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Thread: Backpressure: The myth and why it's wrong.

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    Backpressure: The myth and why it's wrong.

    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.




    Copyrighted from HCF (Sacicons)



    found this on another forum, thought I'd share it with you.
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    Performance Exhaust systems

    Performance Exhaust systems are a collection of altered and/or improved parts that add up to make an entire exhaust system and replaces what would have been a regular system.
    Performance exhaust is often referred to as aftermarket exhaust because it is relatively easy to acquire aftermarket exhaust upgrades. In fact it is much more common to see vehicles with aftermarket exhaust parts than vehicles with performance exhaust from the factory.
    A brief explanation on the regular exhaust system will be helpful in understanding the performance exhaust. A basic exhaust system is used to reduce noise (silencer/muffler), reduce emissions (catalytic converter) and to measure the oxygen content (sensors) as the gases leave the engine.
    Reducing noise and emissions restricts the free flow of the exhaust gases and this creates a back pressure all the way back to the engine and affects the four stroke cycle.
    Performance exhaust increases peak horsepower by reducing the amount of back pressure the exhaust sends by allowing a free flow of the gasses from the engine to the outside of the vehicle.
    Several methods can be used to achieve this free flow. One way is using a type of manifold called a header. This uses individual pipes from each cylinder of approximately the same length that merges into the down pipe and smoother curves.
    The equal length of the pipes creates a vacuum effect that pulls the gases out of the cylinder on each exhaust stroke then the smoother curves means less resistance of the airflow.
    Another way is by increasing the diameter of the downpipe and also using a minimum amount of curves and smooth angles for the few curves it may have. The muffler/silencer is also altered to reduce the back pressure and ultimately does less silencing.
    This change of sound from the vehicle results in a more desirable sound for a performance oriented vehicle. Some exhausts systems are even acoustically tuned to increase the noise and give the vehicle a louder or unique sporty sound especially during accelerating, downshifts or even when you take your foot off the accelerator pedal.
    Aftermarket exhausts also creates a visual appeal as the muffler of the vehicle will be different from a muffler on a non performance vehicle. There are persons who change the muffler on their vehicle just for that visual effect and the performance/aftermarket muffler is one method of identifying the vehicle as performance oriented.
    In any exhaust system the airflow and back pressure must be controlled as reducing back pressure increases peak horsepower but it reduces low end torque at low speed.
    Some performance exhaust systems use two stage mufflers with internal valves that can restrict airflow at low revs but at higher revs and speeds when too much pressure starts to back up the valve opens to allow a free flow.
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    Quote Originally Posted by JoeTheBoss View Post
    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.
    Will it? I mean, I know it'll come out at a higher pressure, but I'm sure the amount of water coming out is exactly the same (or a little less, depending how fat my finger is). I'm also assuming that flow rate is measured in terms of Volume / Time. So I can't see that the finger over hose technique has a positive impact on flow rate, it surely just maintains the existing flow rate (or lessens it) but increases the pressure that the exiting water is under?
    Dean_T likes this.

 

 

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