How is exhaust pipe measurement referred to?

[A383Wing]

fairly simple question I guess, but I'm old and am getting conflicting answers

when asking or measuring for exhaust pipe to be added to my car, how does one measure for the pipe needed, inside or outside diameter of said pipe?

Bryan

[XH29N0G]

I am almost 100% sure it is OD.  Tubing is normally measured this way, and this would standardize the measurement needed for the ID of slip on tubes and mufflers much more easily.

[John_Kunkel]

  Yes, O.D. What we often call pipe is actually tubing.

[Ghoste]

I gues I could goggle it but since Im already here, what determines pipe and tubing?

[HPP]

Tubing is structural and a very precise OD, ie roll bar tubing or bridge supports.  Pipe is a containment vessel and a nominal OD that caries fluids of vapors, such as exhaust pipe or gasoline pipe.

[John_Kunkel]

Pipe is measured by the I.D. The O.D. of pipe varies with the wall thickness of the pipe just as the tubing I.D. varies with the wall thickness of the tube.

[A383Wing]

and we are getting conflicting answers here as well....I always thought that the exhaust pipe was measured by the I.D. of the pipe.....

[ws23rt]

and we are getting conflicting answers here as well....I always thought that the exhaust pipe was measured by the I.D. of the pipe.....

Maybe the conflict is that what we are used to calling "exhaust pipes" should (more correctly) be referred to as exhaust tubing?

The OD of tubing tends to be a more precise number/size then the ID of pipe.--It is engineered for OD fit as opposed to ID flow.

[el dub]

Whatever flows through the pipe can only be calculated using the I.D.

[ws23rt]

 I've heard iron workers refer to the stuff as "O beam"  And millwrights call it  "hollow bar"
Electricians use another kind of stuff "conduit"

[GPULLER]

Tubing measured O.D.
Pipe measured I.D.

[ws23rt]

Whatever flows through the pipe can only be calculated using the I.D.

That is a true statement.  However for any given "pipe size" the Id varies (significantly) by schedule or wall thickness.
http://www.mcnichols.com/?pageCode=pipedims

For tubing the OD is a more closely held dimension and the variations from wall thickness are all reflected in the ID.
http://www.mcnichols.com/?pageCode=tubedims

[A383Wing]

my head hurts now....please make it stop!!!   

[skip68]

    So when you hear someone say they're running 3" exhaust I've always presumed they're talking about the inside diameter. 
An actual 3" opening.    Makes sense to me.    
Unless it's the overall size of the pipe.   

[ws23rt]

  So when you hear someone say they're running 3" exhaust I've always presumed they're talking about the inside diameter.  
An actual 3" opening.    Makes sense to me.    
Unless it's the overall size of the pipe.  

Yup. ----A 3" exhaust refers to the OD of the "tubing" which is used to make the---Item that the exhaust flows through.

The actual 3" opening (inside diameter) that receives a 3'' (outside diameter) tube is not a normal tube size and is generally a stretched piece of tube made to fit the outside of standard tubing.

Tube size refers to OD.  --- Pipe size is a messy mix of ID and OD.

[skip68]

This got me thinking because not long ago I was fabricating exhaust and muffler on my son's buggy and remembered what a muffler guy told me.  
The pipe is measured from the outside.
The mufflers are measured from the inside.  

Mr kunkel is right again as usual.    

[birdsandbees]

You going for turbulent flow or laminar? Sorry, the mechanical engineer coming out in me! lol

[ws23rt]

Laminar please.  What we want is all about getting it done.

[odcics2]

This is easy to remember: You can thread metal pipe but not metal tubing.

Also, you can bend metal tubing, but you'd snap metal pipe!

[John_Kunkel]

Actually, there are metal pipe benders (Harbor Freight sells one) but many are disappointed when they try to use if for tubing...the difference in the diameters becomes apparent when trying to use a pipe bender to bend tubing (and vice versa).

[XH29N0G]

We have seen the artwork that Keepat has done with his bullitt build tube bending, right? 

Well, I found the strangest bend on a mock up of a 1/4 inch tube manifold I have ever seen just last week.  When I asked about how that occurred, the builder said he couldn't find the right size tube bender....   So even if you are using a tubing bender, it had better be the right size.......

[daveco]

Wow, a person not knowing the difference would be totally confused by these answers.

Pipe is referred to by the nominal diameter. This diameter doesn't necessarily correspond to the actual O.D or I.D. but is more akin to saying small/medium/large. Pipe goes by trade size in fractional inches plus the schedule. Schedule refers to the relative wall thickness. Schedule 10 being typically the thinnest available with the wall thickness getting thicker with higher schedule numbers. Typical schedule numbers being 10, 20, 40, & 80.
80 being very thick, how thick?, well that depends on a lot of things and I don't feel like writing a book here. Suffice to say there are all sorts of considerations and exceptions in the land of pipe sizes. When you hear "pipe", think plumbing, as in toilets.

Typical automotive exhaust "pipe" is made from tubing as in tubular headers (ever heard of pipeular headers?). Calling it exhaust pipe is just a colloquialism. It is tubing.
Tubing is specified by it's actual O.D. and its actual wall thickness.
Automotive mufflers are not pipe or tubing, mufflers are assemblies made to fit standard tubing sizes or specific applications and are not held to any particular standard.
Generic, or aftermarket mufflers for non-specific applications will typically have the inlet and outlet configured to accept standard size tubing.

About pipe:
http://www.mcmaster.com/#about-selecting-and-measuring-pipe/=105kzw5

About tubing:
http://www.mcmaster.com/#about-metal-tubing/=105l0i2

[ws23rt]

^^^  A concise answer to the original question without getting into the many odd exceptions.
I like the clarity of these charts for this discussion so I brought them here again in case they were missed.


http://www.mcnichols.com/?pageCode=pipedims
http://www.mcnichols.com/?pageCode=tubedims