[Scrawny]

I want to hear your thoughts on this subject.

[matty87]

back pressure is a myth end of story.

[Scrawny]

I pinched this from another forum.... It pretty much sums it all up





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).

[Westie]

I have been told that some cars if you open the exhaust up to much it will loose performance. But when you watch all these dyno shows the first thing they do is wack a 3-3 1/2" exhaust to get more power.

When I got the Vit done I wanted 2" pipe and they said NO as they ended up putting a resonator on it as well to keep the pressure there.

Personally I do agree some need it and some do better without it.

LPG cars they say need a larger exhaust because the gas burns at a higher temp.
On LCOOL site I was interested in opening the 100 series up when we LPG it and after reading some of the stuff on there went off the idea. The people that placed extractors and 3" on lost heaps of power in towing and they said it was due to the extractors and mufflers so the went back to the std headers and put a res on the back of the system and it went better.


So who knows it depends who you talk to , and it always seems the louder the note the better it performs or is that the HOON in us all

[royce]

I think velocity is a very very important word there

[Scrawny]

More reading here....

I find this stuff really interesting. Yes, there is some truth to the back pressure idea but the terminology is wrong. It's not back pressure you want, it's velocity.

Apparently bigger is not always better

[BUZOOKA]

so it is tru that if u go tobig of a pipe it turns to shit


so the muffler place knew wat he was on about jsut in the wrong area of the problem

[royce]

yes the correct size is whats needed, hence why the towing guys think extractors are bad, they want as much happening at low revs as they can, without worrying about what happens up higher

Scrawn, ive got a pretty good book on it all if you want a look

[want33s]

I've seen a 351 with twin 2.5" pipes make 410hp on a dyno.

The same car, same engine, same 4 into 1 extractors with a 3" Y pipe, 3" to the diff and twin 2" tailpipes made 425hp.

Not sure if the first setup was too large but the second sure worked better. The guy that built it said the smaller 2" tailpipes made the difference because as the exhaust cools it shrinks in volume and smaller pipes keep the velocity up.

[Damo]

want33s wrote:

I've seen a 351 with twin 2.5" pipes make 410hp on a dyno.

The same car, same engine, same 4 into 1 extractors with a 3" Y pipe, 3" to the diff and twin 2" tailpipes made 425hp.

Not sure if the first setup was too large but the second sure worked better. The guy that built it said the smaller 2" tailpipes made the difference because as the exhaust cools it shrinks in volume and smaller pipes keep the velocity up.

Was it was quieter with the 2" tail pipes too?

[want33s]

Damo wrote:

Was it was quieter with the 2" tail pipes too?

I wouldn't say quieter but it sounded better.. Took the clappiness out of it.

[royce]

its the same with intakes and even ports as well, sometimes a lot smaller port will make more power than jsut grinding the crap out of it all

[eldo]

i have seen heaps of bikes over the years turn to shit due to owners putting pipes on, you always need to tune the exhaust to what ever part of the power band you intend on using the vehicle in. in other words low down torgue for 4x4 and outright high end for racing etc.

[mightymouse]

Just another thought on the same theme, If you create a better area for the fuel to burn, that is cleaner environment you will need some improvement on the intake side too. To explain this just look at a 2 stroke with basanni chambers on it the shape creates a scavenging affect therefore cleaning all the burnt gas from within the combustion chamber, ltherefore making it necessary to add extra fuel to fill it again, for another burn.

[IN2DEP]

want33s wrote:

I've seen a 351 with twin 2.5" pipes make 410hp on a dyno.

The same car, same engine, same 4 into 1 extractors with a 3" Y pipe, 3" to the diff and twin 2" tailpipes made 425hp.

Yeah but say that was it done the same day, did the temp drop so on so on, My cuzin dyno his Turbo gti we got about 8 runs in and he went from 110kw to 107 then back up to 115kw

The car wont have the same power through out the day or night, But i do belive with more power the bigger exhurst you need, Better flow

[matty87]

the really long post above is really good at summing up the back pressure velocity deal and its very correct.

