Performance Exhausts for the BMW Z3
May 31, 1999
By: Robert Leidy
Inside each cylinder, the BMW Z3’s electronic control module injects a calculated mixture of air and fuel. This mixture is then ignited which produces power. However a by-product is also produced from this process which is commonly referred to as exhaust. The burnt gas fumes (exhaust) exit the cylinder and travel through a pipe commonly referred to as a header. The header pipes from each cylinder are then combined and channel the exhaust fumes from the engine into a catalytic converter. The catalytic converter is a device filled with a metallic-mesh-like filter that removes some of the pollution from the exhaust fumes. Once the exhaust has passed through the catalytic converter, it is channeled through a single pipe to an exhaust resonator, which reduces some of the sound produced by the engine. Once the exhaust has passed through the resonator it is channeled to a muffler to “muffle” additional sound from the exhaust. Once the exhaust travels through the muffler it exists the Z3 via tail pipe(s) under the rear bumper.
The theory behind performance exhausts is that each device that the exhaust fumes pass through cause resistance, which in turn increases the amount of air pressure inside the exhaust. The pressure built up also effects the cylinder because “back pressure” from the exhaust is putting additional effort on the cylinder as it is handling the next mixture of air and fuel. A performance exhaust is designed to reduce the amount of resistance in the exhaust making the exhaust flow more freely and reduce and amount of “back pressure”. In order to accomplish this, those devices within the stock exhaust that cause resistance can either be removed or redesigned to be less restrictive. However each component of the stock exhaust is there for a reason. The muffler is designed to remove sound at the cost of exhaust resistance. You can redesign a muffler to have less resistance but in general you will also be decreasing the mufflers ability to “muffle” sound. As with most things in life it is a give and take relationship. Finding the correct balance of give and take is a judgment call, so it can be different for different personal tastes.
There are varying degrees an owner can take to reduce the pressure in the exhaust and increase performance of the engine. Perhaps the easiest way is to just replace the muffler. Or for a little more performance, replace the muffler and resonator. Professional racers like Mark Hughes remove all of these resistance-causing devices. However removing the catalytic converter would keep a Z3 from being street legal (which is not a problem for the Z3 Race Team). The most common after-market exhaust systems are called “cat-back exhausts”. With the design of the Z3 these “cat-back” systems bolt right onto the stock catalytic converter and replace everything “back” from there (keeping the Z3 street legal). The new pipes are larger, the resonator is removed and the muffler is less restrictive, so exhaust can exit the engine/exhaust with less resistance. This has two effects on the Z3, it increases the performance and it makes the Z3 louder (more sound from the exhaust).
So now that we’ve covered the theory behind performance exhausts lets look at the type of decisions an owner would need to make in evaluating after-market performance exhaust systems:
Stock Engine/Exhaust Design:
The theory behind performance exhausts is that reducing the amount of pressure increases performance. So it only makes since that the amount of potential performance gain is directly related to the amount of resistance/air-pressure-buildup in the stock exhaust. In other words if the stock exhaust is very restrictive then there is a lot of potential performance gain.
Assuming that between the different Z3 engine configurations the restrictiveness of the various BMW stock exhausts components is roughly the same, we can make some general observations looking at the different designs. The 1.9 Z3 is a 4-cylinder engine, the exhaust output from all four cylinders is combined before entering the single catalytic converter. So roughly 1.9 liters of exhaust output is sent through the stock exhaust each time the gas inside the cylinders is ignited.
Compare that to the 6-cylinder 2.8 Z3, which has the exhaust output from all six cylinders combined before entering the single catalytic converter. So roughly 2.8 liters of exhaust output is sent through the same pipe. Comparing these two it would appear that the 2.8 would benefit more from a performance exhaust than the 1.9 would since 47% more exhaust is traveling through the exhaust.
To continue the comparison, the 3.2 Z3 is also a 6-cylinder engine. However the exhaust output is split in half, the output from three cylinders is sent to the first catalytic converter, with the exhaust output from the other three cylinders is sent to a second catalytic converter. The exhaust output from each half is never mixed so in reality the each exhaust is only handling 1.6 liters of exhaust output. Comparing this output it would appear the 3.2 engine would benefit the least from an after-market exhaust.
There are also differences in the type of metal used in exhaust systems, the big differentiation is it, or is it not stainless steel. The big advantage to stainless steel is its durability. If you live in an area where salt is used on roads then you know that rust can eat up car parts. For these areas stainless steel will last a lot longer so the increased price is easily justified. The other advantage to stainless steel is that it conducts 2/3 less heat than mild steel, which helps to keep temperatures in the exhaust to a minimum. However stainless steel also expands 40-45% more than mild steel when heated so fitting a stainless steel exhaust is slightly more difficult. (The stock exhaust system is not stainless steel).
Choosing exhaust tip style is usually a 90% cosmetic decision. However the type of exhaust tips also effect how the exhaust will sound. If the exhaust tip is angled up the sound will generally be louder, if the exhaust tip is angled down the sound will generally be quieter. Most exhaust tips point straight back just like the stock exhaust does.
In the United States, five different engine configurations have been built in the Z3:
M44: model year 1996-1998
The M44 1.9 liter engine was the first Z3 sold in the United States. Several companies make performance exhausts for this Z3. However this engine configuration is no longer being made for the US market so it is doubtful that any additional aftermarket performance exhausts will be added to the list.
M52: model year 1997-1999
The M52 2.8 liter engine was the second Z3 engine configuration to be sold in the United States. Several companies make performance exhausts for this Z3. However this engine configuration is no longer being made. BMW now has a new M52TU 2.8 liter engine that is different in design, so exhausts for the M52 Z3 will not work on the M52TU Z3 and via-versa.
S52: model year 1998-2000
The S52 3.2 liter engine was the third Z3 engine configuration to be sold in the United States. Several companies make performance exhausts for this Z3 (officially called the M roadster and M coupe).
MXXTU: model year 1999-2000
The MXXTU 2.3 liter engine is the forth engine configuration to be sold in the United States. MZ3.Net does not know of any M52TU Z3 “cat-back” performance exhaust systems at this time.
M52TU: model year 1999-2000
The M52TU 2.8 liter engine is the most recent engine configuration to be sold in the United States. Supersprint is the only company that MZ3.Net has heard that currently has a cat-back exhaust ready for the new 2.8 liter engine. Supersprint’s part numbers for the new exhausts are 78.67.06 or 78.67.66. I assume the two different numbers are for different exhaust tip options.
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