does BACK-PRESSURE increase MPG vs straight thru...

[07WYarisRS]

Originally Posted by GeneW

You've actually seen this in the real world? I would want to see such data.

yes
But again i deal with bikes on a daily basis not cars but these engines all basically work the same.
many of my race engine are dialed in using CHT and EGT as well as proper plug reading...
Simple changes like header size and diameter, header shape and length, exhaust size and silencer can and will alter how efficiently the engine runs..
Even a simple slip on/bolt on muffler often requires rejetting.
With EFI bikes the fuel maps are changed to correct any lean or rich mixtures when changing exhaust set ups
There would be no need to rejet or re-map if the efficiency of the engines combustion never changes.
An engine runs leaner when more burnt gasses are allowed to escape, when burnt gasses are kept in the combustion chamber this dilutes the mixture and the power output... Some engines respond much better then others to minor changes and require much larger changes, other it makes very little but a lot of that has to do with valve overlap etc.