I am curious to know why exhaust mod can increase engine HP. I thought normally engine output can only be changed by either increasing the capacity or the efficiency of the engine. And exhaust is only the bit that passes gas!!??

An engine is really a big air pump with mechanical bits that turn that air flow into mechanical energy. Because of noise and emmission requirements, the exhuast system is usually less than ideal for optimal air flow and usually one of the biggest obsticles in flowing more air, which in turn provides more power. For example, most aftermarket systems do not have the second set of catalytic converters, a requirement for Porsche. Eliminating them makes the gasses flow more easily through the system which lets the engine pump more air, make more power.

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Gator Bite
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Good explanation Gator Bite, never really looked at it quite so simplistically

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allan@motordrive.com

If you are climate conscious, cat-back exhaust systems (short for catalyst-back exhaust systems) could fit your needs. There are many such after-market cat-backs. [Catalystic convertors reduce the harmful hydrocarbons and heavy nitrogen gases produced by the engine to (slightly) less lethal gases. The more the number of cats, better pollution control.]

These cat-back exhaust systems leave the emissions-control gear in place, but allow you to put large-diameter exhaust pipes and low-restriction performance mufflers into the system after the catalytic converter.
The net result is lower auto exhaust backpressure, more horsepower, more torque, and a much hotter sound, all while staying perfectly legal.

To see the parts and functions of an exhaust system try the diagram below:
http://www.kwik-fit.com/assets/jpg/g...work_large.jpg

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Wow thanks, so it's the airflow that makes the difference. Now I understand why we mess with the intake and exhaust to squeeze more HP. But then again intake has nothing to do with pollution, why would Porsche put anything there to restrict airflow?

It goes right back to your original post of increasing capacity or efficiency...flowing more air in and out not only effects efficiency, but also has a big effect on dynamic dispacement. the static capacity of you motor is measured at rest and is a fixed value, but during operation numerous factors change that volume such as ignition and valve timing, valve angle, and size, as well as intake variables and exhaust...not just the removal of restrictions, but the diameter and leangth of the tubing on both ends effect the flow and wave pulses dramatically. This all effects how much air is actually in the chamber at combustion...and power output. Small changes in the length and diamer to of the intake tube alone can significantly move the power band up or down, etc...

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Atlas Gray Cayman S

On the intake side, the restriction keeps debris out of your engine and, to a lesser extent, mutes the induction howl.

On the exhaust side, the point is to keep the engine reasonably quiet and to keep emissions legal.

IMHO with any aftermarket intake or exhaust system you are going to make some compromises for the increased flow. Those compromises may or may not be acceptable to you.

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Originally Posted by K-Man S
elmo,
I never said I was mistaken about anything, I said that my understanding of your explanation was different from what I first thought.

On the intake side, the main restriction is the air filter. It is essential to prevent dirt that would destroy the engine from reaching the engine. Noise can also be a factor, as can the physical space needed to make an ideal intake system.

In addition, air will flow differently through the system at different speeds. One intake manifold might be great for high air flows, but not great at low air speeds. Or vice versa. That's why the Cayman intake has two vacum actuated flap valves inside of it that are opened and closed at different RPMs. This allows the ECU to 'tune' the intake for best airflow at all air speeds.

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Gator Bite
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From all of my engineering courses, engine power (given a static displacement) has always been limited by the amount of air you can "cram" into the cylinders. That is why cars generally run stronger on those cool, fall mornings (cold air means it is more dense and therefore more can fit into the cylinders). Superchargers and turbos are another means of "forcing" more air in. Now, another way to help air in, is to move the spent air (exhaust) out of the way...modified exhaust. The modified exhaust usually has less restriction as mentioned above. Hope this helps

Thank you for all the great responses So the EVO air intake that is popular here works by combining a less restrictive air filter with the removal of the airbox enabling colder air to get inside the engine. Am I right?

Not exactly. The EVO air filter doesn't eliminate the airbox. It goes inside it. It simply replaces the EOM filter with one that is both less restrictive and shaped differently. Both of which are expected to allow more air flow than the OE filter.

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Gator Bite
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