Back pressure: The myth and why it’s wrong.
One of the most misunderstood concepts in exhaust theory is back pressure. People talk about it 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 “Engines need back pressure” when discussing exhaust upgrades. Sadly, that phrase is completely inaccurate and a wholly misguided notion.
1. 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; 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. Back pressure can be loosely defined as the resistance to positive flow – in this case, the resistance to positive flow of the exhaust stream.
2. Back pressure and velocity
Many people mistakenly believe that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It’s not hard to see how this idea would be appealing – as wider pipes have the capability to flow more than narrower pipes. However, this omits the concept of exhaust VELOCITY. Here is an analogy…a garden hose without a spray nozzle on it. If you let the water just run unrestricted out of the hose it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will spray 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. (see below) 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 travelling 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 also want make sure the pipe is wide enough so that there is as little back pressure as possible while maintaining 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 is to have a pipe that that is as narrow as possible while 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 back pressure at high rpm. Thus if your power band is located 2-3000 RPM you’d want a narrower pipe than if your power band is located at 8-9000RPM.
Many engineers try to work around the RPM specific nature of pipe diameters by using set-ups 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 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 has two paths; one path is closed at low RPM but both are open at high RPM.
3. The Manifolds
The manifold is an assembly designed to collect the exhaust gas from the engine cylinders into exhaust pipes, (either one or two with Morgans). Manifolds are often made of cast iron in stock cars. Many have material-saving design features to use the least metal, to occupy the least space necessary, and/or have the lowest production cost. These restrictions often result in a design that is cost effective but that does not do the most efficient job of venting the gases from the engine.
Inefficiencies generally occur due to the nature of the combustion engine and its cylinders. Since cylinders fire at different times, exhaust leaves them at different times, and pressure waves from gas emerging from one cylinder might not be completely vacated through the exhaust system when another comes. This creates back pressure and restriction in the engine’s exhaust system and limits the engine’s true performance possibilities.
A header (aka branch manifolds or extractors) is a manifold specifically designed for performance. Engineers create a manifold without regard to weight or cost but instead for optimal flow of the exhaust gases. These designs can result in more efficient scavenging of the exhaust from the cylinders. Headers are generally circular steel or stainless tubing with bends and folds calculated to make the paths from each cylinder’s exhaust port to the common outlet all equal length, and joined at narrow angles to encourage pressure waves to flow through the outlet, and not backwards towards the other cylinders. In a set of tuned headers the pipe lengths are carefully calculated to enhance exhaust flow in a particular engine revolutions per minute range and married to the firing sequence.
3. How did the myth about back pressure and big pipes come to be?
I believe it is/was a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a Honda Civic decides he’s going to upgrade 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 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 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 back pressure with a much narrower pipe – in that way he would not have lost all his flow velocity.
4. 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 article 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 travelling 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).
Many designers will increase the length of the exhaust, trying to achieve a faster flow and a larger area of low pressure. Short pipes create a smaller low pressure area..
Conclusion. Each engine will before function its best best with a exhaust system balanced to its needs and characteristics with a goal of to producing the the highest exhaust gas speeds.