You're talking about 4 different Flowmaster mufflers:
1. The Flowmaster 40 series
are two chambers.
2. The 50 series are three chambers.
3. The 40 series Delta are the new design of two chambers.
4. The 50 series Delta are the new design of three chambers
The Delta Flow mufflers have a different baffle configuration, using "Delta" shaped baffles just inside the inlet end. It reduces resonance by about 30%, and it's supposed to provide a little more power than the old design.
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Moxie Racing