New S4 with Supercharger 27 mpg - how?
 

Has anyone been reading about the new S4? They are saying that the new engine is supercharged vs. turbo'ed. Since I had a SC'ed M3, I thought this would be nice, but the turbo would ultimately give more power.

Usually, SC'ed cars suffer from poor gas mileage since the SC is causing "drag" on the engine. Turbo cars typically get better mileage. So how does this new S4 get such good mileage at 27 mpg? Does the SC have a "clutch" that disengages it?

I know the engine is direct fuel injection, the DSG with 7 speeds gets better mileage than a manual, etc. But I was thinking that this car would have gotten 21 mpg or so with the weight around 3600 lbs. When my M3 was supercharged, I was getting about 22-23 mpg on the highway. My car was 3175 lbs.

I must say that the wife and I are seriously going to consider the S4 at this point. We didn't before since the V8 gets such shitty gas mileage. The S4 gets the same highway mileage as the 4cyl A4 on the highway!

Iv been following the new S4 closely, i was initially disappointed when the first information was released about the supercharger. I was really hoping for a twin turbo a la B5. Now i see the light so to speak, the B8 S4 looks unbelievable. I will be test driving one as soon as they hit the dealerships near me.

The fuel econemy really comes down to the FSI direct injection. It allows the roots type supercharger to be placed in the V of the engine behind the throttle plate so then the throttle is closed the supercharger is in vacume, the result is increased efficiency. I think parasitic drag is 20HP at the 333hp rating for the car. The supercharger itself is also a relatively new & efficient design, IIRC its twin screw, 4 lobes per screw, with very tight clearances and 2 intercoolers. Variable valve timing is another notable addition that helps improve fuel efficiency.

Beyond that audi is making an effort to reduce friction within the car making it more efficient overall.

7 speed only nets you 1mpg on the highway. Ill take 3 pedals and a 6 speed.

If your interested here are 2 good articles. The first is a more recent road test, the second is a little older but has all the tech info. Theres really alot of good stuff in this car! And i like the look too, even though some people say it looks like a BMW. The lip on the trunk and A5-esq tail lights really look good to me.

http://www.audiworld.com/news/08/b8-s4-first-drive/

http://www.audiworld.com/news/08/s4-in-depth/athlete/

http://www.audiworld.com/news/08/b8-...ve/B8S4076.jpg
http://www.audiworld.com/news/08/b8-...ve/B8S4008.jpg
http://www.audiworld.com/news/08/b8-...ve/B8S4040.jpg

^+1

Audi has really taken direct injection to a new level -- I read an article a while back (I think it was posted over at Edmunds), but I can't find it. It gave a nice breakdown of all of the perks of FSI, notably it's improved efficiency.

Here's an article posted a while back when the 2.0 FSI was released.

http://www.audiworld.com/news/01/iaa/fsi/content.shtml

I'll see if I can find the article I read a while back, I believe it was written when the 4.2 was developed for the RS4.

Fuel rail pressure is 150 bar on the s4 that equals 2175 psi. Thats crazy.

For sure!

I was amazed to see that kind of pressure on a gas engine. High pressure fuel applications are no longer reserved for diesels :)

Haha its like a diesel/gas hybrid. I'd like to see the size of the injectors :D

Here are the APR Stage 3 injectors from the 2.0FSI
http://www.goapr.com/images/20ts3_injectors.jpg

Well not as big as I was expecting. Diesel injectors are massive.

The large diesel V8 and I6s have tremendous pressure at the injector tip. Pretty common for them to run 25,000+ PSI -- and no, that isn't a typo.

Not sure what the pressure is at the injector tip on the FSI applications, but I doubt it's that high. I would guess that much pressure on a gasoline application would probably cause some crazy pre-ignition issues.

Any new news on the new S4? I.e. - has the delivery dates changed?