Just found this great graph. It plots A/F at down pipe of a Subaru as well as the simultaneous tail pipe A/F.

It seems to show:

1. Tail pipe A/F accurately represents manifold A/F
   
2. Tail pipe follows manifold by pipe-propagation-delay time.
   
3. Manifold more accurately captures fast transients. These transients are reduced in amplitude at the tail pipe.
   

So for carb a/f monitoring, use the manifold for accelerator pump transient tuning. The tail pipe is fine for all other carb tuning.

Turbocharged Subby or non?

It says "down pipe" (DP) so I assume turbo.

That's neat stuff. I've wondered about that.

I like your conclusions as well. Makes sense for some small amount of additional mixing and smoothing of transients by the time gasses reach the light of day, but it should still be FIFO for the most part.

Cool!

Was the Subaru a race car? Is that rich a mix what you want at high rpm? Also wondering about the big drop in AFR right at 3400 rpm.