do turbos kill mileage? bypass for mpg?

[stillsearching]

I've never seen a turbocharged car which gets even comparable mileage to a nonturbocharged car under the same specifications YET. Even if the total package may be a little better (like the Chevy Cruze Eco using it to pull higher gears) I would assume if you deactivated the turbocharger that even though you couldn't drive under normal habits, it might actually improve mileage assuming you didn't normally see hills and such anyways.


On that subject I was wondering, why not have a turbo bypass built in? Then it's there when you felt you needed it, and could be cut out when you were in Eco mode. I can't think of any downside not to except the expense of the turbo if it's added on and not stock.

[oil pan 4]

Turbos help the fuel economy on diesels.

They do have a bypass for the exhaust, its called a waste gate.
The intake also has a bypass valve, called a blow off valve, its operated by intake vacuum, anything less than about 5''Hg and the blow off valve is open.

Diesels do not require blow off valves.
I intend to use a BOV on my diesel to allow air to by pass the compressor at low speed before the turbo lights off, I measured measured 1''Hg in the intake piping so far, driving it kind of easy.

Other wise turbos help diesels about 90% of the time. You wouldnt want to deactivate it.

[thomason2wheels]

Not to puttoo fine a. Point on it but there is a bypass of sorts.....your right foot. Having owned several turbo cars and also their normally aspirated brothers, the nonturbo versions in every case had more usable torque under 2000 rpm than the rurbo cars did. Above 2k, the turbo cars would run circles around their na brothers. My feeling is the turbo costs about 2 to 3% fuel economy for performance sake. Even if you could bypass the turbo, the lower static compression of the turbo engine would have a negative impact on fuel economy and bottom end torque. So while you can be gentle with the right foot and largely stay out of the boost except for long steep hills, there is a penalty paid for having that hardware.
Another sonsideration is cam timing. Most gas turbo cars use a milder camm grind than their higher compression na brothers. Bottom line is the laws of physics remain in force. The so called free energy driving the turbo is a mechanical load imposed on the engine as a pumping loss on the exhaust side. I guess there really is no such thing as a free lunch.

[NickelB NL]

I have a diesel turbo, but when i drive just under the turbo spoolup rpm is use about 0.5L/100km less then when my turbo spools up. The turbo spoolup is just above 1800rmp and max torque is at 1900rpm. But my engine is from around the year 2000. One of the first electronicly controled turbo dieseld. My mate has a 2007 saab 93 1.9 turbodiesel 6speed. In gear 6 at 120kmh it uses more then 130kmh is 6. This is because the turbo spools up at 125. So i think you can say that newer diesels need their turbos, and the old could do without.

[thomason2wheels]

Guess i should. Have said that i have never owned a diesel and given the premium prices they seem to go for and the cost of diesel fuel, prob never will. But they do seem to have some real efficiency attributes. Wish icould afford one as a toy to play with....

[NickelB NL]

The balance here i completly different. Diesel is about 20% cheaper at the pump. And whit better mpg it could save money. To bad thet we have to pay al lot of road taxes here in the netherlands. I pay around 1500+ dollar a year for my diesel. The same car whit a gas engine is 700 a year

[thomason2wheels]

Originally Posted by NickelB NL

The balance here i completly different. Diesel is about 20% cheaper at the pump. And whit better mpg it could save money. To bad thet we have to pay al lot of road taxes here in the netherlands. I pay around 1500+ dollar a year for my diesel. The same car whit a gas engine is 700 a year

Fascinating that it would be so different there. And the taxes....yikes,that is crazy. You'd haveto drive a lot of miles in that diesel to make the higher taxes worth it.

[Vekke]

Some of the time turbo helps to lower fuel consumption like when accelerating etc. But when driving steady speeds you dont need it and it hurts the FE little. However you can remap your ECU programming to avoid that boost in steady throttle.

My lupo did have 0.5 bar boost when driving 100 km/h or 62 MPH. That boost if there so the EGR system could work. Now with custom ECU program there is no boost even at 120 km/h speeds. When you have done this modification turbon is always more fuel efficient.

[thomason2wheels]

Sounds like retuning the wrx ecu would buy me better economy....hnmmmmm

[serialk11r]

Usually the wastegate is activated when the pressure reaches a certain level. An electronic wastegate can be opened at all times to provide said bypassing effect at cruise to improve fuel economy. One might wonder, a turbo is supposed to recycle waste energy in the exhaust, so why does it even create backpressure? Well a turbo's main objective is to increase power, and then it needs to be driveable as well. For driveability (turbo lag/transient response) the turbine is extremely restrictive to be able to quickly produce boost.

Engines with continuously variable intake duration can benefit from the pressure produced by the turbo that would otherwise be wasted via blow off valve.