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So I hear lots about back pressure and the neccessity of it all and I see a lot of bikes from across the pond and some on this forum that have taken the exhaust down to the minimum. The Destroyer can't have much of a collector if any. (I may be wrong, just going by looks, and yes, I realize it's a race bike). So my question is just how important IS that collector and can back pressure be achieved without one?
Thanks for keeping the violence to a minimum.
:hidesbeh:
 

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Soundhound said:
So I hear lots about back pressure and the neccessity of it all and I see a lot of bikes from across the pond and some on this forum that have taken the exhaust down to the minimum. The Destroyer can't have much of a collector if any. (I may be wrong, just going by looks, and yes, I realize it's a race bike). So my question is just how important IS that collector and can back pressure be achieved without one?
Thanks for keeping the violence to a minimum.
:hidesbeh:
:rofl2:
 

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I guess as far as importance I can give you an example. I run Sampson street sweepers on my bike. When we did the Dyno shoot out at gails, I was anywhere from 4 to 10 hp lower than every other pipe in my class. It was LOUD though. My pipes have no baffles at all
 

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back pressure is important unless you want all the power up the top end.
I have tried 5 exhuasts on my bike and ones that give back pressure give better rideability.
 

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A gasoline fueled internal combustion engine like you find in a V-rod is at is most basic just and air pump. If you put an electric motor to the crank and turn it, it will pump air, in one side and out the other. The throttle on it is by a restriction, most commonly by a butterfly type valve or valves. This is a restriction in the intake side of the pump. By restricting input flow, you can control the amount of power and the rpm of the engine.
On the output side, the less restriction you have, the more efficient the engine is. So, no restrict is better. On race engines with multiple cylinders by combining the exhaust tracts the effect of scavenging can be used. The gases coming out of the engine will be drawn out of the cylinder because there is lower than atmospheric pressure as seen by the exhaust port and the gas pulses combine for less resistance to the gas moving out of the port into the exhaust through the plumbing.
Mufflers are desirable in most situations where people are around the running engines because of the sound pressure that is generated. Vehicles are sold with them to comply with governing bodies requirements concerning noise levels. The limited space most motorcycles have to carry a muffler and the cost to manufacture them ends up causing resistance to exhaust flow and produces what is called back pressure. The up stream effects require changes in air fuel ratios to accommodate this configuration.
When somebody changes the configuration like changing the exhaust to less restrictive core or a different setup that uses collectors and induces scavenging by combining gas pulses, the air fuel ratio is probably not going to be close to the optimum for the engine.
Just changing back pressure is not the key to this situation. Anytime you change something that affects the intake or exhaust flow characteristics requires altering the fuel delivery to correct the a/f ration in the cylinder at combustion time. If the engine flows more air, it will need to deliver more fuel volume to keep the a/f ration the same as before the change.
If you change the air filter to flow more air and the exhaust to a better flowing configuration you will probably lose power unless you add more fuel too. In this day of very lean a/f ratios from the factory for EPA compliance, you need to pay attention to these changes or it could make large differences to performance and efficiency or even cause failures due to internal parts breakage.
 

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http://www.team-integra.net/sections/articles/showarticle.asp?ArticleID=355

Pretty much answers any question about "backpressure".

Long story short: Backpressure is bad. Over-scavenging (from valve overlap) is bad. This over-scavenging and reversion is what causes loss of power at the lower RPMs. Having backpressure at cam overlap alleviates the symptoms of over-scavenging and cylinder reversion, thus improving torque. Other than at that point, backpressure is bad.


Notice the positive (backpressure) spike at the far left as the exhaust valve just opens at BDC. The exhaust gases must now push against this POSITIVE (back)pressure before it can leave the combustion chamber. The pressure tracing is upwards and positive. Energy must be used up in order to overcome the initial positive (back)pressure in the exhaust system before the exhaust gas is pushed out of the combustion chamber.

After we are able to overcome the positive backpressure, you see that the exhaust gas begins to travel faster and creates a NEGATIVE pressure. The pressure tracing in the diagram is downwards or has a negative value. The more negative a pressure becomes means that you are creating more suction or a vacuum in the system. The system is literally sucking or pulling out exhaust gas from the combustion chamber or cylinder. This sucking or "SCAVENGING" effect not only helps remove more exhaust gas from the cylinder. It also helps suck in more intake air & fuel mix at cam overlap. The faster the exhaust gas travels the more vacuum it creates. We want to get as much as negative pressure created before cam overlap.

At cam overlap, there is a reflected pressure wave travelling backwards towards the engine. This reflected wave or "REVERSION" is what contaminates the intake charge at cam overlap and reduces or dilutes the oxygen content coming into the cylinder. Less oxygen going in means less power. Notice the pressure at the exhaust valve is still negative but less negative than before. This reflected exhaust wave or pulse is the second BACKpressure we experience and again reduces exhaust flow speed or energy because the exhaust pulse must now push against this pressure to move forward. A loss in flow speed means less negative pressure, or vacuum, or suck
 
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