http://www.thumpertalk.com/forum/showthread.php?t=659727 here: 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 "Engines 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 "Engines 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).
when the exhaust valve opens, a high pressure pulse travels down the exhaust pipe at the speed of sound. the length of the exhaust pipe determines 'when' the high pressure wave traveling at the speed of sound, hits the atmoshere (or the collector's 'wide spot'). 'when' it hits the atmosphere it reflects a low pressure wave back 'up' the pipe - also at the speed of sound. get the lengths right and the low pressure wave hits the valve face just is it starts to open for the next pressure pulse. The exhaust gases are then 'sucked' out of the cylinder by the reflected, low pressure wave. its 'harmonics' not back pressure and not (primarily) pipe diameter. Although pipe diameter does come into play to a smaller degree with respect to frictional losses which creates the unwanted backpressure. exhaust gases are a compressable fluid - they do not vary their velocity based on pipe diameter. the pulse travels at the speed of sound - irregardless of the pipe diameter. Speed varies with pipe diameter only for incompressable fluids (water). Rgds, Vincenzo
Thanks for the write up. I was under that same misconception for the last 20 years and now I know better, but how to explain to people in less than 10 words... there is the question?
Vincenzo- Exhaust flow is neither supersonic (~1000fps) nor transonic (~920-1020 FPS). The velocity is way lower than that. Taz Terry Phillips
My Duesy has a lever that adjusts exhaust back pressure. The owners manual says to use it when heading UP long hills to increase low speed torque.
This may help... from: http://en.wikipedia.org/wiki/Choked_flow "When a flowing fluid at a certain pressure and temperature flows through a restriction (such as the hole in an orifice plate or a valve in a pipe) into a lower pressure environment, under the conservation of mass the fluid velocity must increase for initially subsonic upstream conditions as it flows through the smaller cross-sectional area of the restriction." It goes on to say: "For homogeneous fluids, the physical point at which the choking occurs for adiabatic conditions is when the exit plane velocity is at sonic conditions or at a Mach number of 1" ("choked flow" means that no additional gas can flow thru a hole no matter how much differential pressure is present.) Down at the bottom of the article, you can see some typical Pu/Pd (upstream to downstream pressure) ratios for some sample gases that are required to generate sonic flow across a choke (ie: your exhaust valve). All this translate into a simple understanding.... when the upstream to downstream pressure differential across your exhaust valve is in the 1.5 to 2.5 range, the gas will go to a sonic condition as it exits thru the orifice (the exhaust valve opening). This sonic flow condition is what generates the sonic pulse (think of it as a sonic blast - - the sound of the explosion of gas out from the cylinder) and powers it down the exhaust pipe and thru the gases therein. As for the gases themselves, once they pass the orifice (valve seat) the Pu/Pd ratio goes to near zero and the gases quickly go subsonic. That said however, the sonic pressure pulse continues thru the subsonic gases at sonic levels. The sonic pulse will actually 'bounce' back and forth (high presssure waves then reflect back a low pressure wave etc etc) in a well tuned system. Don't confuse the sonic pulse speed with the gas speed as it travels thru the pipe. The 'harmonics' are based on the speed of sound and the sonic pressure pulse - not the physical speed of the overall volume of gas. I hope this helps clear it up.... its a fun science! Rgds, Vincenzo PS When I re-read my previous post I said: "exhaust gases are a compressable fluid - they do not vary their velocity based on pipe diameter." to be more clear it should have read: "exhaust gases are a compressable fluid - the sonic pulse does not change velocity based on pipe diameter."
most interesting. i'd seen that article years ago but never considered the finer points. BTW, as relates to 360/430, the reason the high rpm exhaust (muffler) valve is there is for noise regulations, not for engine tuning.
Vincenzo- Much more accurate. The exhaust noise, or sonic pulse, as you describe it, travels at a velocity dependent on the medium through which it passes. Slower in partial vacuum, and faster in compressed gasses, than through the open atmosphere. The subsonic exhaust gas flow is a mixture of partial vacuum and compressed gasses. The gas velocity is a function of pipe size and restrictions. Taz Terry Phillips
Really nice and interesting thread. OK then did Ferrari do a good job designing the exhaust for the CS and 430? It seems like they tried to address the variable demands of engine RPM by using a valve and two different lengths of pipes. Although there are problems with the pre-cats and heat and vibration related metal cracks, do the aftermarket systems offer a more sophisticated system?
MG- Yes they did. An exhaust system works like the nozzle on a rocket engine. The idea is to keep the nozzle or exhaust filled for best performance, without slowing down the flow. At low throttle and low exhaust flow, you want less pipe to optimally fill the exhaust, just like rocket nozzles need to be small (expansion ratio) at low altitude and high atmospheric pressure for best performance. As you increase throttle in a Ferrari, the pneumatic valves open, effectively giving more pipe volume for the exhaust gasses. At high altitude and low atmospheric pressure, you want a big nozzle (expansion ratio) on a rocket so the plume can fully expand and provide maximum performance. Taz Terry Phillips
Ray- Exhaust leaks do two things: first they introduce turbulence into the system, which impedes smooth airflow, and second, they tend to depressurize the exhaust gas flow and keep it from flowing smoothly and extracting the exhaust smoothly. Leaks also create a local hot spot as hot exhaust gas mixes with oxygen rich air and this can widen the leak and, depending on location, create the possibility of a fire. But it all comes down to normal exhaust is a series of pulses of dense gasses and partial vacuum between the pulses, flowing smoothly down the exhaust pipes. Any disruption lowers efficiency. Salzburg- If you have the valves open all the time, at low speeds there is not enough exhaust flow to completely fill the exhaust system and turbulence will occur at the outer edge of the pipes, decreasing flow efficiency. So you will get more exhaust sound, but lower overall efficiency at low speeds. Most people only care about the sound, at any rate. Taz Terry Phillips
Our engines (Ferrari engines) need back pressure in the lower RPMs or we (Ferrari drivers and almost all cars) will have no power or torque. Without back pressure, at lower RPMs the intake stroke will be mixing with exhaust gasses thus lowering power on the up stoke. BTW, having a smaller diameter exhaust will increase back pressure.
Thanks Terry! if I apply a lot of throttle at very low rpm(<1500rpm), especially going uphill, the muffler would starting shaking and vibrating heavily. Can you explain that please?
You need to remember that the wave is a wave of pressure and moves at the speed of sound in the gasseous environment that it is in. This just happens to be a gas of between 1600 and 1300 degees F. The speed of sound is considerably higher at these leevated temperatures found in the header. Here, the SoS is closer to 1300 fps than 1050 fps at STP {59dF sea level pressures 50% humidity}.
So, when I read that Fabspeed has a product such as the new tips without "valves", does that mean they are eliminating the valves, and if so, doesn't that mean that the torque should be way down at the low revs? Their dyno charts show the torque up throughout the entire RPM range.
correct. all the corvette z06 aftermarket exhaust guys also have dyno showing increased torque without the valve, in fact using the stock exhaust and removing the butterfly valve fuse (to force it open always) increases torque. the butterfly valve is there solely for noise laws.
Hmmm, this puts a serious dent in the theory that back pressure is not required ... and yet nobody has commented on this?? More researched required Pete
