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The Myth Of Backpressure

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    Posted: 20-May-2010 at 3:03PM
  …is probably the most widely misunderstood concept in engine tuning.  IMO, the reason this concept is so hard to get around lies in the  engineering terms surrounding gas flow. Here's the most impotant ones  you need to be aware of to understand the things I'm about to say:
  BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI.
  DELTA PRESSURE (aka delta P): Describes the pressure drop through a  component and is the difference in pressure between two points.
  One other concept needs to be covered too, and that's the idea of air  pressure vs. velocity. When a moving air column picks up speed, one of  the weird things that happens is it's pressure drops. So remember  through all this that the higher the air velocity for a given volume of  gas, the lower it's internal pressure becomes. And remember throughout  all of this that I'm no mechanical engineer, simply an enthusiast who  done all the reading he can. I don't claim that this information is the  absolute truth, just that it makes sense in my eyes.
  Ok, so as you can see, backpressure is actually defined as the  resistance to flow. So how can backpressure help power production at  any RPM? IT CAN'T. I think the reason people began to think that  pressure was in important thing to have at low RPM is because of the  term delta pressure. Delta pressure is what you need to produce good  power at any RPM, which means that you need to have a pressure DROP  when measuring pressures from the cylinder to the exhaust tract (the  term "pressure" is what I think continually confuses things). The  larger the delta P measurement is, the higher this pressure drop  becomes. And as earlier stated, you can understand that this pressure  drop means the exhaust gas velocity is increasing as it travels from  the cylinder to the exhaust system. Put simply, the higher the delta P  value, the faster the exhaust gasses end up traveling. So what does all  this mean? It means that it's important to have gas velocity reach a  certain point in order to have good power production at any RPM  (traditional engine techs sited 240 ft/sec as the magic number, but  this is likely outdated by now).
  The effect of having larger exhaust pipe diameters (in the primary,  secondary, collector and cat-back exhaust tubes) has a direct effect on  gas velocity and therefore delta P (as well as backpressure levels).  The larger the exhaust diameter, the slower the exhaust gasses end up  going for a given amount of airflow. Now the ***** of all this tech is  that one exhaust size will not work over a large RPM range, so we are  left with trying to find the best compromise in sizing for good low RPM  velocity without hindering higher RPM flow ability. It doesn't take a  rocket scientist to understand that an engine flows a whole lot more  air at 6000 RPM than at 1000 RPM, and so it also makes sense that one  single pipe diameter isn't going to acheive optiaml gas velocity and  pressure at both these RPM points, given the need to flow such varying  volumes.
  These concepts are why larger exhaust piping works well for high RPM  power but hurts low RPM power; becuase is hurts gas velocity and  therefore delta P at low RPM. At higher RPM however, the larger piping  lets the engine breath well without having the exhuast gasses get  bundled up in the system, which would produce high levels of  backpressure and therefore hurt flow. Remember, managing airflow in  engines is mainly about three things; maintaining laminar flow and good  charge velocity, and doing both of those with varying volumes of air.  Ok, so now that all this has been explained, let's cover one last  concept (sorry this is getting so long, but it takes time to explain  things in straight text!).
