Exhaust Pressure Testing

[GregO]

There’s a simple explanation on the flow difference.
The straight pipe isn’t expanded on the air draw through end whereas the muffler has both ends expanded for slip connections.
If the 18” straight tube had both ends expanded exactly like the muffler the air flow measurement would be nearly identical.
Just my observation.

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[Angrey]

There’s a simple explanation on the flow difference.
The straight pipe isn’t expanded on the air draw through end whereas the muffler has both ends expanded for slip connections.
If the 18” straight tube had both ends expanded exactly like the muffler the air flow measurement would be nearly identical.
Just my observation.

That's not really it. The lack of a slight flare on the exit end will have a very minor/negligible impact on flow. Even if you were to take that length and create a perfect coned diffuser it's not enough to account for 5% flow improvement.

He explains it KINDA okay in the video, but essentially by creating small voids in the tubing all, once the pressure in the chamber matches the pressure through the tube column, the air is slipping past with less friction and disruption as it slides past other air molecules rather than against the tubing wall. It's a similar principle as to why dimpled golf balls achieve a higher velocity and maintain their velocity better than a smooth golf ball.

We also observe this in fish and torpedoes. Sharks have very tiny nodules on their skin which help to break up the water as it flows by and that increases their kinetic efficiency in the water. Similarly, it's counter intuitive, but if you take a jet nozzle and redirect some of a torpedo's thrust to to the front of the torpedo (a small portion) even though it's aimed in the opposite line of travel, it helps to create enough fluid turbulence that the fluid experiences less friction as it passes along the body of the torpedo. TOTALLY counterintuitive to what you would expect, but it works, the result is torpedoes that have that run slightly faster than the same torpedo without it.

 

[GregO]

That's not really it. The lack of a slight flare on the exit end will have a very minor/negligible impact on flow. Even if you were to take that length and create a perfect coned diffuser it's not enough to account for 5% flow improvement.

He explains it KINDA okay in the video, but essentially by creating small voids in the tubing all, once the pressure in the chamber matches the pressure through the tube column, the air is slipping past with less friction and disruption as it slides past other air molecules rather than against the tubing wall. It's a similar principle as to why dimpled golf balls achieve a higher velocity and maintain their velocity better than a smooth golf ball.

We also observe this in fish and torpedoes. Sharks have very tiny nodules on their skin which help to break up the water as it flows by and that increases their kinetic efficiency in the water. Similarly, it's counter intuitive, but if you take a jet nozzle and redirect some of a torpedo's thrust to to the front of the torpedo (a small portion) even though it's aimed in the opposite line of travel, it helps to create enough fluid turbulence that the fluid experiences less friction as it passes along the body of the torpedo. TOTALLY counterintuitive to what you would expect, but it works, the result is torpedoes that have that run slightly faster than the same torpedo without it.

Until they test that straight pipe expanded on both ends the test is inconclusive.
I agree, dimpling the surface reduces boundary layer friction. Expanding the inlet also moves the boundary layer away from the flow column resulting in a flow increase. To what percent we won’t know until that test is performed with both ends expanded.
The flow bench is draw through not blow through. That straight pipe has a lot working against it getting the air to draw into it.

Maybe someone can do a CFD on a 3” tube with inlet straight vs. expanded at the tested CFM.

 

[Angrey]

Until they test that straight pipe expanded on both ends the test is inconclusive.
I agree, dimpling the surface reduces boundary layer friction. Expanding the inlet also moves the boundary layer away from the flow column resulting in a flow increase. To what percent we won’t know until that test is performed with both ends expanded.
The flow bench is draw through not blow through. That straight pipe has a lot working against it getting the air to draw into it.

Maybe someone can do a CFD on a 3” tube with inlet straight vs. expanded at the tested CFM.

It's not going to result in 35 cfm difference. Like I said, you could take either pipe, take the extruded length and make it a perfect cone diffuser and it's not going to improve CFM by 35 cfm.

While I don't disagree with your apples/oranges catch, the idea that what's making the muffler flow better is a 3 inch section that's been flared 1/8" is MORE dubious that just recognizing the principle they're trying to show. I don't think they're intentionally trying to be deceptive or misleading.

If flaring the end of a straight pipe resulted in 35 cfm improvement (at test pressure) you'd see everyone in the world with flared ends on everything.

 

[GregO]

While I don't disagree with your apples/oranges catch, the idea that what's making the muffler flow better is a 3 inch section that's been flared 1/8" is MORE dubious that just recognizing the principle they're trying to show. I don't think they're intentionally trying to be deceptive or misleading.

Correct, not deceptive or misleading. Simply an oversight and lack of understand the dynamics of airflow eddying at the blunt end tube inlet.
Expanding the inlet for a distance of 3” by just .125” in diameter will result in an airflow improvement IMO.

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