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Originally posted by snowfox

Now do you see why the stock DP is choking your 2L? You really don't don't need a header on the street. They tend to live pretty short lives, too. They heavy cast iron manifolds do a much better job of surviving life on the street.

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quote:

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Originally posted by mdwelsh
For the past eighteen months I have viewed numerous posts and replies regarding the installation of a twin downpipe. What I have yet to view is actual dynamometer readings in performance gains using this exhaust system. A twin downpipe that reduces into one within two feet is a "restriction" any way you slice it. Please do not misunderstand but mathematics would prohibit any substantial gains as least in my view. Pi times radius squared equals the area of a circle so indeed the area of the twin will be greater until you reduce it back into one. Conceivably, again in my view, there may be a reduction in exhaust manifold operating temperatures but it is doubtful that noticeable increases in horspower and fuel mileage will be achieved.
Now if you were to extend the twin exhaust to the terminal point this would obviously increase performance and fuel mileage as well.

Until I view actual dyno readings relating to the twin versus single downpipe I will save my money and labor. Now if the application increases the HP to 85 and 3 additional mpg then I am down with this.

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quote:

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Originally posted by snowfox

quote:

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Originally posted by mdwelsh
For the past eighteen months I have viewed numerous posts and replies regarding the installation of a twin downpipe. What I have yet to view is actual dynamometer readings in performance gains using this exhaust system. A twin downpipe that reduces into one within two feet is a "restriction" any way you slice it. Please do not misunderstand but mathematics would prohibit any substantial gains as least in my view. Pi times radius squared equals the area of a circle so indeed the area of the twin will be greater until you reduce it back into one. Conceivably, again in my view, there may be a reduction in exhaust manifold operating temperatures but it is doubtful that noticeable increases in horspower and fuel mileage will be achieved.
Now if you were to extend the twin exhaust to the terminal point this would obviously increase performance and fuel mileage as well.

Until I view actual dyno readings relating to the twin versus single downpipe I will save my money and labor. Now if the application increases the HP to 85 and 3 additional mpg then I am down with this.

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Long time, no see around here-

Several points are off here and I have boldfaced them.

First, the secondary runners on the DP shown above are much closer to 4 feet in length than the 2 feet you mention. (The transverse cars' DP's are more like the 2' you mention - because of their layout. Look here to see what a 'short' secondary runner looks like: http://forums.vwvortex.com/zerothread?id=2782737) Yes, there is some restriction, but as I have stated before, some backpressure (a.k.a., restriction) is required for proper, efficient running of these motors.

Both horsepower and efficiency are gained through the reduction of restriction - but don't fall into the 'more is better' way of thinking; some 'free-flowing' is good, so more must be better is only true up to a point. The way that both these street tuned systems and tuned racing headers make more horsepower is through the tuning. There is a lot more going on than just letting air flow through a bigger pipe. The 'tuning' of these systems allows them to 'scavenge' exhaust gasses from the combustion chambers. The systems are built in such a way that as one exhaust pulse is traveling down the pipe, it has momentum that creates a reduced pressure zone behind it. The next exhaust pulse that is released/expelled from the cylinder that it is paired with then encounters a low pressure environment which allows it to have higher velocity (and momentum) on exit and create a similar low pressure zone in its wake - which then benefits the next exhaust pulse that the original cylinder will release, ad nauseum... This is a dynamic system involving the tuning of waves and balancing of constructive and destructive interference. For more information on how a 4 to 2 to 1 exhaust system works system works, Google a little. It may be helpful if you see this explained with diagrams. Long story short - the mathematics involved are a lot more involved the the simple geometry you have referenced - believe me. (I'm a Mechanical Engineer with a concentration in machine design.)

The last point is pretty well covered by the "some is good - more is better" way of thinking - it's only valid to a point. Period. This method would also have to be tuned. Since there is no symbiotic scavenging between cylinders, the tuning of this type of exhaust will only make good power and efficiency in a VERY narrow RPM range. Not good for the street. You'll notice that dragsters use this method - one pipe per cylinder, but they have a very limited application.

Sorry for the long post, but there is a lot more involved in an exhaust system than you are considering. I'll look around for some dyno information (from someone other than the company selling the exhaust system - for objectivity reasons, obviously)to try to quantify the types of gains provided by a properly tuned system.

