why do turbos ALWAYS kill mpg? (& why not bypass)

[oil pan 4]

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

[pgfpro]

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.

[Fat Charlie]

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?

[Stubby79]

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.

[Fingie]

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 .

[sael]

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

[cRiPpLe_rOoStEr]

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.

Originally Posted by Stubby79

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.

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.

[teoman]

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

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

[niky]

Originally Posted by teoman

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.

[NHB]

Originally Posted by Baltothewolf

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.

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.