More backpressure doesn't affect the mixture.
It affects the EGR and modulator. More exhaust in the intake = less NOx.
Look at the modulator and EGR and hoses in Snowtoy's post.
When the bottom of the 'T' is closed, full available vacuum is applied to the EGR (if the BVSV is working correctly, BVSV is so EGR doesn't work when cold, you can bypass as smog test is for warm conditions)
Backpressure at the bottom of the modulator and vacuum in the middle will close the bottom of the 'T'
But (this is from memory) the middle chamber also has a controlled vacuum leak. So if there's low backpressure and not much vacuum the bottom of the 'T' is never (or rarely) closed. So you never get much vacuum to the EGR valve to open it up.
Toyota designed the system for the stock exhaust and muffler. Any changes to the exhaust and you've messed up Toyota's calibration.
The plate I made was just a educated guess and it worked................