There is however a huge deal more to exauhst technology than just that. For example as mentioned above the two systems on the 351 with the the y pipe system is a far more advanced system and the crossover in it means that one set of 4 cylinders help to scavange the other bank its all about creating lower pressures at the exhaust valves.

you may also have heard about heat reflective coatings on exhausts to keep engin bays cool, well these coatings also help the exhaust function better too, as the longer you keep the gasses hot the faster the flow will be because as the gasses cool the contract and thus slow down, that is why ceramic coatings are raved about on high end systems.

now a bit off topic but turbo exhausts are completly different as what is wanted here is for all the pressure drop to be across the turbine in the turbo and as such all cother components in the system are made such that they dont cause any pressure drop.

[2stroker]

I love this topic.......

No exhaust system is ideal for all applications. Depending on their design and purpose, all exhaust systems compromise something to achieve something else. Before performing exhaust changes or modifications to increase performance, it is critical to determine what kind of performance you want.
• Do you want the best possible low-end and mid-range power or maximum top-end power?
• Will you be using an aftermarket cam with different lift, duration, timing and overlap?
• Have you investigated the relationship between torque (force) and horsepower (amount of work within time)?
• Do you want a cosmetic exhaust system or a performance exhaust system?
Without careful thought about these variables, an exhaust system can yield very disappointing results. On the other hand, a properly designed and tuned exhaust system that is well-matched to the engine can provide outstanding power gains.

The distinction between "maximum power" and "maximum performance" is significant beyond general discussion. Realistically, one exhaust system may not produce both maximum power and maximum performance. For a vehicle to cover "X" distance as quickly as possible, it is not the highest peak power generated by the engine that is most critical. It is the highest average power generated across the distance that typically produces the quickest time. When comparing two horsepower curves on a dynamometer chart (assuming other factors remain constant), the curve containing the greatest average power is the one that will typically cover the distance in the least time and that curve may, or may not, contain the highest possible peak power.

The potential power of an engine is determined by the proper amount of fuel available for combustion. However, the efficiency of combustion and engine pumping processes is profoundly influenced by the exhaust system. A properly designed exhaust system can reduce engine pumping losses. Therefore, the design objective for a high performance exhaust is (or should be) to reduce engine-pumping losses, and by so doing, increase volumetric efficiency. The net result of reduced pumping losses is more power available to move the vehicle. As volumetric efficiency increases, potential fuel mileage also increases because less throttle opening is required to move the vehicle at the same velocity.

Much controversy and confusion surrounds the issue of exhaust "back-pressure".

For performance purposes, "backpressure" in an exhaust system increases the engine-pumping losses and therefore decreases available engine power. Most engines are mechanically tuned to "X" amount of exhaust gas velocity and flow and will show a loss of low-end torque if velocity is decreased by increasing the flow, but may show greater mid-to-high RPM power when that flow is increased, reducing the volume flowed to increase the velocity is commonly refered to as backpressure.

The objective of most engine modifications is to maximize air/fuel flow into the engine and maximize the exhaust flow out of the engine. The inflow of an air/fuel mixture is a separate issue, but it is directly influenced by exhaust flow, particularly during valve overlap (when both valves are open for "X" degrees of crankshaft rotation).

Theoretically, in a normally aspirated state of tune an engine’s maximum power potential is directly proportional with the volume of air it flows. This means that an engine of 80 cubic inches has the same maximum power potential as an engine of 100 cubic inches, if the 100 cubic inch engine is restricted to the same flow volume as the 80 cubic inch engine. The powerband characteristics of the two engines may be quite different but the peak attainable power is essentially the same.

