DIY Sidepipe Fabrication Question
 

I am in the planning stages of making my own 289 FIA exhaust system. I've done some research on mufflers. Early on I bought a pair of Hedman louvered mufflers. Now after additional research I find that this style is not as efficient as perforated core. Should I just sell the Hedman mufflers and buy the perforated or just use the Hedman? I have no plans to track the car but still don't want unnecessary impediments to performance if I can avoid it.

Thanks

It won't make that much difference. Will quieten it down more.

Try Lo Bak Mufflers ...
They are not glass or stainless steel packed.

Give a look to Classic Chambered Mufflers.

I am running the Cobra Mufflers ° No Glas Pack° on my FIA 289 N.A.F.

Engine 302
Flow core 2.5"
Muffler OD 3.5"
in/out 3.0"
Muffler length 28"

Verry happy and awesome sound from idling up to high revs and down shifting.

AC Cobra 289 NAF - YouTube

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

Louvers vs. Perforations
 

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.

Mr. Jody,

Laminar vs turbulent flow is not a choice at this point in the exhaust. All flow transitions from laminar to turbulent at some distance, based on Reynolds number and surface roughness. In an exhaust tube like this the transition occurs within a few inches for smooth walls, so surface roughness is effectively inconsequential as it only causes the onset of turbulent boundary layer to occur a couple inches sooner.

You are correct that louvers on the outside of the flow passage will be less restrictive. it will also be less effective at reducing sound, so there is a trade. In this case, with a 3" tube ID, the vena contracta will still be around 2.25", given the boundary layer thickness, so the OP will have plenty of "effective" cross-section to have considerable flow. Certainly enough for 331cid. If he were to have say 540cid, thats another story ;)

The spiral pattern is not likely to have any effect on reducing overall flow restriction, but may prevent strong wave patterns from standing in the tubing, basically causing a reduction in noise due to destructive wave cancellation.

Now back to your regularly scheduled programming...

Great info Mr. E!

Some of the terms in your post certainly bring back the memories (a.k.a. nightmares) of the predictability, or perhaps unpredictability of fluid flow. I was indeed curious as to what could be considered as the "effective" diameter with the louvers in place. Something else that I was just thinking about is how exhaust flow is actually a pulsating flow instead of a steady, constant velocity flow as is typically dealt with, and then the pulse frequency changes with RPM, which only further complicates things for exhaust designers. **)