I'd like to highlight some of the key points I can see from the cutaway muffler photo Tom has provided. These are my opinions based on the fun I have had with mufflers.
1. Note that this muffler has no drastic diameter reductions in either the internal tube or external bushings. This adds a lot of airflow potential, especially when you tax it with a stroked (larger displacement), higher rpm potential engine. Remember that back in 1968 when 340s came out, they were redlined at around 6000 rpm and running them to 6500 and higher was not as prevalent as it is today. With good rods and pistons, it is not uncommon to buzz our engines today to the 7000 range and higher. At these higher rpms, a muffler’s design becomes very important. Larger diameter pass tubes reduce backpressure; this Accurate muffler has them.
2. See how the internal tubes (pass tubes) of the tri-flow section are expanded over the tubes that feed into them? Why is this so important? If Accurate would have reduced the internal tubes to fit into (instead of over) the tubes that are feeding exhaust into them, the sharp edge and diameter reduction would cause flow restriction (backpressure) which equals reduced flow. If drastic enough, this joint could also cause high frequency hissing (think about those old, massive Ford luxury cars that hissed at wide open throttle). This expanded joint is good for exhaust gas entry but what about exit? The way Accurate did it is basically like butting two identical tube diameters end to end; there is not much diameter change to cause restriction and hissing.
3. Observe the large open chambers shown above the ACM 912 decal and in the bottom of the muffler? These two chambers are “turn arounds” and they are where the exhaust has to make the drastic direction change. If Accurate had put smaller volume areas in either location, they could have reduced some more sound level but it would have drastically cut the performance potential. Think about it this way: do you want exhaust to expand into a chamber the size of a baseball or a basketball before it changes direction? You want larger volume for flow. I can’t tell exactly but it looks as if one volume is smaller than the other which helps tune out different sound frequencies. Always remember that the gasses really do not want to change direction and this is most true at high gas volume flows (high loads or rpms) so anything one can do to make this easier and smoother, you gain flow potential. The best thing you can do is give the end chambers lots of volume and this muffler does this.
In summary, for a good sounding, nice flowing, all around performance street muffler, you want a tri-flow configuration and this is what Accurate is providing. To get low backpressure, you want open turn arounds, you need unrestricted flow tubes, you need good cross bleeding (holes between tubes in the tri-flow section), you want a short tri-flow length (this one is around 12 to 13”), and you want good muffler volume (these should be 4.24 by 9.25 by 18” roughly).
I have never heard these but based on what I see, I am betting they rumble at idle only slightly more than stockers did, they are only slightly louder across the rpm range, they have very little, if any resonation in the 2000 to 2200 rpm range, they flow 50% more than stock ones did (roughly 65 to 70% of an open pipe of the same diameter compared to 30 to 40% for the stockers). Think about this 65% to 70% number: the chambered mufflers you hear so often on Mustangs that have this offset inlet-center outlet orientation will flow about 65% of an open pipe but the sound level will be at east 10 dBA higher in the ranges you tend to drive. More importantly, and this is why you would buy this tri-flow muffler, the chambered mufflers will almost always resonate whereas the good designed tri-flow will not. When you add the chrome tips most performance Dodge and Plymouths came with, this muffler will sound good, rumble, flow a lot, and be essentially resonance-free.
And as Tom said, if you want maximum flow, you would always want the best straight-through muffler you could get. Like a tri-flow, you want large flow tubes, generous and correctly sized and shaped perforations for sound control. Additionally, I would want sound absorbing fiber, some sort to filter (st. st. sock) to keep the fiber in, and definitely, I’d want no sharp edges. UltraFlos, Magnaflows, XLERATORs, Max-Flows and some others provide these characteristics. But, it is hard to totally eliminate resonance with these mufflers even though st. st. UtraFlos do a good job with their extra tuned chambers and perforated partitions which act as Helmholtz resonating chambers.
Tom Hand
There are a lot of excellent points in this thread regarding mufflers and balancing flow with sound qualities. The first poster pictured a muffler design we used that was about 10 generations back (14 years). It served the market well with a reasonable exhaust flow/exhaust note and lasted for many years without coming apart even when being abused. Our most recent ACM-912 design (pictured) are for those wanting excellent flow and sound control that sounds close to factory. The ACM-912 mufflers have replaced these earlier units. For those wanting all out performance we have the "Stainless Magnum" series muffler which is similar to the Ultraflow and Magnaflow straight through designs but these , like most similar designs can resonate under load and certain rpms. A few issues that face those of us in the exhaust industry is marketing a muffler that is not too loud in the passenger compartment, flows reasonably well and fits properly.