Flow Volume & Flow Velocity
One of the biggest issues with exhaust systems, is the relationship between gas flow volume and gas flow velocity (which also applies to the intake system). An engine needs the highest flow velocity possible for quick throttle response and torque throughout the low-to-mid range portion of the power band. The same engine also needs the highest flow volume possible throughout the mid-to-high range portion of the powerband for maximum performance. This is where a fundamental conflict arises. For "X" amount of exhaust pressure at an exhaust valve, a smaller diameter exhaust pipe will provide higher flow velocity than a larger diameter pipe. Unfortunately, the laws of physics will not allow that same small diameter pipe to flow sufficient volume to realize maximum possible power at higher RPM. If we install a larger diameter pipe, we will have enough flow volume for maximum power at mid-to-high RPM, but the flow velocity will decrease and low-to-mid range throttle response and torque will suffer. This is the primary paradox of exhaust flow dynamics and the solution is usually a design compromise that produces an acceptable amount of throttle response, torque and horsepower across the entire powerband.

A very common mistake is the selection of an exhaust system with pipes that are too large in diameter for their engine's state of tune. Bigger is not necessarily better and is often worse.

Equal Length Exhaust
The effectiveness of equal length exhaust is widely debated. Assuming that an exhaust system is otherwise properly designed, equal length pipes offer some benefits that are not present with unequal length pipes. These benefits are smoother engine operation, tuning simplicity and increased low-to-mid range torque.

If the pipes are not equal length, both inertial scavenging and wave scavenging will vary among engine cylinders, often dramatically. This, in turn, causes different tuning requirements for different cylinders. These variations affect air/fuel mixtures and timing requirements, and can make it very difficult to achieve optimal tuning. Equal length pipes eliminate these exhaust-induced difficulties. "Tuning", in the context used here, does not mean installing new sparkplugs and an air filter. It means configuring a combination of mechanical components to maximum efficiency for a specific purpose and it can not be overemphasized that such tuning is the path to superior performance with a combination of parts that must work together in a complimentary manner.

In an exhaust system that is properly designed for it’s application, equal length pipes are generally more efficient. The lengths of both the primary and main section of pipes strongly influence the location of the torque peak(s) within the powerband. In street and track performance engines with longer pipes typically produce more low-to-mid range torque than shorter pipes and it is torque that moves the vehicle. The question is... Where in the powerband do you want to maximize the torque?
• Longer pipes tend to increase power below the engine’s torque peak and shorter pipes tend to increase power above the torque peak.
• Large diameter pipes tend to limit low-range power and increase high range power.
• Small diameter pipes tend to increase low-range power and to some degree limit high-range power.
• "Balance" or "equalizer" chambers between the exhaust pipes tend to flatten the torque peak(s) and widen the powerband.
Among the more astute and responsible exhaust builders, it is more-or-less understood that pipe length variations should not exceed 1" to be considered equal. Even this standard can result in a 2" difference if one pipe is an inch short and another pipe is an inch long.

Exhaust Scavenging and Energy Waves
Inertial scavenging and wave scavenging are different phenomena but both impact exhaust system efficiency and affect one another. Scavenging is simply gas extraction. These two scavenging effects are directly influenced by pipe diameter, length, shape and the thermal properties of the pipe material (stainless, mild steel, thermal coatings, etc.). When the exhaust valve opens, two things immediately happen. An energy wave, or pulse, is created from the rapidly expanding combustion gases. The wave enters the exhaust pipe traveling outward at a nominal speed of 1,300 - 1,700 feet per second (this speed varies depending on engine design, modifications, etc., and is therefore stated as a "nominal" velocity). This wave is pure energy, similar to a shock wave from an explosion. Simultaneous with the energy wave, the spent combustion gases also enter the exhaust pipe and travel outward more slowly at 150 - 300 feet per second nominal (maximum power is usually made with gas velocities between 240 and 300 feet per second). Since the energy wave is moving about 5 times faster than the exhaust gases, it will get where it is going faster than the gases. When the outbound energy wave encounters a lower pressure area such as a second or larger diameter section of pipe, the muffler or the ambient atmosphere, a reversion wave (a reversed or mirrored wave) is reflected back toward the exhaust valve without significant loss of velocity.

