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Power may be tuned based on exhaust length (archive)

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Posted by davidcosine on June 29, 2001 at 12:12:24:

In Reply to: what exactly does a resonator do? posted by Reckless on June 27, 2001 at 22:43:18:

........there was a big thread on this in the past. The length of the exhaust system determines the frequency of harmonic waves that create pressure or vacuum and travel at the speed of sound through the exhaust system. The length of the exhaust tube determines the frequency of the harmonic waves (think of the exhaust like a flute in that you close a hole and the pitch changes). If this frequency is "tuned" correctly (i.e. exhaust is the right length), then the wave corresponding to a partial vacuum (as opposed to pressure wave) will arrive at the exhaust valve end of the pipe when the exhaust valve opens. This helps to suck exhaust gasses out. Supposedly the optimal length for an exhaust pipe in the 02 has it ending about midway back on the car. The "resonator" fools the exhaust system into thinking it ends at that point. This optimizes the hp of the system.
........Correct me if I'm wrong, as I am paraphrasing others (can't find exact archive thing I'm referring to)

Below is some stuff I found:

Posted by XBeteNoirX on December 16, 1999 at 01:20:09:

In Reply to: Do I need or want a resonator? posted by Mike Sexton on December 14, 1999 at 22:25:51:

Yes you want and need a center resonator. It is far more important than which muffler you select. The size and location
of the center resonator helps determine the exhaust system pressure tuning. Please do not confuse noise with power.
They are not the same.

Posted by XBeteNoirX on December 19, 1999 at 02:06:03:

In Reply to: Re: please explain posted by delk on December 16, 1999 at 06:36:16:

While it is often possible to replace an old rusted, semi plugged resonators with an incorrect new system and see more
performance, this rarely occurs when apples are being compared to apples.
In order to optimize the BMW exhaust pipe length for maximum performance it must be much shorter. This causes a
problem since regulations require that the pipe go to the back end of the car. The solution was to insert a large volume
resonator at the right location to fool the exhaust pulses into thinking they have reached the end of the pipe.
If you saw an improvement by simply removing the resonator, it might mean your entire system was choked, rusted or
undersized. Whenever we have fitted an entire new exhaust system, it has always performed better with the resonator
than without. Sometimes being a little off tune masks the observation.

Posted by XBeteNoirX on December 12, 1998 at 13:48:30:

In Reply to: Re: BEST EXHAUST MANIFOLD posted by mike on December 11, 1998 at 21:59:00:

Re: Exhaust headers for 2002

Exhaust headers and systems have confused enthusiasts and engine rebuilders for many years. NO, repeat, NO exhaust
system can create horsepower. The best that a properly engineered system (system, not mere parts) can do is unlock the
power already designed into the engine. A poorly designed exhaust system can choke an engine’s performance. BMW
factory stock systems (not aftermarket knockoffs) are NOT poorly designed. You will NEVER see a 15 HP increase
over a factory system unless your current system has a potato stuck in it somewhere.
Proper exhaust system design MUST consider the following:
Induction tuning
Cylinder and engine displacement
Camshaft lift, duration, overlap
Valve sizes
Target RPM range
Noise limits
Packaging and installation
Low reving engines require longer exhaust system lengths in order to maximize torque. But long pipes don’t flow as
well and resonant pressure pulses are not in phase with exhaust valve events at high RPMs. High reving engines need
shorter pipes to achieve high end horsepower (torque and RPM) but often suffer an intolerable loss of torque at low
and mid range. Larger cylinder displacement and/or increased volumetric efficiency (cams, valves, inlet manifold, carb,
etc.) require a larger diameter exhaust pipe.
To improve engine power performance, changes must be made in engine size, camshaft, valves, and/or induction. Once
done, an exhaust system (header, downpipe and resonator) can be designed/selected which will allow maximum
performance. In exhaust systems (unlike women) one size does NOT fit all.
From a performance standpoint, virtually every aftermarket tubular exhaust is junk. Some are welded better, some have
thicker tube walls but none are properly designed. These are fighting words to some people. But most performance
claims are anecdotal and undocumented – proof by assertion. When rigorous, controlled dyno tests are performed- the
results are very clear. JUNK. If real power gains could be obtained from cheap aftermarket exhaust systems, the BMW
factory would have quickly installed them instead of redesigning engines.
At a torque peak (not redline) below 5000 RPM a Tri-Y system is superior. At a higher RPM torque peak, a four into
one configuration is better. And best yet is a four into two system with dual tailpipes. Take a look at the exhaust system
of the BMW 318is (M-42) for an example of how it should be done.
Even though most all aftermarket systems are of very low quality, most breakage is due to improper installation not
poor fabrication. Since the engine has flexible mounts, the exhaust system must move with the engine. A rigidly
mounted exhaust system will always break, sooner or later. But usually at the worst possible time, like at the track or
on a cross country vacation.
Cast iron and tubular headers are often subject to excessive heat, which then weakens the metal and causes failure.
Improper combustion is caused by retarded ignition timing or excessively rich fuel mixture. And that’s what eats
exhaust valves as well.
If you’re running a 2002 and have improved the breathing, use the factory Tii cast iron header, tubular downpipe and
factory stock resonator. The can type muffler doesn’t matter; it doesn’t affect performance unless its too small.
If you’re running a Tii, you’re screwed. There is no readily available product on the market which will outperform the
stock system. The best you can do is match the ports and cut off the tailpipe shortly after the resonator. Keep the inside
clean, free of carbon and rust.
Or you could hire a competent automotive engineer to design a proper system for you.

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