DIY Sidepipe Fabrication Question

[ItBites]

What Inner Diameter (ID) are your louvered Hedmans? If the ID is large enough, louvers vs perforations won't make a difference. Turn the louvers downstream, though. That reduces the thickness of the boundary layer and increases the effective ID where the bulk of the flow occurs, as compared to pointing the louvers into the flow. With a 331, if the ID is nominally 2.5inches or larger, you will be OK.

[Bill D]

Originally Posted by ItBites

What Inner Diameter (ID) are your louvered Hedmans? If the ID is large enough, louvers vs perforations won't make a difference. Turn the louvers downstream, though. That reduces the thickness of the boundary layer and increases the effective ID where the bulk of the flow occurs, as compared to pointing the louvers into the flow. With a 331, if the ID is nominally 2.5inches or larger, you will be OK.

In/out is 3". If I reverse them I should be ok. Might be a bit louder, but I'm not worried about that

[Mr Jody]

I was just looking at something along those lines myself, but for mufflers for a pickup instead of a Cobra. I too am skeptical of the louvered design vs. the perforated. I noticed the Magnaflow site makes mention of the perforated design being less restrictive as well.

I recall from some of my mechanical engineering classes oh so many years ago that turbulent flow is more restrictive than laminar flow, and it seems to me that the louvers will act to induce turbulence in the pipes and thus be more restrictive than a straight through, perforated pipe. From looking at the Classic Chambered pipes, which use louvers, it looks like a better design would be to put the louvers on the OUTSIDE of the inner pipe instead of the inside. This would leave a smooth interior flow surface except the dimples where the louvers are located, which could possibly enhance flow from a boundary layer theory perspective (think dimples on a golf ball). But at any rate, this would be less restrictive than the internal louvers protruding up into the flow.

Another thing I noticed about the Classic Chambered design is that the louvers are cut into a slight spiral pattern. While this is probably intended to create a vortex affect and assist in scavenging the exhaust, it also lengthens the flow path, with the swirl adding turbulence. Getting each element of exhaust through and out of the pipes in the quickest manner will always be the path of least resistance, which means the straightest flow path each element can take through the pipe is best.

While that straightest flow path is also the loudest, I think the amount and type of packing used, and the length of the muffler can play greater factors in the loudness of the mufflers than the internal design.

I don’t mean to step on any Classic Chambered fans and owners toes here. I agree that they sound great, and perhaps the internal diameter is such that the turbulence effects aren’t too large, and it would probably take some side by side testing for accurate comparison, but just giving a little more in depth look and my two cents worth on the big picture of louvers vs. perforations.