  This last concept is why low velocity gas flow and backpressure hurt  power production. Understand that during the exhaust stroke of a 4  stroke engine, it's not only important to get as much of the spent  air/fuel mixture out of the chamber (to make room for the unburnt  mixture in the intake system), it's also important that these exhaust  gasses never turn around and start flowing back into the cylinder. Why  would this happen? Because of valve overlap, that's why. At the end of  the exhaust stroke, not only does the piston start moving back down the  bore to ingest the fresh mixture, but the intake valve also opens to  expose the fresh air charge to this event. In modern automotive 4  stroke engines valve overlap occurs at all RPM, so for a short period  of time the exhaust system is open to these low pressure influences  which can suck things back towards the cylinder. if the exhaust gas  velocity is low and pressure is high in the system, this will make  everything turn around and go the opposite direction it's supposed to.  If these gasses reach the cylinder they will dilute the incoming  mixture with unburnable gasses and take up valuable space within the  combustion chamber, thus lowering power output (and potentially pushing  the intake charge temp beyond the fuel's knock resistance). So having  good velocity and therefore low pressure in the system is absolutely  imperative to good power production at any RPM, you just have to  remember that these concepts are also dependent on total flow volume.  The overall volume of flow is important because it is entirely possible  to have both high velocity and high pressure in the system, if there is  simply not enough exhaust piping to handle the needed airflow.
  It's all about finding a compromise to work at both high and low RPM on  most cars, but that's a bit beyond the scope of this post. All I am  trying to show here is how the term backpressure is in reference to a  bad exhaust system, not one that creates good low RPM torque. You can  just as easily have backpressure at low RPM too, which would also hurt  low RPM cylinder scavenging and increase the potential for gas  reversion. And understand that these tuning concepts will also affect  cam timing, though that is again probably beyond the scope of this  post. At any rate, hope this helps, peace.
  Here's a reply to the above post:
  "I've been seeing a resurgence of the backpressure misnomer, but didn't  have the time or inclination to write it up. So, again, thanks.
  There is one thing I'd like to add to texan's work:
  Exhaust Scavenging
  In essence, this is the opposite of the exhaust reversion that texan describes.
  Reversion: at the beginning of the intake stroke during cam overlap,  exaust gas in the header is under high pressure (negative delta P) and  is pushed back into the cylinder, diluting the new air/fuel charge.
  Scavenging: at the beginning of the intake stroke during cam overlap,  the momentum of the exiting exhaust gasses creates a brief vacuum  (positive delta P) in the header, pulling out the remaining exhaust  gases from the combustion chamber, and allowing the new air/fuel charge  to be full-strength.
  Scavenging is also the reason for differently shaped headers (4-2-1,  4-1) and collectors. We use the momentum of exiting exhaust from one  cylinder to scavenge exhaust from another that is next in the firing  order! The different shapes allow for this to happen at different  airflow velocities thus at different RPM bands.
  Scavenging takes advantage of the momentum of the exiting gasses. In  essence, the fast moving exhaust pulse pulls a vacuum behind it.  Momentum is mass times velocity. So not only do we need to keep the  velocity high to prevent reversion - but it greatly improves the  scavenging effect.
  Thus we have a balancing act (as others have pointed out). We want to  minimize friction to lower the backpressure as much as possible -  larger pipes have less friction because they have less surface area per  unit volume. But we want to increase the delta P as much as possible to  prevent reversion and increase scavenging effects - smaller pipes  increase delta P because they increase velocity.
  There are lots of tricks to try to widen the useful RPM band (stepped  headers) or to increase the overall effiency (ceramic coated exhausts),  but it's still subject to this basic tradeoff:
  Friction vs. Velocity
  AKA: Backpressure vs. Delta Pressure
  You want low friction and high velocity.
  You want low backpressure and high positive delta pressure. "
  Credit given to Texan and Fritz for their info on this topic.

