No, I can't think of a reason why a smaller engine should have a higher exhaust gas temp/pressure. (I assume the the exit port is near atmospheric). The cooling effect comes from the turbo taking energy from the exhaust gas... therefore its temperature or pressure should drop... this can be seen in pictures of the manifold before and after the turbo. Anything that cools the exhaust gas and puts that energy back into the engine should increase thermodynamic efficiency. I think the big gain is the reduced weight of the engine/car/ -> rolling resistance etc. of smaller engine cars. The amount of unburnt fuel should be negligible (assuming the ECU is correct or the carb is properly jetted) so the thermodynamics become quite simple and reflected only in inlet and outlet temperatures and pressures (and waste heat). At least that's how I see it (being simplistic).

Cheers

Why has the turbocharged car got a lower effective compression ratio?

Do they not still run lower compression ratios than N/A?

The geometric compression ratio of the piston may be lower, but then the turbocharger pushes it up beyond where an N/A engine would be

Ah, well, if you're running at a power when the turbo isn't making boost, then the larger N/A engine will be well throttled back, pulling the maifold pressure waaaay down. I'm trying to dig out an old spreadsheet out of interest to see how the efficiencies pan out...

The problem with new engines is that they are driven by the same old drivers.

Magic new technologies sometimes require the user to use them sympathetically. Expecting to get the same performance and better economy at the same time is unreasonable. However with a downsized engine you have the option of the same performance or improved economy, depending on how you drive it.

The driver has to adapt to new technology to benefit from it. We do it in other areas of life which is why we don't use mobile phones by tieing a string between two of them and shouting.

You know the people who hold you up on nice twisty b-roads? They are driving slower and more cautiously, and to them a downsized turbo engine makes sense. They might even change up when the indicator on the dash tells them to. Your typical PHer will want to drive fast and should maybe stick to manly sized engines. PHers are not a representitive cross section of drivers.

If a new technology doesn't work for you and you don't want to change then you don't have to use it. It doesn't make the new technology bad, it just means it isn't for you. Although with the never ending increase of fuel costs maybe people will want to change up a bit earlier, and not floor the throttle all the time.

How do you know the engine was boosting at 55mph on a steady throttle in top gear? Did it have a boost gauge? If it is making boost in those conditions I'd happily describe it as a rubbish engine. It'd need less than 30bhp to maintain that speed, and a 1.4 shouldn't need a turbo to develop that sort of power unless the gearing is rediculous.

After that you lost me at "make normal progress".

I like this thread. Lots of technical discussion and no one has called each others cars gay yet.

With regards to the Astra above, I would guess that it's not on boost cruising at 55 but will be if you need to touch the throttle at all.

True.

False.

I have been wondering about this for a while and I'm disappointed with manufacturers shifts towards smaller capacity turbo engines to meet co2 targets but I guess that's their way of playing the game.

I would like to run a back to back test comparing my BMW 2.5 straight 6 against the fiat 500 twin air turbo on a variety of routes. My money would be on the BMW using less fuel on a comparative on a real road test, not in the science lab.