Exhaust upgrade


Cost: a lot...


BMW E 34 535i Racing Dynamics headers

The Bmw E34 exhaust is not really restrictive to start with.

The very basic idea behind exhaust tuning is that the exhaust gas coming out of your engine isn't a stream of gas, it is made up pulses of gas. These pulses are essentially shock waves, with high pressure at the front and low pressure behind it. The low pressure end of one shock wave essentially "pulls" the next shock wave along. This means the engine has to do less work to get the exhaust gas out of the car, thus increasing power. The trick is that all of these pressure waves only line up at a certain RPM, at and around that RPM is where you will get the most power increase. The tube length and diameter is critical for this tuning. This tube length must be exactly the same for each pipe connected to each exhaust outlet. A larger diameter tube will tune at a higher RPM, a smaller diameter tube will tune at a lower RPM. A longer tube length before the collector will tune at a lower RPM, a shorter tube will tune at a higher RPM. Tube length is more critical to tuning than tube diameter.



The BMWs have an exhaust manifold whose job it is to collect the exhaust gases from each of the exhaust ports of the head to which it is attached. The manifold is cast iron to reduce noise and provide some longevity. In a typical exhaust mainfold, the individul "runner" for an exhaust port is virtually non-existent and the exhaust gases from all cylinders (on the same bank) merges within just a few inches - at most.

A header, is composed of dedicated "runners" (tubes) for each exhaust port. In nearly all header designs, there is an intent to keep the length of each tube identical and the same as tubes attached to the other head (if any). There is a great deal of science around header design and like most other engine designer choices - it is an issue of compromises. Length of the tubes and diameter of the tubes as well as the collector can be varied to optimize torque and HP at different RPM ranges. Of course all of this extra *plumbing* usually has to fit in a rather confined space - which presents additional challenges.

Installing aftermarket headers from RD or supersprint will give you a performance gain. Reducing the pressure from the cat back makes it easier for your engine to push exhaust gases out, this really on helps at high rpms. Headers makes the exhaust gases come together more smoothly than stock before entering the cat. Exhaust systems to have small gains on stock engines, but if you are planning on NOS, turbo, or supercharger, you can expects your 10 hp exhaust to produce something like 30 and 50hp.


Does Muffler give more HP?

Sorry to disapoint you, but the answer, simply, is no. The most efficient mufflers can only employ the same scavenging effect as a header, to help slightly overcome the loss of efficiency introduced into the system as back pressure. But no engine makes more power with a muffler than an open header exhaust.

There is three types of muffler:
Absorption mufflers are the simplest. the construction consists of a perforated tube that goes through a can filled with a packing material, such as fiberglass or steel wool. This is similar to simply punching holes in your exhaust pipe, then wrapping it up with insulation.
Restriction mufflers: Obviously, a restrictive muffler doesn't require much engineering expertise, and is almost always the least expensive to manufacture. Thus, we find restrictive mufflers on almost all OEM exhaust systems.
Reflection mufflers: Probably the most sophisticated type of muffler. They often utilize absorption principles in conjunction with reflection to make the ultimate high-performance silencer. Sound is a wave. And when two like waves collide, they will "cancel" each other and leave nothing to call a corpse but a spot of low-grade heat.


An exhaust resonator is used in some exhaust stystems to change the tone and timber of the exhaust sound. The resonator helps reduce the annoying vibrations of exhaust gas pulses with a length of perforated tubing inside an expansion chamber. The exhaust gasses partially escape the tubing diameter through the perforations into the expansion chamber. In the expansion chamber, the exhaust gasses expand and change tone. When the gasses re-enter the tubing, they rejoin the exhaust flow and dampen the vibrations of the combined flow, creating a more pleasing sound.

(Thanks to & Justin)


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