Exhaust back pressure testing part 2

Follow up on the earlier blog post where I measured the backpressure on the OEM exhaust I now have some data for two new setups, both with the OEM rear muffler and GW 1.5” roadstersport header:

  1. 2.5” GW roadstersport non catted resonated midpipe.

Max pressure as measured after the header was 6 psi.
If we assume that the header and straight trough midpipe gives no more than 1 psi, the stock rear muffler adds 5 psi back pressure alone.

  1.    2.5” I.L motorsport catted and resonated mid pipe.

The reading says 4 psi, but since I measure AFTER the CAT there is probably some psi of back pressure in front of the CAT. I would expect the total to be more than the 6 psi measured on the non catted midpipe..


  • Stock exhaust is not usable for FI with 11 PSI after the primary CAT.
    The fact that a large chunk of the CAT fell out during testing could mean that the pressure up stream was considerable more.
  • Even with a non catted 2.5” midpipe and 1.5” primary non catted header the stock muffler seems to add 5 psi back pressure.
  • A spun metal “race cat” adds back pressure. Hard to tell how much, but I would guess at ~ 3 psi.

If someone does similar testing with results BEFORE a catted mid pipe please post the result on miata.net.


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