Results at idle are pretty meaningless...
I tested at 2000 RPM as well, following this guide:
How Exhaust Backpressure Affects Your Engine - https://www.aa1car.com/library/exhaust_backpressure.htm.
"Backpressure readings at idle on most engines should generally be less than 1.5 psi (10 kPa). This will vary somewhat from one vehicle to another depending on the design of the exhaust system, the size of the pipes, how restrictive the converter, muffler and/or resonator is, and whether it is single or dual exhausts. We've seen some idle readings as high as 2.75 psi on a few vehicles, but for most 1.5 psi or less at idle is normal.
A partially restricted converter, muffler or pipe may flow enough exhaust at idle not to cause a problem, but chokes breathing at higher engine speeds. So to test this possibility, you need to rev and hold the engine at 2000 rpm. A "good" reading on most engines at 2000 rpm should be 3 psi (20 to 21 kPa) or less. Again, there may be some vehicles that will read a little higher that don't have a problem, but the reading should not be significantly higher.
Pay close attention to what the backpressure reading does while you are holding it at 2000 rpm. If it remains steady, chances are there is no restriction. But if the reading gradually increases, it means backpressure is building up and there may be a blockage.
If you want to rev the engine higher, say to 4000 rpm and hold it, the backpressure numbers will shoot up. Most stock exhaust systems will show backpressure readings from 4 to 8 psi (27 to 55 kPa), or even higher. As before, if the backpressure reading is unusually high or it continues to climb at a steady rpm, it usually means there is an abnormal restriction causing an unhealthy increase in backpressure."
