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In Reply to: "Performance" Exhausts... A Must Read... posted by Joe ///M on August 13, 2000 at 21:10:27:
It seems a lot of people are putting "free flow" exhausts after their stock
cats. Thats no good the stock cat is THE RESRICTION its like putting a 6inch pipe behind a 3inch, the gas velocity will slow in the 6 but it wont flow more.
The other problem is the two branches (each three cylinders) come together too soon with stock cat.(Look under a new M3 you will see two exhausts system effectively)
This is why 3inch systems can be a problem and Euro’s keep each manifold separate as long as practically possible due to some very complex resonance/wave effects on a straight six.
The Motronic CAN adapt especially running closed loop the O2 sensor will of course richen a mixture that has become too lean due to better volumetric efficiency.
Some manifolds (including BMW) are a bad design where the down pipe attaches (3 bolt flange) is very restricted to make room for the bolts. So again any free flow down stream is wasted and gives people these funny ideas about back pressure.
Think about it any work the piston has to do on its way up to push against back pressure is a negative.
The trick is to have a system with no or very little back pressure BUT still keep down noise. This is better achieved with dual exhaust on a BMW, problem is it can make the system a lot more expensive and therein lies the big part of the problem.
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