do turbos kill mileage? bypass for mpg?

[YukonCornelius]

I think with a gas engine it hinders it via the lower compression. I was always disapointed in the mileage that friends got with their WRXs an STi's.

[oil pan 4]

That D-serries thread looses a lot of creditability on about page 8.

[euromodder]

Originally Posted by thomason2wheels

Seems as though from what you folks are saying that the higher initial cost is outweighed several times over by the mean time between failure.

Was outweighed.
The latest diesels have become very complex machines.
As a result they also have more issues.

My opinion about automotive diesels, as opposed to trucks, has unfortunately been skewed by the early GM diesels and their rather sketchy reliability.

Things have changed a bit since - especially with European diesels

Dunno if you'll ever have the chance to drive say a 2011 or 2012 BMW 3L 6-in-line "330" diesel, but that's a good example of what modern diesels can be.

[Diesel_Dave]

Originally Posted by stillsearching

Can you find me even one vehicle on fueleconomy.gov that shows the same MPG figure for a turbo engine vs a nonturbo engine if gas powered?

No, I can't...but I can show you two with improved FE. The Chevy Cruze & the Dodge Dart. It comes with the option of 2 different engines: a 1.8L naturally aspirated engine and a 1.4L turbocharged engine. Both have exactly the same horsepower rating according to Chevy (138 hp):
2012 Chevy Cruze | Powertrain & Chassis | Chevrolet

fueleconomy.gov rates the NA engine at 22 mpg city & 35 highway = 27 mpg combined. The turbocharged engine is rated at 26 mpg city & 38 highway = 30 mpg combined--a 10% improvement in FE.
Compare Side-by-Side

The Dodge Dart is very similar--two engines with the same exact 160 hp power rating: 1.4L turbo & 2.0 L NA (http://www.dodge.com/en/2013/dart/). The 1.4L turbo beats the 2.0L NA engine by 10% (http://www.fueleconomy.gov/feg/Find....32403&id=32404).

Originally Posted by stillsearching

Those are being driven about as easily as possible in the mileage loop.

Research the EPA test procedure--there's a whole lot more to it than just driving around easily in a loop.

[niky]

Originally Posted by Lbar

I think with a gas engine it hinders it via the lower compression. I was always disapointed in the mileage that friends got with their WRXs an STi's.

AWD and tuning for power (read: rich) will do that. If the turbos are more realistically sized, perhaps fuel economy would benefit.

[meanjoe75fan]

For a turbo that does better, fe-wise, than the N/A version: the '88 Saab 9000 turbo matches it's N/A version EPA combined, and betters it by 2MPG on the highway. Same car, same engine....one had a turbo bolted; one didn't.

It was also extremely laggy, which shows the path to success: a really big, heavy turbine wheel.

Diesel dave: you mentioned compressor/turbine combos at 50% efficiency...better tgan piston power. Then wny not delete the piston portion and go with a pure turbine engine? I was told in my flying days that a piston would handily beat a turbine in "power-specific fuel consumption": is that no longer true?

[thomason2wheels]

Originally Posted by meanjoe75fan

For a turbo that does better, fe-wise, than the N/A version: the '88 Saab 9000 turbo matches it's N/A version EPA combined, and betters it by 2MPG on the highway. Same car, same engine....one had a turbo bolted; one didn't.

It was also extremely laggy, which shows the path to success: a really big, heavy turbine wheel.

Diesel dave: you mentioned compressor/turbine combos at 50% efficiency...better tgan piston power. Then wny not delete the piston portion and go with a pure turbine engine? I was told in my flying days that a piston would handily beat a turbine in "power-specific fuel consumption": is that no longer true?

The wrx is pretty 'laggy' too. Turbo starts working around 2k, cruising rpm at 55 is about 2200, so its in boost range at normal highway speeds. Next set of tires will be taller to force slightly lower rpms in top gear. I think getting it down to 2k if possible would give me 10% improvement at eco speeds but when needed i would stillhave boost available above 55.

[niky]

Some of the more economical turbos I've driven have very little lag. Lag sometimes indicates that the turbo was not optimized for the rpm range you're driving in.

For terrible lag, nothing beats the previous STI automatic. Absolutely nothing in first or second gear below 5k rpm. Sucks like a hoover to drive around town.

[oil pan 4]

Gas turbines have a very narrow high effeciency margin.
Usually you want the piston engine at its most efficient speed and load while the turbo is usually running at 10% or 20% efficiency.
You could size both the pistion and turbine engine to be most efficient at cruising speed but that would be all the vehicle would be good for. Every other driving condition would be less drivable. You would have to have a tiny engine turning about 2500rpms with a properly sized turbo boosting it to between 10 and 30 psi.

[mort]

Hello meanjo

Originally Posted by meanjoe75fan

Diesel dave: you mentioned compressor/turbine combos at 50% efficiency...better tgan piston power. Then wny not delete the piston portion and go with a pure turbine engine? I was told in my flying days that a piston would handily beat a turbine in "power-specific fuel consumption": is that no longer true?

What Dave was referring to is mechanical efficiency, not thermodynamic. The best low power turbines can return about 20% thermodynamic efficiency. And conversely the mechanical efficiency of a piston as compressor and expansion engine is better than 95%.
-mort