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quote:

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Originally posted by mdwelsh

quote:

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Originally posted by snowfox

quote:

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Originally posted by mdwelsh
For the past eighteen months I have viewed numerous posts and replies regarding the installation of a twin downpipe. What I have yet to view is actual dynamometer readings in performance gains using this exhaust system. A twin downpipe that reduces into one within two feet is a "restriction" any way you slice it. Please do not misunderstand but mathematics would prohibit any substantial gains as least in my view. Pi times radius squared equals the area of a circle so indeed the area of the twin will be greater until you reduce it back into one. Conceivably, again in my view, there may be a reduction in exhaust manifold operating temperatures but it is doubtful that noticeable increases in horspower and fuel mileage will be achieved.
Now if you were to extend the twin exhaust to the terminal point this would obviously increase performance and fuel mileage as well.

Until I view actual dyno readings relating to the twin versus single downpipe I will save my money and labor. Now if the application increases the HP to 85 and 3 additional mpg then I am down with this.

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Long time, no see around here-

Several points are off here and I have boldfaced them.

First, the secondary runners on the DP shown above are much closer to 4 feet in length than the 2 feet you mention. (The transverse cars' DP's are more like the 2' you mention - because of their layout. Look here to see what a 'short' secondary runner looks like: http://forums.vwvortex.com/zerothread?id=2782737) Yes, there is some restriction, but as I have stated before, some backpressure (a.k.a., restriction) is required for proper, efficient running of these motors.

Both horsepower and efficiency are gained through the reduction of restriction - but don't fall into the 'more is better' way of thinking; some 'free-flowing' is good, so more must be better is only true up to a point. The way that both these street tuned systems and tuned racing headers make more horsepower is through the tuning. There is a lot more going on than just letting air flow through a bigger pipe. The 'tuning' of these systems allows them to 'scavenge' exhaust gasses from the combustion chambers. The systems are built in such a way that as one exhaust pulse is traveling down the pipe, it has momentum that creates a reduced pressure zone behind it. The next exhaust pulse that is released/expelled from the cylinder that it is paired with then encounters a low pressure environment which allows it to have higher velocity (and momentum) on exit and create a similar low pressure zone in its wake - which then benefits the next exhaust pulse that the original cylinder will release, ad nauseum... This is a dynamic system involving the tuning of waves and balancing of constructive and destructive interference. For more information on how a 4 to 2 to 1 exhaust system works system works, Google a little. It may be helpful if you see this explained with diagrams. Long story short - the mathematics involved are a lot more involved the the simple geometry you have referenced - believe me. (I'm a Mechanical Engineer with a concentration in machine design.)

The last point is pretty well covered by the "some is good - more is better" way of thinking - it's only valid to a point. Period. This method would also have to be tuned. Since there is no symbiotic scavenging between cylinders, the tuning of this type of exhaust will only make good power and efficiency in a VERY narrow RPM range. Not good for the street. You'll notice that dragsters use this method - one pipe per cylinder, but they have a very limited application.

Sorry for the long post, but there is a lot more involved in an exhaust system than you are considering. I'll look around for some dyno information (from someone other than the company selling the exhaust system - for objectivity reasons, obviously)to try to quantify the types of gains provided by a properly tuned system.

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I would concur that tuning is of vast importance and the two-foot length is for an examples sake. Why do not one of you foxers develop a "four into one" downpipe? Following the theory if two is good then four would be better would it not? As I stated I have yet to see any actual proof; dyno readouts which you state that you will research and provide. To state that there is more than mathematics is erroneous in that every component on a naturally aspirated or any fueled engine is ALL mathematics. Every component of an engine has numbers; compression ratio, valve clearence, cam lift and duration, rpm, etcetera, etcetera, etcetera. Closed headers will only obtain slight gains until they are opened at the drag strip. A person could spend all of the money on these ABA swaps, cams and other mods that I have read about and know absolutely nothing about. However, a very good turbocharging system would outperform all of the aforementioned mods if one were available. Regardless of either a 400BHP versus a 500BHP the Fox will only reach a maximum terminal velocity because of the non-aerodynamic box design. As you are fully aware being an engineer, horsepower is meaningless without other components considered during the research and development stage.

Personally, I have a stock exhaust manifold but I have completely removed the catalytic converter and the resonator and run 2" straight from the exhaust manifold to a Flowmaster High-Flow muffler with nothing but tubing in between. The car runs considerably better and the fuel mileage increased dramatically and I must say that she does sound sweet! Just some thoughts for discussion and for myself to learn.

