Impreza Off Boost?

Never claimed otherwise.. but (at the time) it was worth it.

"PC Paul" wrote in news:Lo6Me.1155$Wq4.379 @fe1.news.blueyonder.co.uk:

Let's face it, turbocharging sacrifices economy just as much. My S70's handbook still has a little card with the Volvo mpg figures on it. Combined mpg for my T5 is 25.2. A modern, normally aspirated 2.3l engine would do much better than that. Basically, more power equals less mpg. The only way to improve speed without sacrificing fuel economy is to reduce weight (i.e. get a motorbike).

When I was looking into getting a bike to save money while maintaining performance, I found it was going to cost about the same to run as the car, once you include servicing it every 3000 miles, a new rear tyre frequently, etc.

And fuel economy was crap compared to what I expected! Almost all only had an average of 35-40 mpg.

"PC Paul" wrote in news:N37Me.1373$ snipped-for-privacy@fe1.news.blueyonder.co.uk:

Not to mention the fact that you're far more likely to die. My mate at work is now looking at buying a TVR, after a near-death experience with his Kawasaki Ninja. The largest remaining piece of the bike was the fuel tank!

If you drive normally, a turbo actually saves fuel. This is because of the positive displacement effect; the turbo makes a small engine behave like a larger engine, but without the extra friction losses of the larger engine. The spin of the turbo wheel is almost for free. In practical terms, you can reduce the rpm and drive in higher gear, just as if it was a bigger engine.

No in 90% of cases.

the turbo acts as a restriction in the exhaust so the engine is breathing with a clogged lung effectively, and as petrol turbo motors tradtionally ran with a lower CR- thermodynamic efficiency is lower than a NA higher CR engine would be, so again it will use more fuel.

A petrol engine tradionally operates at 14.7:1 fuel air mix, if you add more air, you have to add more fuel- mpg decreases, so allowing the turbo to boost is not good.

However if you look at the very latest crop of low or medium blow turbo's with 10:1 CR's, and lean burn technology, and direct injection, such as VAG's 2.0l FSI turbo motor, they give very nearly as good mpg as the equivilant NA engine, but only when running at little or no positive boost.

Diesels are another matter. Almost without exception, if you take a stock NA diesel, and turbo it, leaving the fuelling as stock, or even increasing it slightly, the turbo will present better mpg's.

PSA's XUD 70ps vs the XUDT 90ps motors are a very good example.

Tim..

Because of the hot gas temperatures. The high volume of expanded gas is as much restricted as in a NA engine. In the turbo, the gas works against the turbo wheel instead of the exhaust. After the thermodynamic work on the turbo wheel, the gas becomes cooler and denser and flows throw the exhaust at lower speed than for a NA. Hence the loss due to the turbo restriction is minimal.

Not if you use a lower rpm. Since you add both air and fuel, you can end up with the exact same volume and mix (power) but at lower rpm. This is the 'positive displacement' effect. The engine does the same as before, but at a lower rpm, hence less friction and pumping losses.

If you use the engine as a 'performance' engine, then yes you will naturally use more fuel. If you just use the engine as usual, then the turbo will be more economical.

Yes, but the same principle can be applied to petrol turbo, e.g. Saab's light pressure turbos.

Except in a diseasel.

The Turbo Escort Diesel has BETTER fuel economy than the non-turbo, and