It doesn't quite work that way. Most of your time on the street or highway is spent idling or cruising (keeping a steady speed on a relatively level surface), so the engine spends most of it's time at part throttle and lower rpms. When you get a bigger exhaust for more flow at higher rpms, you actually decrease flow at lower rpms and part throttle.

In other words, you've traded off engine efficiency at lower rpms to gain efficiency at higher rpms. And since you spend most of your time idling/cruising (something like 95% and higher), you will actually decrease your mileage. Even at WOT, you're still pumping in as much gas. You'll free up a little more power, but you won't get any more mileage.

Take a look at a magazine's before and after dyno comparisons of a stock vs. aftermarket exhaust on an NA car. You'll notice that your "peak" gains are at the top of the rpm band, but you will also lose some torque down at the lower rpm ranges. Part throttle response will suffer like low rpm WOT.

This is the main reason why economy cars like Civics come with small diameter exhausts. Honda tuned those engines for the best mileage, not peak HP. And that's why Civic owners get such big gains from simple exhaust and intake mods.

---jps

 

We do! It gets us nowhere. So much for 'democracy'. And to add insult to injury we're an oil producer too!

If you knew anything about the state of British public transport you'd soon realise that cars are a necessity here too.