Ok gents, after me and Steelers1377 hijacked a thread on this subject I figured it would be better to start a thread devoted to the subject.
My point is that most street driven cars need a little bit of backpressure due to the scavenging affect of a good exhaust system.
And I believe his point is that you want the most free flowing exhaust possible to get the most power.
What is your view on this subject??
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-1994 Mustang GT... K&N filter,GT40 tubular intake,BBK Full Length Headers,modified midpipe & Flowmaster muffs, timing at 12*. Best time- 14.74 @ 93.16 -Check profile for pics of our car collection! -Member of MBMCustoms Car Club - AmericanMuscle ROCKS!! - "Save a horse, drive a Mustang!"
some backpressure is good on a street car, but on the other hand, my fathers 70 mustang is a drag car wit a 351w and has open headers with a 5 inch outlet, theres almost no backpresure, so it depends on your application.
so in other words your both right,
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1993 lx hatch, 5 speed, all stock except k&n air filter and flowmaster mufflers
Here is something for you guys to chew on and digest......
Some basic exhaust theory Your exhaust 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
will have 4 distinct pulses per complete engine cycle, a 8 cylinder has 8 pulses. 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.
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.
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.
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).
__________________
Reggie
Real Racers Don't Street Race
PERIOD.......
'88 LX Hatch With A Little Of This & A Little Of That...... 10.39 @128 MPH Soon to be faster....
Some backpressure is needed to maintain exhaust velocity. Backpressure does NOT mean it's restrictive, backpressure is a measurement of the velocity.
Try this. Hold a piece of paper at arms length, then blow a puff of breath at it. Then do it again but do it through a straw. The puff is more powerful through the straw. Exhaust systems do that very same effect on the engines output.
__________________
MOOG Certified Tech. For mods and sound clips, see profile and gallery.
Ask me why I ran "only" a 13.54.
Member: NMRA Member: White Mustang Registry #362 Member: Fuzzy Dicer
Here is something for you guys to chew on and digest......
Some basic exhaust theory Your exhaust 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
will have 4 distinct pulses per complete engine cycle, a 8 cylinder has 8 pulses. 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.
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.
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.
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).
hmm im not sure if that information is correct, please look at a statement made by Joel5.0
Backpressure = high torque down low = There is no such thing as "too big" of an exhaust system. When such a claim/observation is made it literally translates to a "too small of an intake" air and fuel wise.
Key component in the exhaust = header (long tube) and collector + extension design.
Exhaust system is one that requires pulse tuning NOT VELOCITY.
Exhaust system is one that requires free flowing characteristics after the headers NOT VELOCITY.
Characterization of full exhaust systems sizing based on engine HP categories is wrong.
This is an area in which erring towards the too big side is better. Rather than trying to correct such an error by restricting the exhaust, it's always more productive to correct the exhaust via increasing the engine's intake capabilities.
hmm im not sure if that information is correct, please look at a statement made by Joel5.0
Backpressure = high torque down low = There is no such thing as "too big" of an exhaust system. When such a claim/observation is made it literally translates to a "too small of an intake" air and fuel wise.
Key component in the exhaust = header (long tube) and collector + extension design.
Exhaust system is one that requires pulse tuning NOT VELOCITY.
Exhaust system is one that requires free flowing characteristics after the headers NOT VELOCITY.
Characterization of full exhaust systems sizing based on engine HP categories is wrong.
This is an area in which erring towards the too big side is better. Rather than trying to correct such an error by restricting the exhaust, it's always more productive to correct the exhaust via increasing the engine's intake capabilities.
the info i provided is not the end all and be all it was just fodder for conversation.....
The information that joel provide is specifically direct at the high performance SBF.....
So in your attempt to discredit info ......
You have only provide another rational point of view......
__________________
Reggie
Real Racers Don't Street Race
PERIOD.......
'88 LX Hatch With A Little Of This & A Little Of That...... 10.39 @128 MPH Soon to be faster....
Try this. Hold a piece of paper at arms length, then blow a puff of breath at it. Then do it again but do it through a straw. The puff is more powerful through the straw. Exhaust systems do that very same effect on the engines output.
but....
you will not be able to "blow through a straw" as fast as you could just blow out normally, which would limit the engine...
If you read it carefully, the info given by RLG34750 and Joel5.0 are pretty much saying the same thing.
1) Backpressure = good low-end torque. This is true but the I find the way it was written to be misleading. An engine at low PRM will do very well with small exhaust diameters because the small pipe provides a good balance between high velocity flow and low restriction down low. On the top end though, this engine would suffer because the exhaust would fail to provide adequate flow and restriction (backpressure) would be excessive due to the inability of the small pipe to remove the larger volume of exhaust gasses.
Put straight pipe on it....run it hard....where the discoloration ends cut the pipe off....perfect exhaust flow is achieved. On a race motor the exhaust removes heat from the engine compartment via the exhaust flow...it does NOTHING else!
A street motor is a compromise.....emissions....noise...fumes....and yes, a little torque....
A thread about backpressure. 
