I have been testing the backpressure on my AMG C43.

I did not start before I chopped off the AMG muffler and resonator though.

But with the Magnaflow "X" chamber muffler with 2.5" dual in and 2.5" dual out where and stock resonator was and then straight dual 2.5" piping out to the bumper I measured 1 psi just ahead of the muffler and in back of the stock cats.

Today I measured the pressure ahead of the stock 2nd cat and found 4psi of backpressure. I believe 4psi is quite a lot of backpressure.

The backpressure rises with engine load and rpm. So at 2000rpm the backpressure is low, say 1/2psi but climbs to say 2psi at 3500rpm and to 4psi by 5000 rpm.

Jeff