"Needing Backpressure - Myth or Reality?

The goal of any exhaust system is to efficiently remove burnt gases from the combustion chamber, prevent reversion at overlap, and by enhancing exhaust gas velocity leaving the chamber, create a vacuum to help draw or scavenge in more intake charge volume at cam overlap.

The key is maintaining exhaust gas velocity or energy as the gases leave the exhaust port when the exhaust valve opens.

So as the exhaust gas leaves the exhaust port in a 4 stroke engine , it creates a series of pressure waves travelling at the speed of sound that move towards the exhaust tip (or forwards) and then some reflects back. Like the water waves coming onto the beach, forward and back, forward and back. The main overall direction is forwards but there is some reflection back to the exhaust port (reversion).

Simple enough...everyone knows this. So what's new and groovy?

The problem is at cam overlap (when both the exhaust valve and intake valve are both partially open and when the pressure in the chamber is greater than in the intake port).

If a high pressure wave is reflecting back and arrives at the exhaust port at the wrong time (i.e. when burnt gases still need to leave), it blocks the flow out. You see these instances when a high pressure wave is reflected back at the wrong time as dips in the torque curve AT REGULAR INTERVALS (usually in the midrange rpms).

If a low pressure wave is reflecting back at the correct time at the exhaust port it actually helps pull burnt gases out of the chamber and also helps pull in more intake air/fuel at overlap. You see these favourable low pressure reflected waves occurring on your torque curve as small torque increases AT REGULAR INTERVALS.

Now here's the first bone of contention and a source of debate between exhaust makers.

1. Is a reflected high pressure wave always bad?

Most of the experienced people I speak to and read on the various boards say YES! You never want backpressure and you want it as low as possible for as long as possible. The low backpressure assists in maintaining that high exhaust gas velocity. They then design anti-reversion chambers and/or place steps (increases in diameter at various proprietary points along the length of the header) to prevent the reflected waves from travelling back to the head.

There are also some pretty smart people who believe slightly differently ...They believe that if you have a high pressure reflected wave arriving a few milliseconds before exhaust valve closure, you prevent the loss of intake air:fuel out the exhaust valve at cam overlap. The exhaust backpressure at this crankshaft degree in the exhaust stroke prevents leaking out or bleeding out of you intake charge into the header and ensures all of it goes into the chamber for combustion.

However, these people do NOT use the exhaust diameter as a way to create this backpressure. That would be too crude or less precise, since the backpressure would exist at all times and they only want this backpressure over the few crankshaft degrees when the exhaust valve is just about to close ,when the intake valve is opening further, and the piston has reached TDC and starts downward for the intake stroke. Using an exhaust just to have backpressure then is like cutting butter with a chain saw.

The people who agree with this will often tell you that combustion chamber and intake port pressures are higher than the pressure in the exhaust just before exhaust valve closure . So some intake flow into the chamber can get pushed out the closing exhaust valve by the higher combustion chamber pressures.

So all you guys that say backpressure is a good thing...I don't believe so...not at all crankshaft degrees which is what you get with a restrictive diameter exhaust. You don't want to have too big a diameter (actually it's cross-sectional area) that will slow or kill velocity or energy. But no backpressure most (99%) of the time is good.

2. How do we get low pressure waves and high pressure wave to arrive at the correct time?

The conventional way to get the exhaust gas harmonic to do this dance of low pressure to pull in more intake charge and high pressure to prevent bleeding off all at the right time is by changing the tube layout on the header: using lengths, diameters, collectors with various merge angles. But these are limited to one harmonic or exhaust gas speed.

So some Japanese engineers at Yamaha (figures, it's always some genius engineer at some bike manufacturer that comes up with these wild ideas) thought: "What if you have an exhaust throttle valve (located in the header collector or at the entrance to the secondary tubes in the first merge collector) that could control the pressure wave behaviour?".

The throttle valve angle would vary as the speed of the exhaust gases changed to control the reflected waves. In an 11,000 rpm bike, the valve opens progressively as the rpms climb as the tubes are "in step" with the engine harmonics and less reflected waves occur but at around 7000 rpm, the valve is closed down to 40-60% of wide open when the harmonic is "out of step" with the engine and at 8500 rpm the exhaust throttle valve is progressively opened. How much to change the throttle angle is based on crankshaft angle input or ignition signal input to an ECU with then controls the throttle valve angle knowing the harmonics of the engine.

We see these in the Mercedes McLaren F1 car. If you think this is somebody's Frankenstein pipe dream then guess again. The new Suzuki GSXR1000, Honda Fireblade, and Yamaha R1 already have these. And those are today's street bikes! Can the new RSX and Civic Si be that far away from the next stage forward for more power? The impetus will not be performance oriented but the drive to bring this to the market place will likely be more practical, as this throttle valve (the first one was called the Exup or Exhaust Ultimate Power by Yamaha in the late 80's) gains better emissions and lower exhaust noise (less pollution is good ...admit it Kyoto is the right thing to do).

So the new toy for exhaust makers will be like variable valve timing and variable cam timing...the mating of electronics to optimise exhaust harmonics at each rpm as the harmonics change with the rpms climbing. It won't be just cut and try any will be cut try and reprogram. Welcome to the new millenium. "

Credit Given to Tuan at SHO.

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.
* * *

All info taken from this website

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