the way when you're running on a road bump. It is annoying and you will get scratches on the bottom of the pipe, or worse bent pipes. Piping that follows the original route is the best. While it gives you stock appearance, many people claim that it gives better low end to midrange power compare to straight piping. While weak at low end, straight piping somehow tend to give better acceleration power because exhaust gas gets out easily because of the shorter pipe compare to stock.
Muffler is more to cosmetic rather than performance. There is minimal difference in power output if we compare straight-thru type muffler (also known as N1 type) with s-flow type muffler (which usually have big resonator box). The later is seemed to be more preferable for street usage because more silent, thus giving more pleasant driving experience especially while in cruising mode. Personally, I would recommend Tanabe s-flow like the G-Medallion series because they are very silent when idling and cruising. This avoids attention from the authority. If you are going for N1 type, make sure it is made of good quality. Avoid imitations, they are widely sold by Wei Yip (tidak tipu customer) at surprisingly cheap price. Good exhaust doesn't come cheap but they are worth it. Try to spend some time to go to 'kedai potong' and try look for used branded exhaust. They are worth it. Other than that, exhaust with JASMA tag (not JASMA brand) should be good enough for street. From my experience and observations, JASMA approved exhaust tend to be quiet, unless the used mufflers are already running out of fiber. If your setup is still noisy, it is advisable to install a resonator (bullet silencer) in the middle.
The newer Proton Saga/Iswara has very good example on bad muffler setup. The exhaust tends to get noisy and noisy after awhile because of low quality fiber inside the muffler. Besides, normal cars should not have noisy exhaust, it is lame. They are even louder than my Tanabe even when my car is in VTEC mode!
6. Catalytic converter...to remove or not?
It depends. Cat converters are the biggest restriction to the exhaust gas flow. Removing it will definitely release some more power, but it is bad for environment. If your engine is badly tuned or always running rich, I suggest that just leave them on. Cat converters do not contribute anything to exhaust noise.
7. What is bullet silencer?
Some people also refer this as 'cherry bomb'. It is a small cylinder shaped resonator which usually placed in the middle of the exhaust piping. It functions as a muffler, to reduce exhaust noise. Due to its size, it is not as good as the rear muffler but still helpful because usually there is no space below the car that allows one to put a big muffler. To choose a correct size, the inner diameter should be more or less equal to the overal piping diameter to avoid restriction or bottleneck.
8. Flexible pipe
Usually installed after the extractor. The purpose is to minimize exhaust piping vibration and avoid the piping joint from being cracked. This is because the engine tends to vibrate and flexes backwards when rammed, so a flexible pipe will make sure that exhaust piping is not bent.
Somehow, rear wheel drive engines are exceptional because of the longitudinal engine layout that flexes to the left or right instead of front and rear.