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Old 01-30-2003, 10:46 AM   #1
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Default Back Pressure FAQ

This FAQ should hopefully answer all back-pressure questions.

One of the most misunderstood concepts in exhaust theory is back-pressure. People love to talk about back-pressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need back-pressure" when discussing exhaust upgrades. That phrase is, in fact, completely inaccurate and a wholly misguided notion.

I. Some Basic Exhaust Theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficiently. Exhaust gases are not produced in a smooth stream; they originate in pulses. A 4-cylinder motor will have four distinct pulses per complete engine cycle, a 6-cylinder has six pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Back-pressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

II. Back-pressure and Velocity

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more air than narrower pipes. So, if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrades? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose without a spray nozzle on it. If you let the water run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible; you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases will be, you want make sure that the pipe is wide enough so that there is as little back-pressure as possible while simultaneously maintaining a suitable exhaust gas velocity.

Back-pressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick then, is to have piping that is as narrow as possible while also having as close to zero back-pressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at higher RPMs. Thus, if your power band is located within the 2-3000 RPM range you'd want a narrower pipe than you would if your power band is located within the 8-9000 RPM range.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect to a change in pipe diameter on the fly. The most advanced system is Ferrari's, which consists of two exhaust paths after the header. At low RPMs, only one path is open so as to maintain exhaust velocity, but as the RPMs climbs and exhaust volume increases, the second path is opened to curb back-pressure. Since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method: there is a single exhaust path to the muffler; the muffler itself has two paths; one path is closed at low RPMs but both are open at higher RPMs.

III. So How Did this Myth Come to Be?

I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it stems from a misunderstanding of what happens with the exhaust stream as pipe diameter changes. For instance, someone with a Civic decides to uprade the exhaust with 3" diameter piping. Once it's installed, the owner notices that the car seems to have lost a good bit of power throughout the power band. He makes the connections in the following manner: "My wider exhaust eliminated all back-pressure but I lost power, therefore the motor must need some back-pressure in order to make power." What he didn't realize is that he killed off all of his flow velocity by using such a ridiculously wide exhaust pipe. It would have been possible for him to achieve close to zero back-pressure with a much narrower pipe; in that way he would not have eliminated his flow velocity.

IV. So Why is Exhaust Velocity So Important?

The faster an exhaust pulse moves, the better it can scavenge out all of the gases spent during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this FAQ but the general idea is that a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle; dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas, giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

VI. Conclusion

So it turns out that Hondas really don't need much backpressure; instead, they need as high a flow velocity as possible with as little back-pressure as possible.
92 Prelude Si
H23A3, self-installed

Last edited by 94accordex; 04-03-2007 at 10:30 PM. Reason: Added additional formating
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