The reversion wave moves back toward the exhaust valve on a collision course with the exiting gases whereupon they pass through one another, with some energy loss and turbulence, and continue in their respective directions. What happens when that reversion wave arrives at the exhaust valve depends on whether the valve is still open or closed. This is a critical moment in the exhaust cycle because the reversion wave can be beneficial or detrimental to exhaust flow, depending upon its arrival time at the exhaust valve. If the exhaust valve is closed when the reversion wave arrives, the wave is again reflected toward the exhaust outlet and eventually dissipates its energy in this back and forth motion. If the exhaust valve is open when the wave arrives, its effect upon exhaust gas flow depends on which part of the wave is hitting the open exhaust valve.

A wave is comprised of two alternating and opposing pressures. In one part of the wave cycle, the gas molecules are compressed. In the other part of the wave, the gas molecules are rarefied. Therefore, each wave contains a compression area (node) of higher pressure and a rarefaction area (anti-node) of lower pressure. An exhaust pipe of the proper length (for a specific RPM range) will place the wave’s anti-node at the exhaust valve at the proper time for it’s lower pressure to help fill the combustion chamber with fresh incoming charge and to extract spent gases from the chamber. This is wave scavenging or "wave tuning".

From these cyclical engine events, one can deduce that the beneficial part of a rapidly traveling reversion wave can only be present at an exhaust port during portions of the powerband since it's relative arrival time changes with RPM. This makes it difficult to tune an exhaust system to take advantage of reversion waves which is why there are various anti-reversion devices designed to improve performance. These anti-reversion devices are designed to weaken and disrupt the detrimental reversion waves (when the wave's higher-pressure node impedes scavenging and intake draw-through). Specifically designed performance baffles can be extremely effective, as well as heads with D shaped ports. Unlike reversion waves that have no mass, exhaust gases do have mass. Since they are in motion, they also have inertia (or "momentum") as they travel outward at their comparatively slow velocity of 150 - 300 feet per second. When the gases move outward as a gas column through the exhaust pipe, a decreasing pressure area is created in the pipe behind them. It may help to think of this lower pressure area as a partial vacuum and one can visualize the vacuous lower pressure "pulling" residual exhaust gases from the combustion chamber and exhaust port. It can also help pull fresh air/fuel charge into the combustion chamber. This is inertial scavenging and it has a major effect upon engine power at low-to-mid range RPM.

There are other factors that further complicate the behavior of exhaust gases. Wave harmonics, wave amplification and wave cancellation effects also play into the scheme of exhaust events. The interaction of all these variables is so abstractly complex that it is difficult to fully grasp. There does not appear to be any absolute formula that will produce the perfect exhaust design. Even super-computer designed exhaust systems must undergo dyno, track, and street testing to determine the necessary configuration for the desired results. Last but not least, the correct choices and combinations of carburetor, air cleaner, cam shaft, ignition, and exhaust used in the proper relationship to each other for the intended riding application will always produce the finest quality results.

[Derzky]

All i can say is that i bought a ex-race gemini that had a 2.2lt with twin dd webbers, stage III head and extractors with a 2 1/2" collector followed by a short 2 1/2" pipe with a tiny muffler with a dump pipe welded on the other side of it.

This thing idled at around 1100 rpm when i got it (any lower and it stopped)
It was loud as all hell

I took it for a spin up the road to show my mates before i got it registered and i ripped the muffler off going into my mates driveway.

With no muffler on the system the car would not idle by itself.
I had to keep the revs at approx 1800 - 2000rpm and when i drove it to the exhaust shop it wanted to run at approx 4500 - 5500+ rpm, any lower and it lost "power"

It sounded red-hot
lets just say that the boys at the exhaust shop heard me coming 5min before i got there

I thought i was peter brock for 10min

[MacDaddy]

matty87 wrote:

back pressure is a myth end of story.

