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关于排气背压，发一篇老外的看法。
一
Backpressure: The myth and why it's wrong.
I. Introduction
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure 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 backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.
II. Some basic exhaust theory
Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.
III. Backpressure 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 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 upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just 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, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure 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 is to have a pipe that that is as narrow as possible while having as close to zero backpressure 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 high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.
Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - 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 has two paths; one path is closed at low RPM but both are open at high RPM.
IV. So how did this myth come to be?
I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.
V. So why is exhaust velocity so important?
The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is 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 don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.
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大致的翻译如下，如有什么错误请见谅。
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背压：神话和为什么它是错的
1.介绍
在排气理论里一个被广泛误解的概念是背压。 人们喜欢在论坛上讨论背压，却没有真正理解它的含义以及它的由来。我相信很多人在讨论排气系统改装时听过这样一句话：“Honda需要背压”。这句话事实上完全是不正确的。
2. 基本的排气理论
排 气系统的设计目的就是为了尽快和尽可能有效地从燃烧室里派出废气。废气不是平滑地流动着，而是有一个脉冲。一个4缸的发动机每个循环有4个不同的脉冲，一 个六缸的发动机有6个，诸如此类。产生的脉冲数越多，排气流动越连续。背压可以大致定义为对正向流动的阻碍 - 在排气系统中就是对排气气流的阻碍。
3.背压和速度
有 些人在错误观念的误导下，认为粗的管子比细的管子能更有效地排气。产生这样的误解并不奇怪 -- 粗的管子比细的管子能流的更多。那么如果粗的能流的更多，为什么不是“粗的是好的”？ 一句话 --速度。 我相信你们一定玩过花园里的水头头， 如果你直接让水流出来它会流的相当慢，然而如果你用手指堵住龙头口的一部分，水流出的速度会快的多。
明智的排气设计者知道你必须在流量和 速度之间保持平衡。你想让废气以尽可能高的速度排出气缸--你需要更快速的气流。如果你有两个等体积的排气管，一个是2寸管一个是3寸管。那么两寸管里的 速度要快的多。当你知道越细的管子排气速度越大时，你想确保管子足够宽，以便有尽可能少背压力，同时保持适当的排气速度。背压在极端情况下可以导致排气气 流的反向，这是不好的。这里面的技巧是在你想要的转数范围内使用尽可能小的管子同时保证尽可能小的背压。排气管的直径只对某一个转数范围是最合适的。较小 的管径在低转数下产生较高的排气速度，却会造成高转速下较大的背压。因此如果你想要发力（？英文是powerband）的转速是在2-3千转，你需要一个 较窄的管子。 如果你需要的转数在8-9千转， 则需要大一点的管子。
许多发动机试图在管径固定的情况下，使用某种装置来创造像调节管径 这样类似的效果。比较高级的是Ferrari的包含两种排气路径的管子： 在低转数区只有一个路径打开以保持排气速度。但转数上升排气体积增大时，第二条路径打开以减小背压-- 因为这时候气体流量大不会造成速度损失。BMW和Nissan使用相对简单的方法：只有一条排气通道至消音器，在消音器里有两条路径，低转数时一条关闭， 高转速时全开。
4. 神话的来源。
****略过
5. 为什么排气速度如此重要。
排气脉冲运动的越快，越 能卷走（scavenge）在进排气阀重叠期间的燃烧过的废气。怎么样卷走废气有点超出了我们讨论的范围，但是通常的办法是快速运动的气流能在后部创造一 个低压区。这个低压区像真空泵一样抽走它后面的气体。 一个简单的例子是车辆在有灰尘的路上高速行时，它的尾部会产生低压区，灰尘颗粒被吸这个区域并粘在后面车身上。这种效果再卡车和掀背车上尤为显著。 （***）
6. 结论
因此结论是“Honda不需要背压”， 而是需要竟可能高的速度和竟可能小的背压。
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