Hey smart people!!(twin charge)
 

random question:

I understand why superchargers are not going to give you better millage, so why does vw make twin charge fi engines that seem to be all about gas millage and performance at the same time. The 1.4 makes 175 hp (forgot what torque it makes but I remember It was good) with over 40mpg. I also remember reading something that said the 2.0 was comparable to the vr6 in power

I think they use the supercharger at low rpms and turbo at high.

so, is the supercharger only helping performance, while the low displacement, and turbo are keeping it efficient?

Smaller engines are more efficient in general. A supercharged 300hp 2.4 liter 4 cylinder is going to be more efficient than a naturally aspirated 5.7 liter engine making the same power. There's much more to it than that but it's a good general rule. Less cylinders and less displacement = less fuel used.

Both the turbo and the charger are there for the performance, not for economy. The split between the charger and the turbo is probably to fight the turbo lag, since the engine is so small. This is a way for VW to make fun cars that are fairly good on gas.

VW is not pushing these engines as economical and efficient engines, at least not here on their home turf. They are just saying that it is more efficient then a larger NA engine with pretty much the same HP. Of course, the 1.4L NA version has a lot lower consumption. With gasoline at a price of 1.4€/L (yeah, you guys go ahead and check what that makes in $/gallon…) the added price of a charger on the new car is not a big deal, if you get the limited consumption of a 1.4L engine.

Gets better mileage for these reasons (mostly guesstimates)

1. it's a 1.4 liter engine in a car which normally takes 2.0-3.2l engines.
2. IIRC the SC can be disconnected on the fly (clutch)
3. With direct injection (if it has it) it can use less fuel (leaner AFR) in boost. Moreover, with FSI (the overseas version, fuel stratified injection) at idle and part throttle AFRs are seriously leaned out since the injector forms a "fuel cloud" around the plug, but runs very hot and lean so we can't use it here.

IIRC they disconnect the supercharger (clutch) when not in use ... so, there is probably a very minor lag when the supercharger gets connected vs rotating all the time, but that car's main goal is to save fuel and not to be raced on a track/autocross ...