X 2
Back pressure is a 100% Myth !!!

Have any of you watched the Red Bull Air Race, when they do the super slow motion of the planes, if you look at the exhaust, the exhaust gases come out in puffs, as each cylinder fires and opens, this is what creates the pulse wave through the exhaust, easy to understand, they also have a very short pipe, because the engine creates Max power and torque at max rev's...

In my understanding/my opinion -
The puffs of exhaust gases are high pressure, they are then followed by a low pressure between the puffs, in a exhaust system, the low pressure actually helps the high pressure move forward at greater speed, almost like pulling or dragging it along, the trick is to get the right diameter pipe at the right length, to help this high and low pressure, move efficiently and quickly, which is velocity.

If the exhaust pipe is to big, say 3inch, the high pressure puffs, have more room to expand, and have a lower pressure, the end result is the velocity is slower.
With a say 2inch pipe, it still contains the high pressure puffs enough so they keep their high pressure, and still have the low pressure between puffs, to give it the high velocity.

The normal stock exhaust has to find a balance, which in the end, has loses at one or either end of the RPM spectrum, as they aim for medium velocity.

The length of pipe comes into it also, as hotter air moves quicker than colder air, so the idea is to get the right diameter pipe, to keep the pressure difference between puffs high, and the right length, to maintain heat, to get the higher velocity, and then get the performance boost.

This is my understanding at its most basic... as has been said, there are many other factors that come into play... But, if this basics can be worked out, it will be a much better exhaust system over a standard one, and you will get/notice an increase in power/torque, i certainly did with my Vit

[royce]

you are close, think more along the lines of the original pulse and the low pressure behind it creates a bit of a vacuum inside the cylinder by dragging all the gas out, combined with a higher overlap cam so the intake and exhaust valves are open together will draw some intake charge in

so rather than a bit of exhaist still hanging around in the cylinder dirtying it all up its a lot cleaner intake charge so more power is made

[MacDaddy]

Derzky wrote:

All i can say is that i bought a ex-race gemini that had a 2.2lt with twin dd webbers, stage III head and extractors with a 2 1/2" collector followed by a short 2 1/2" pipe with a tiny muffler with a dump pipe welded on the other side of it.

This thing idled at around 1100 rpm when i got it (any lower and it stopped)
It was loud as all hell

I took it for a spin up the road to show my mates before i got it registered and i ripped the muffler off going into my mates driveway.

With no muffler on the system the car would not idle by itself.
I had to keep the revs at approx 1800 - 2000rpm and when i drove it to the exhaust shop it wanted to run at approx 4500 - 5500+ rpm, any lower and it lost "power"

It sounded red-hot
lets just say that the boys at the exhaust shop heard me coming 5min before i got there

I thought i was peter brock for 10min

Yep, racing and high performance engines are designed to run at high RPM to get max power and torque, hence they have much shorter tuned exhaust
system, they do not run well or best at low RMP, which is why, when the muffler fell off and made it even shorter, you had to keep the rev's higher just to keep it running... drag cars have one of the shortest exhaust pipes...

[MacDaddy]

royce wrote:

you are close, think more along the lines of the original pulse and the low pressure behind it creates a bit of a vacuum inside the cylinder by dragging all the gas out, combined with a higher overlap cam so the intake and exhaust valves are open together will draw some intake charge in

so rather than a bit of exhaist still hanging around in the cylinder dirtying it all up its a lot cleaner intake charge so more power is made

X 2

Yep, like totally agree with you dude !

[2stroker]

Matty 87 and Mcbear I don't understand what you are saying.

You both say that backpressure is 100% myth.

Are you saying there no such thing as backpressure therefore it is a myth because it does not exist.

Or are you saying it is a myth that backpressure plays any part in the efficient running of an engine.

Because after saying this you both go on to describe in varying ways that there needs to be some restriction of the flow volume to achive the ideal flow velocity.

