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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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Old 08-31-2016, 10:38 AM   #2 (permalink)
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Quote:
Originally Posted by stillsearching View Post
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.
Most turbochargers either have a wastegate (aka turbo bypass valve) or the variable vane turbochargers you can adjust how much boost you get with the variable vane setup. Both are typically vacuum actuated and it is very easy to manipulate them or replace their actuators with an electric actuator to gain full control over the turbo and how much boost pressure you get.
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Old 08-31-2016, 10:48 AM   #3 (permalink)
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Turbos kill mpg when you get into the turbo.

In applications designed for higher mpg they help by giving a puny engine more punch when it's really wanted- but staying in the powerless, non-turbo zone is where the savings actually are.
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Old 08-31-2016, 11:34 AM   #4 (permalink)
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Quote:
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Turbos kill mpg when you get into boost.

In applications designed for higher mpg they help by giving a puny engine more punch when it's really wanted- but staying in the powerless, non-boosted state is where the savings actually are.
Fixed your wording a little.

Anyway, I'm tired and didn't feel like responding, but thought I would because Old Tele Man, I don't know how extensive your knowledge is on forced induction engines, but you are most certainly wrong. When off-boost the effective CID does go up, but so does engine efficiency (which increases exponentially as boost builds, btw). The engine does not have to work as hard to 'ingest' the air so the engine is running at a higher efficiency. More efficiency = less throttle required to produce the same amount of power.

Just as an example, on my turbo insight, when climbing my 6% grade I used to have to use 34-38% throttle to maintain 70. Now I use 26-32% to maintain the same amount of speed. Why? I'm not producing boost and my vacuum levels are the same. It's because the engine itself is running at a higher efficiency and the extra back- pressure in the exhaust caused by the turbo restrictions is creating more torque, which again is great for efficiency.
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Old 08-31-2016, 01:47 PM   #5 (permalink)
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Turbochargers NEVER kill MPG, unless it's implemented wrong.

To get the same power out of a naturally aspirated engine, the displacement and / or number of cylinders would have to be increased. The normally aspirated engine would get worse MPG than the turbocharged one.
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Old 08-31-2016, 02:28 PM   #6 (permalink)
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All things being equal, yeah. But turbocharged engines are very rarely designed equally to naturally aspirated engines. Typically, they have lower compression ratios at the very least. They also have beefier internals which I'm sure doesn't have a gigantic effect on efficiency, but its got to have some. They're also typically smaller than the normally aspirated engine offered in a car. If we get into fuel management, they also dump fuel (open loop operation) to prevent detonation which generally isn't an issue with naturally aspirated engines (unless you really wind them up). So, its quite hard to do an apples to apples comparo.
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Old 08-31-2016, 02:31 PM   #7 (permalink)
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Perhaps the basic reason is that a turbocharger allows you to get more power from the same displacement. More power means you have to burn more fuel (simplistically). If you really wanted to use a turbocharger to increase a car's mpg, you would do it by downsizing the engine.
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Old 08-31-2016, 07:53 PM   #8 (permalink)
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6 answers and still not answered.

Not asking about power, or controlling boost, i'm not asking about people using their left foot to ruin MPG, not talking about hillclimbing with more power at lower rpm... i'm talking about i've never seen ever any automaker put out two versions of the same engine (one with turbo and one without) where the turbo version even matches the MPG of the nonturbo one. The EPA driving cycle accelerates and loads all engines about the same. I would assume that a light foot and staying out of boost should in theory leave you with the same mileage. I would assume its "right foot controlled" but i've not seen that.

I don't know if it's just the backpressure worsening mileage (when it's not in boost) or whether the turbo reducing the vaccum in some way impairs it. (I'd heard for mileage to drive at steeper vaccum if you have a gauge) Or maybe the presence of the turbo prevents that super high load condition (like used by pulse and glide drivers) where the BSFC island is at it's best?


SOOOOO i'm wondering why there isn't just a complete turbo bypass ever implemented, similar to having exhaust cutouts on some cars, even if it has to be heat resistant. For that matter you might even have a three way cutout - two different turbos for different load conditions (lower rpm torque, higher rpm performance) plus a total cutout for best MPG keeping high load unladen.

I'm hoping someone who knows the physics of turbos better maybe could tell me why it's not a good idea to put in a cutout.

PS Balto cute avatar. And in the theory of the Saab Light Pressure Turbo and the Ford Ecoboost I should be wrong, but according to the EPA figures for basically every turbo engine i've seen it doesn't show under their test cycles.
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Old 08-31-2016, 10:29 PM   #9 (permalink)
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Quote:
Originally Posted by stillsearching View Post
6 answers and still not answered.

Not asking about power, or controlling boost, i'm not asking about people using their left foot to ruin MPG, not talking about hillclimbing with more power at lower rpm... i'm talking about i've never seen ever any automaker put out two versions of the same engine (one with turbo and one without) where the turbo version even matches the MPG of the nonturbo one. The EPA driving cycle accelerates and loads all engines about the same. I would assume that a light foot and staying out of boost should in theory leave you with the same mileage. I would assume its "right foot controlled" but i've not seen that.

I don't know if it's just the backpressure worsening mileage (when it's not in boost) or whether the turbo reducing the vaccum in some way impairs it. (I'd heard for mileage to drive at steeper vaccum if you have a gauge) Or maybe the presence of the turbo prevents that super high load condition (like used by pulse and glide drivers) where the BSFC island is at it's best?


SOOOOO i'm wondering why there isn't just a complete turbo bypass ever implemented, similar to having exhaust cutouts on some cars, even if it has to be heat resistant. For that matter you might even have a three way cutout - two different turbos for different load conditions (lower rpm torque, higher rpm performance) plus a total cutout for best MPG keeping high load unladen.

I'm hoping someone who knows the physics of turbos better maybe could tell me why it's not a good idea to put in a cutout.

PS Balto cute avatar. And in the theory of the Saab Light Pressure Turbo and the Ford Ecoboost I should be wrong, but according to the EPA figures for basically every turbo engine i've seen it doesn't show under their test cycles.

Thanks, that's my fursona :3.

I have never seen a factory turbo car that has the same gearing ratio as the NA version. The turbo version always has a shorter ratio to make it feel more 'sporty'. Thus, Killing the mileage. Also no manufacturer in the past has economy in mind when tuning the ECU and other systems on a turbo version.
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Old 08-31-2016, 11:39 PM   #10 (permalink)
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The problem is, you can't compare an X-NA to an X-Turbo engine. The X-Turbo engine, granted, will never get the exact same efficiency as the X-NA version. Even if you tried to completely bypass the turbo, you're introducing a more convoluted exhaust path that robs it of power (compared to the smoother flow on newer NA engines) and which causes lag and driveability issues. And that's not taking into account the intake path, which would have to also bypass the turbo unit completely.

Then there's the higher pressure fuel rail, the bigger injectors, the stronger clutch and drivetrain, all that extra weight, the need for a richer air-fuel mixture to prevent detonation, the lower compression versus NA variants, etcetera, etcetera, etcetera.

In the end, that X-Turbo engine is meant to replace a Y-NA motor that's bigger and... theoretically... less fuel efficient. Comparing it to the X-NA is comparing apples to oranges.

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