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quote:

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Originally posted by snowfox

quote:

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Originally posted by mdwelsh

quote:

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Originally posted by snowfox

quote:

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Originally posted by mdwelsh
For the past eighteen months I have viewed numerous posts and replies regarding the installation of a twin downpipe. What I have yet to view is actual dynamometer readings in performance gains using this exhaust system. A twin downpipe that reduces into one within two feet is a "restriction" any way you slice it. Please do not misunderstand but mathematics would prohibit any substantial gains as least in my view. Pi times radius squared equals the area of a circle so indeed the area of the twin will be greater until you reduce it back into one. Conceivably, again in my view, there may be a reduction in exhaust manifold operating temperatures but it is doubtful that noticeable increases in horspower and fuel mileage will be achieved.
Now if you were to extend the twin exhaust to the terminal point this would obviously increase performance and fuel mileage as well.

Until I view actual dyno readings relating to the twin versus single downpipe I will save my money and labor. Now if the application increases the HP to 85 and 3 additional mpg then I am down with this.

---

Long time, no see around here-

Several points are off here and I have boldfaced them.

First, the secondary runners on the DP shown above are much closer to 4 feet in length than the 2 feet you mention. (The transverse cars' DP's are more like the 2' you mention - because of their layout. Look here to see what a 'short' secondary runner looks like: http://forums.vwvortex.com/zerothread?id=2782737) Yes, there is some restriction, but as I have stated before, some backpressure (a.k.a., restriction) is required for proper, efficient running of these motors.

Both horsepower and efficiency are gained through the reduction of restriction - but don't fall into the 'more is better' way of thinking; some 'free-flowing' is good, so more must be better is only true up to a point. The way that both these street tuned systems and tuned racing headers make more horsepower is through the tuning. There is a lot more going on than just letting air flow through a bigger pipe. The 'tuning' of these systems allows them to 'scavenge' exhaust gasses from the combustion chambers. The systems are built in such a way that as one exhaust pulse is traveling down the pipe, it has momentum that creates a reduced pressure zone behind it. The next exhaust pulse that is released/expelled from the cylinder that it is paired with then encounters a low pressure environment which allows it to have higher velocity (and momentum) on exit and create a similar low pressure zone in its wake - which then benefits the next exhaust pulse that the original cylinder will release, ad nauseum... This is a dynamic system involving the tuning of waves and balancing of constructive and destructive interference. For more information on how a 4 to 2 to 1 exhaust system works system works, Google a little. It may be helpful if you see this explained with diagrams. Long story short - the mathematics involved are a lot more involved the the simple geometry you have referenced - believe me. (I'm a Mechanical Engineer with a concentration in machine design.)

The last point is pretty well covered by the "some is good - more is better" way of thinking - it's only valid to a point. Period. This method would also have to be tuned. Since there is no symbiotic scavenging between cylinders, the tuning of this type of exhaust will only make good power and efficiency in a VERY narrow RPM range. Not good for the street. You'll notice that dragsters use this method - one pipe per cylinder, but they have a very limited application.

Sorry for the long post, but there is a lot more involved in an exhaust system than you are considering. I'll look around for some dyno information (from someone other than the company selling the exhaust system - for objectivity reasons, obviously)to try to quantify the types of gains provided by a properly tuned system.

---

I would concur that tuning is of vast importance and the two-foot length is for an examples sake. Why do not one of you foxers develop a "four into one" downpipe? Following the theory if two is good then four would be better would it not? As I stated I have yet to see any actual proof; dyno readouts which you state that you will research and provide. To state that there is more than mathematics is erroneous in that every component on a naturally aspirated or any fueled engine is ALL mathematics. Every component of an engine has numbers; compression ratio, valve clearence, cam lift and duration, rpm, etcetera, etcetera, etcetera. Closed headers will only obtain slight gains until they are opened at the drag strip. A person could spend all of the money on these ABA swaps, cams and other mods that I have read about and know absolutely nothing about. However, a very good turbocharging system would outperform all of the aforementioned mods if one were available. Regardless of either a 400BHP versus a 500BHP the Fox will only reach a maximum terminal velocity because of the non-aerodynamic box design. As you are fully aware being an engineer, horsepower is meaningless without other components considered during the research and development stage.

Personally, I have a stock exhaust manifold but I have completely removed the catalytic converter and the resonator and run 2" straight from the exhaust manifold to a Flowmaster High-Flow muffler with nothing but tubing in between. The car runs considerably better and the fuel mileage increased dramatically and I must say that she does sound sweet! Just some thoughts for discussion and for myself to learn.

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See my bold itlaics highlights above...

I didn't say more than mathematics, I said more than the simple geometry you had asserted as a proof.

Done.

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