The term backpressure has been used for a long time to define exactly what you both have said i.e. backpressure is simply any restriction to the flow volume, so what is the 100% myth, it obviously exists and it has a purpose.

Everyone seems to agree that :-

Exhaust too small, not enough flow volume = loss of power in higher RPM
(often described as too much backpressure)

Exhaust too big, not enough flow velocity = loss of power in lower RPM
(often described as too little backpressure)

[royce]

I think we should start an internet revolution and stop saying back pressure start saying velocity, afterall we are talking flow here not pressure, pressure is only measured in flow to determine resistance to flow across a known restriction.

[matty87]

be careful royce pressure is far more than a recistance to flow.

[matty87]

2 stroker you are very right there. i shall explain myself i jumped on the topic a bit half cocked. you are correct in saying that back pressure is not a myth as such what i should have said is the requirement for back pressure is a myth. It is rather a problem to be dealt with and minimized to the best of our ability to gain the best possible outcome. In an ideal world like the one that we engineers learn about at university there are pipes and fluids that are frictionless and therfor no resistance to flow and no pressure drop along them etc etc . But the real world is far more fun and as you know we have all these little barriers in the way of the perfect system.

i hope i have cleared up my original statment

[MacDaddy]

2stroker wrote:

Matty 87 and Mcbear I don't understand what you are saying.

You both say that backpressure is 100% myth.

Are you saying there no such thing as backpressure therefore it is a myth because it does not exist.

Or are you saying it is a myth that backpressure plays any part in the efficient running of an engine.

Because after saying this you both go on to describe in varying ways that there needs to be some restriction of the flow volume to achive the ideal flow velocity.

The term backpressure has been used for a long time to define exactly what you both have said i.e. backpressure is simply any restriction to the flow volume, so what is the 100% myth, it obviously exists and it has a purpose.

Everyone seems to agree that :-

Exhaust too small, not enough flow volume = loss of power in higher RPM
(often described as too much backpressure)

Exhaust too big, not enough flow velocity = loss of power in lower RPM
(often described as too little backpressure)

I would say that the whole back pressure myth/thinking is wrong, back pressure is the wrong word, even calling it restriction, though better, does not describe it properly... its about achieving a high enough pressure in the exhaust, that allows maximum exhaust gas velocity... a sports or high performance exhaust is about having less restriction at a higher pressure...

A standard exhaust has med pressure, and more restriction, to give it high and low RPM performance...

Think of it like a garden hose and water pressure, and the water passing through a big and small hose with a different pressure... change the water pressure and the size of the hose, and you get a better flow/velocity of water, there is no back pressure, just restriction, water behaves different to gases, and this may not be the best comparison, but i think you get the idea...
To me, Back pressure is nothing more than a commercialised myth/phrase used in the Exhaust industry to sell a product, and is not based in fact...

I may be wrong, and i am sure someone will correct me...

[2stroker]

Mbear and matty87, I think what you have said in your last posts sums it up nicely.

Royce that is exactly right, it is the terminoligy that has caused the confusion and led to debate over the years, most people have the right idea but get hung up on the word.

[SuziBlu]

matty87 wrote:

2 stroker you are very right there. i shall explain myself i jumped on the topic a bit half cocked. you are correct in saying that back pressure is not a myth as such what i should have said is the requirement for back pressure is a myth. It is rather a problem to be dealt with and minimized to the best of our ability to gain the best possible outcome. In an ideal world like the one that we engineers learn about at university there are pipes and fluids that are frictionless and therfor no resistance to flow and no pressure drop along them etc etc . But the real world is far more fun and as you know we have all these little barriers in the way of the perfect system.

i hope i have cleared up my original statment

What pipe, or for that matter, anything in the world, is frictionless.

Only something traveling thru a vacuum encounters no friction.

If I'm wrong, will be glad to learn.

[gravity_powered]

I think 'scavenging' is a far more realistic word than 'velocity' for suzuki owners