This is a 2001 Chevy S10, 2.2L four cylinder. The muffler is the original. We replaced it with a Dynomax Super Turbo and decided to check the internals of the OEM muffler. The truck has 2.25" OD exhaust piping, which accounting for some flattening at the curves gives at least a 3.10 square inch cross section. Inside the inlet and outlet tubes are smaller perforated tubes that restrict the cross section down to about 2.25 square inches. But the shocker was that instead of a third tube to pass gas from one end to the other, there was a solid bulkhead with seven apparently randomly placed holes of approximately 7/32-inch diameter. This equates to a total cross sectional area of about 0.3 square inches for the exhaust gas to push through the muffler. Here are the photographs of the bulkhead and holes. As you can see, the outlet pipe is solid wall, so no gas passes through there, and the crimping you see is to hold the smaller perforated tube in place.
