round or square

[2 Fast 4 U]

Does it matter what shape the exhaust pipe has, square or round, if the cross section has the same area??

[Redskinsjbs]

I don't know if the specific profile shape of the tube matters, but the area of the tube, manner in which it is bent, and design of the system are all very important to the flow of the exhaust. If there is too much resistence in the exhaust, it can produce a backpressure, which can prevent the engine from properly 'exhausting' all of the spent fumes in the cylider. If the exhaust creates too much of a vacuum, or low pressure, the result can be far worse fuel economy and sometimes worse engine perfomance.

As for the shape of the tube itself, I don't know. I believe that circles are used because of the ease in manufacturing, design, etc...

However, I do not know if the square tubing would create micro eddeies or vortexs in its corners. Its worth a look. Maybe try it out on a lawnmower before your car.

[jordan]

2.5" rectangle A=bh is 2.5*2.5= 6.25insq
2.5" diameter circle A=pi(r)squared = 4.909insq (roughly)
They do not equate, square tubing would be more turbulent as there will be many varying pressures along the periphery and bending it would prove to be a big issue. The farther you get from round the more issues will arise,

[heimue]

Originally Posted by 2 Fast 4 U ,,,square or round, if the cross section has the same area??

Originally Posted by jordan 2.5" rectangle A=bh is 2.5*2.5= 6.25insq 2.5" diameter circle A=pi(r)squared = 4.909insq (roughly) They do not equate, ...

As of the original question, he wasn't comparing identical dimensions, but identical cross sections to each other.
Theoretically (in an ideal, but nonexisting world) the flow does not depend on the form of the pipe. However, when friction comes in (between gas flow and pipe), it makes a small difference. The flow speed is lower the closer you come to the pipe, and goes to ZERO directly at the pipe. Therefore, the smaller the circumference of the pipe, the faster the air flow. The smallest circumference you can get is in a round pipe. But this is only the smaller picture: Due to the air speed distribution, a round pipe always has a better pressure distribution also. The dynamic pressure is lowest in the center (with fast air flow), while the static pressure is lowest at the pipe wall. The absolut pressure is the sum of dynamic and static pressures, and identical in all places. The different distribution of static and dynamic pressures result in turbulences inside the pipe, creating flow restrictions and higher pressure drops (from inlet to outlet). The turbulences are higher on a pipe that is anything but round, the flow is better in a round pipe.
Generally speaking:

• The larger the pipe sections, the smaller are the differences created by form.
• Pressure drop depends on the length of the pipes (the longer, the higher).
• Pressure drop depends on the number of bends and the bend radius (more bends, smaller bend radius: higher drop).
• Pressure drop depends on the form of the pipe (large circumference at same cross section, opposite walls close together --> higher drops).

I can go deeper in the physics of air flow, if need be.

Dieter

[OpelNewb]

Where did you want to use square tube? Also and I am sure I am not the only one who has noticed this but most intake manifolds and exhaust manifolds even cylinder head ports are square in thier design. I am not saying that makes them work well. Newer vehicles have more rounded ports. But if you were trying to match the port on the head with either your exhaust or intake manifold it would make since to start with the closest size and shape of the port. Then expand to the desired shape or size. I have seen this done but have no pictures. It was interesting to say the least. It was complicated to build but the theory was proven.

Here is why. Picture your head port. Then picture a round tube big enough to cover it on all sides. You end up with huge overlap. So the air exiting would create turbulence crossing past that ledge. That in turn would decrease flow in the worst place.

Newer exhaust manifolds on car and even some older cars are made so the "ledge" is not a issue. Look at Ford Mustang's factory "header" the flange ends of the pipe are squeezed square to fit the shape of the head port better. It's still not perfect but it helps. Here is a picture I found of some on ebay. eBay Motors: 1987-88 mustang 302 factory headers (item 150305706600 end time Oct-30-08 20:01:21 PDT)
Now I am not saying making the whole exhaust out of square tube would be good though it would be very difficult to bend and that would just be the start. I would however recommend having the pipe as close to the same size and shape at the head as possible. then having a smooth transition into round pipe. HTH

[baz]

i think round is used because it is easiest to bend any deg of rotation from datum always has same section with square you are locked in the plain you start in and can only go vertical or horizontal from that start point anywhere from 1* to 89 * of rotation would need an adapter, bends are easy enough with a bending machine but no rotation can be bent in

[Jeepsbeme]

There are a lot of other factors to take into consideration too, such as the exhaust gas velocity, temperature, and cylinder scavenging (although I'm far from an expert on the subject).

I had the pleasure of having lunch with a VP from Borla a few years ago. The one thing I'll never forget is how he mentioned that many of their exhaust systems are made of a larger diameter pipe than is ideal for performance simply because that's what the market demands. Bigger simply has to be better! He explained how they could offer greater performance in many applications by using a smaller diameter pipe (although larger than the diameter of the original equipment pipe) than the product they ultimately released for sale.

[yellaopelgt]

I would think that round pipe would create the least resistance of air flow....a square pipe/tube/whatever,would have too many nooks and crannys to create drag and thus reduce efficiency... not to mention the bendabilty factor.Whats the reasoning for your question? just curious.
Joe

[2 Fast 4 U]

Originally Posted by yellaopelgt .......Whats the reasoning for your question? just curious. Joe

In the future I want to make a stainless steel exhaust for my car and was just curious about the idea.

[heimue]

Originally Posted by OpelNewb Where did you want to use square tube? ...

Because of space-issues!

http://www.opelgt.com/forums/161164-post4.html

Dieter

[blancojp]

Square tubbing is hard to bend for one and it does carry a cutting effect when it comes to flow. Round tubbing is easy to bend and follows the spiral of flow alot better.

When you are doing a conversion and you are tight in space then you improvise and adapt to what is available. The original 140HP V-6 kit we made for the MG back in 1994 had an L-shaped square exhaust hedder. We quickly realized that we lost over 16RWHP by using this setup and fuel economy was not great. Using the same type hedder with round pipe gave us an increase of 22RWHP and ecomony was up to par.

Square tubbing does not allow the engine to breath right.

[wrench459]

One of the questions that I have involve fluid dynamics.
Is the flow of a water pipe half full equal to a full pipe?
The thread is jumping all around skirting the "D" shaped ports.

[namba209]

With my very limited knowledge of pneudraulics, that is the science of fluids in motion, and air is a fluid in this concept. All you gotta do is look at a molecule or the universe. It's all made up of spherical bodies. That means the natural tendency would be to flow in a round pattern. Anything that disrupts that, would be detrimental to flow. To answer Wrench's question, "D" shaped ports are used to create a low pressure area along the straight line to assist in the bending of flow in a desired direction. At least that's what my theorization indicates. As for the half full pipe or a full pipe flow, it depends on the size of the pipe and pressure applied.

[jeff denton]

I think if you use square exhaust pipes it's okay as long as your exhaust valves are square, also.
Just seeing if anybody's awake here.
Smokey Yunick reports in his book that when researching exhaust systems he learned that water flow and air flow behave similarly. He thought water was easier to see and measure, so he used it a lot in his experiments.