Actually they are both straight through, the Magnaflow just has more rock wool in it (we at Magnaflow dont use fiberglass, due to its lack of durability)
The difference is because of the bigger body, we can use more rock wool inside the Magnaflow system then the Magnapack system which makes the vehicle quieter or louder.
They both do have rock wool in them though.
The rock wool is basically processed lava rock with a temperature rating of close to 3000 degrees so you do not have to worry about it burning up over time like fiberglass will-