I’ve seen comments on various articles questioning the extended exhaust tips sometimes seen on prototype cars like this:

In case anyone was curious, these are fitted to prototypes meant for fuel economy and/or emissions certification testing. The reason they use these special tips is that fuel economy is calculated via the “carbon balance” method; the amount of carbon in the exhaust stream is directly proportional to the amount of fuel burned. All of the exhaust gas stream has to pass through the analyzer, so the analyzer’s induction tube has to fit to the tailpipe with a tight seal. Therefore they weld a “standard” test pipe in place of the normal exhaust tips. And, obviously, cut out the fascia if it has hidden exhaust outlets.

This is a lot different than the “sniffer” probe you probably see when you get your yearly emissions test done, which is strictly looking for excessive trace pollutants.

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the upside of testing this way is you can do emissions and F.E. in basically one shot.