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....

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.

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.

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.

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.

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.

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.

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.

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.

Turbocharged gasoline engine typically run lower compression ratio and richer air fuel ratio. So of course fuel economy will be lower.

This might help answer your question.

I have installed around 10 turbo systems on N/A Honda engines. I did not change anything on the engines. Completely stock. These were just bolt on turbo kits and before the larger injectors, stronger heaver clutch and engine management were added the engines got the same fuel mileage as they did N/A at light load and freeway driving. The exhaust back pressure did not hurt fuel mileage at these loads. In fact I measure on two different engines that the turbo back pressure decreased at light load verses N/A because the exhaust system pipe diameter increased from 1.5 to 2.5". So there is no reason to install a exhaust cut out to bypass the turbo at light load.

As others have already said when you see two types of engines of the same family one N/A verses one turbo there's a reason the turbo engine gets poorer fuel mileage.
Lower compression.
Heavy duty internals and drivetrain two handle the HP at high load.
Fuel enrichment to keep detonation suppressed because of the uneducated car buyer.
etc.

The real problem is that at the design stage, factories want to build one thing and sell it several ways instead of building several things to sell one way.

What idiot would come up with a turbo variant of an existing engine that matched it in any performance metric except maybe emissions? How would anyone pitch that to their bosses? We'll slap a turbo on it, but dial it back so it won't add much power in order to match the mpg of the regular engine. Say another engineering crew has a competing pitch: We'll slap a turbo on it, get a bunch of additional horsepower and it'll only cost us a few mpg on the EPA tests. We'll call this trim level the "Sport" and get an extra $5k for it.

Which team do you think is still going to have a job at the end of the meeting?

As has been mentioned, lower compression ratio, plus richer fuel ratio to keep combustion chamber temps down are the main factors. But they can get around these with direct injection these days...examples being GM's 1.4l turbo ecotec engine and VW TDI engines.

My Turbo All-trac Celica Has a 3S-GTE and gets a bit worse MPG than my previous non-turbo celica, which had a 3S-GE.

Have to account for a heftily increased drivetrain loss, though. The turbo variant is lazier on low revs but boy, I can overtake cars in 4th gear :D.

In my experience with a turbo diesel is easy and around of the 90% of the time you are
throttling go to the 75/90% engine load land.
Once you have the speed desired the engine goes to 25/30% engines load.

So turbo chargers are good for throttling in a efficient way

In certain circumstances, such as when an engine would be deemed underpowered if fitted in naturally-aspirated form into a large vehicle, turbocharging actually improves the efficiency regardless of fuel system, be it a carburettor or an EFI.


I guess you mean the VW 1.4 TSI, not the TDI. But anyway, detonation is not so much of an issue nowadays for those newer turbocharged engines fitted with direct injection as it used to be for port-injection engines.

I have one of those fsi engines (passat 2011 1.8 fsi).

How much extra fuel is injected? %1 - %10?

Depends on the situation. Older engines run hella rich (down to the 10:1 range) when in boost.

The newer engines with direct fuel injection run close to stoich at a cruise... or if you have the stratified fuel injection, it's running extra lean, so no extra fuel is being injected to cool down the engine when you're out of the boost.

Under boost, you'll be in the 12:1 range, so probably around that ten percent more. But then, even with naturally aspirated engines, manufacturers tend to run the engines richer when under load for safety reasons. Even those with direct injection.

I believe that you are talking about cars, which have seen the daylight a long time ago. Those old turbos used to be weak a low revs and needed short gearing to compensate their weakness. Modern turbo engine are strong at low revs and have virtually ALWAYS taller gearing than a comparable NA engine.

Here is a list of 1 liter engined cars from years 2015-2016. Speeds at 1000 rpm at the tallest gear are:

Turbo:
Ford Focus - 41.2 km/h
VW Golf - 44.4 km/h
Kia Cee'd - 44.6 km/h
Opel Corsa - 45,5 km/h

NA:
Hyundai i10 - 31.1 km/h
Renault Twingo - 33.5 km/h
Peugeot 108 - 37.1 km/h
Toyota Aygo - 35.1 km/h


Let's take some examples from the year 2013.

Turbo:
Audi A3 1.4 - 46.9 km/h
BMW 320i - 56.7 km/h
VW golf 1.2 - 44.4 km/h

NA:
Honda Civic 1.8i - 37.2 km/h
Mazda6 2.0 - 41.2 km/h

It is very clear, that turbo means taller gearing.

Yes, while that may be the case, the manufacturer still runs more aggressive tunes and lower compression on turbocharged engines. It's a fact of life.

When I had the turbo on my insight, my MPG went up by 3mpg. This was measured at the pump, not going off the FCD.

Why are we still beating this over the head anyway, it's been proven when you add a properly sized turbo/supercharger to an N/A engine, it gains 10-20% MPG due to the increased VE.

It adds efficiency to the engine, but a ricer-rated transmission leads every improvement in combustion efficiency to be wasted :turtle:

Turbo has nothing to do with aggressive tune anymore. Here in Europe almost every model has a turbo. This includes plenty of eco models, MPVs and busses.

Turbocharging can increase or decrease MPG. If you use some home made installation, most like you will loose some fuel efficiency. Better turbo installations can improve FE. For example VW's new 1.5 TSI has a geometric compression ratio of 12.5 thanks to miller cycle, which would not be possible without high boost pressure. Turbo gives also good low end torque, which allows tall gearing.

Allow me to further strike the deceased horse. Home made kits are installed 99% of the time for power, so more aggressive tunes are used, because the install is done improperly. Definitely agree with the low end Torque/taller gearing statement though.

There have been several comments about turbocharged engines being different internally with heavier rods, bearing and other parts. That may be generally true, I don't know.
fwiw During the mid to late 1990's, the turbocharged 5 cyl Volvo engine was mostly the same as the NA engine. Internally the only parts that I can remember being different were the piston (for lower compression of course) and the sodium filled exhaust valve. As for the transmissions and gearing - I have no idea if they were different.

I can tell you that I've been very happy with the mpg of my turbo car. My Saab 9-5 turbo has averaged about 32mpg over the last 13,000 miles. The odometer is showing just over 205,000 miles. As the OP and others have pointed out though, I have nothing to compare it to that would be a fair comparison.

Go back to the year: 1992. get a GM, 6.2 NA or 6.5 TD, same engine(bore diff isn't enough), exhaust, etc.... Turbo had more power AND ECONOMY! same thing for 93. 94 had (somewhat) computer control.