By now you guys/girl(s) know the drill…credit to Ken from NEO
I guess the first question to answer would be why do cars have exhausts? Simply put, gases produced during the power stroke need to be scavenged from the cylinders so they can fill with fresh air and fuel and go kaboom again. So an exhaust system is primarily just a way of scavenging the gases. Now one would think any old pipe, and the shortest possible would be perfectly fine. But no. See, just like you want to have a smooth high-flowing intake to let your engine breath air in, you also want the exhaust to be optimized for getting it out.
The exhaust system is made of 4 parts:
. a header or exhaust manifold, that bolts onto the head;
. the piping
. a catalytic converter
. one or more mufflersThe header is the most important part of the exhaust system. The piping must be of the correct size such that you get good laminar flow, with results in high gas velocity and maximum scavenging. As the pulse of exhaust gas flows out, it actually creates a vacuum behind it, so next time the valve opens, that vacuum will suck out the gas and help it out of the cylinder. A header of a given length will optimize exhaust scanvenging at a particular rpm (pulses coming from each cylinder will “help” each other).
No in addition to this “header” effect, you also want the gas to flow well. Like I said, a smooth laminar flow is best. If the pipe diameter is too small, at high rpm, when the amount of gas flowing out of the engine is high, the exhaust system won’t be able to scavenge properly because of turbulence. Without good scavenging, more exhaust gas will remain in the cylinder, reducing the volumetriv efficiency of the engine. So small piping might be good in the low end, but it will kill in the top end. Notice I am NOT saying backpressure is good. Backpressure is ALWAYS bad. People say it’s needed because an exhaust system that is optimized for the low end with small piping will cause back-pressure in the top end. Because American engines produce a lot of torque in the low end, people assume back-pressure is needed for torque. Not true (in fact, there is very little or no back-pressure at the rpm where the torque is made!).
The converse is also true. If you put large piping to get good scavenging at high rpm, then gases won’t flow well at low rpm: in studying fluid dynamics, you find that you get turbulence when the flow is too high as well as when it is too low. That’s because the large diameter causes a large drop in exhaust gas velocity, which causes turbulence and poor scavenging at low rpm. So a large exhaust will produce more power in the top end, at the risk of killing the bottom end. The ideal exhaust has variable geometry: a smaller pipe coupled to a second pipe that is only active at high rpm. That way you get good scavenging at high and low end. BTW, that’s the same concept as variable intake geometries but for the exhaust. The Maxima is a car that has that type of system.
The catalytic converter is necessary to reduce many of the toxic and polluting gases produced by a gasoline engine. Because of the way it is made (see http://www.icat.dtu.dk/facultystaff/resasco-presentations/lecture7.pdf for an interesting presentation), it does produce an obstruction to flow. However, a properly functioning cat will, because of the way it is made, laminarize the flow, which can be a good thing. Although not wanted for race cars, keep yours on cause you really don’t need the extra few horses you’ll get from removing it. Driving skills alone will be far more important. And your children’s children might thank you later if we haven’t found another way to destroy the planet by then.
The muffler(s). Last but not least, this is meant purely to reduce the sound the exhaust produces. Although the sound will be stringly dependent on the piping diameter, the use of headers instead of a simple manifold (a manifold is like a header, but it’s largely compromised by space considerations so it’s not really optimised like a header: it just collects the gas as conveniently as possible), the muffler is key in reducing the sound your car makes. They work in various ways, typically with baffles (gases go into chambers and exit by some odd route) or louvered cores (gases transfer from one pipe to another by passing through small holes in the pipe and through acoustic deadening material) hereby reducing their velocity and the noise it makes. Because they reduce exhaust gas velocity, they will cause hp losses, but because they are far from the engine, it’s not as bad as it seems.
I hope this answered most of your questions. The unfortunate answer to the obvious question, what piping diameter should I use is, like everything else in engine tuning: it depends if you want better low end or top end response. The following table provides an answer, resulting in a compromise between the two.
Now, another question might be Can piping be too big? Of course! 3" piping might be OK (or optimal) for a 2.4l engine reving at 6-7krpm but it will kill the bottom end, so 4" piping would actually start killing the top end too by slowing down exhaust gases too much: unless the engine produces extremely large quantities of exhaust gas (by displacement, for induction or revving very high like a race car), you don’t need a large exhaust (Mark, where’s that Only one of these two really needs a 4" exhaust picture when I need it?..)
Hope this helps. As usual, questions and coments are welcome.
Ken
