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Old 08-31-2016, 09:12 AM   #1 (permalink)
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why do turbos ALWAYS kill mpg? (& why not bypass)

Something i've often wondered about and decided wonder out loud about...

I would assume the exhaust restriction from having a turbocharger in the exhaust stream if you werent producing any boost should be fairly minimal, but i've never once seen a turbo version of an engine which got even the same equal MPG as the nonturbo version of the engine. There is always a minor penalty of 1-3mpg it seems.

Which made me wonder why not just have an exhaust cutout to completely route around the turbocharger when not wanted or needed? Especially with some of those newer rear mount turbochargers this would not be as inconvenient as doing underhood anymore.

Two cases of turbos possibly not killing MPG:

I think it was Saab used to have a system they called the 'light pressure turbocharger' (keeping higher compression and lower boost like 5psi) which supposedly if combined with steeper gearing, made for better MPG. (the idea being that with slightly lower gearing it kept you from downshifting supposedly providing more economy at the higher load) Having seen no back to back comparison and still seeing lower MPG than comparable cars with similar sized engines I wasn't sure if it was BS though.

Ford's Ecoboost makes the claim that their system supposedly taps into the "high efficiency island" of low BSFC and extends it over a wide rpm and load range. Though without a BSFC chart i'm not sure if that's just theory or reality. It couldn't beat the cyl deactivation chevy v8 in 2014 for mileage until they lightened their trucks with aluminum, so....

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