Offical Audi S4 (B8) Thread

[Sunny]

Originally Posted by Centurion

Do superchargers really hog that power? I was under the impression that they only need 10-15hp to operate. After all they are just small turbines with very efficient ball bearings. I mean a lawnmower does a lot of work with just 4-5hp. I mean the superchargers hooked up to the V8 in the SLR can't be eating 80-120hp or so, no?

I remember reading somewhere the SLR engine actually makes about 700HP, but wastes ~100 to drive the supercharger.

[Deutsch]

I'll choose turbo's over superchargers anyday. Nothing like having instant torque under your foot when you want it. It's always there.

When reading Martinbo's analysis I though of the old MKI Cooper vs. the MKII model and what a difference there is. The MKI felt slightly lethargic in response to your input when depressing the accelerator, and thus didn't feel responsive enough. Swap over to the second generation car and once you squeeze the gas the turbo releases the torque instantaneously with little to no perceptible lag. A real hoot I tell you!

[Poverty]

another main drawback of the supercharger is usually 90% of the time it will significantly lower a engines max rpm. No good for my driving style, but not a problem of whom the S4 is aimed at.

[martinbo]

Originally Posted by Centurion

Do superchargers really hog that power? I was under the impression that they only need 10-15hp to operate. After all they are just small turbines with very efficient ball bearings. I mean a lawnmower does a lot of work with just 4-5hp. I mean the superchargers hooked up to the V8 in the SLR can't be eating 80-120hp or so, no?

Oh absolutely, and furthermore remember that superchargers come in primarily 3 different types or configurations - each of which are sized according to boost output required.

The 3 types of superchargers are:
Roots type; the oldest form of supercharger - its design dates back to the late 19th century where it was intended to ventilate mineshafts. It employs two sets of counter-rotating lobes to squeeze ingested air downwards. It is very commonly used in V-layout engines with the ZR-1, XK-R and this new S4 being examples. Roots blowers are quite efficient but they can be heavy and place a significant load on the engine.

Screw-type; screw type superchargers are the most precisely engineered of the supercharger types. In essence, two tightly fitting counter-rotating screws literally "screw" air from in input side and compress this into an ever smaller space creating the compressed output. These are similar in concept to a Roots-type supercharger. Has a high load on the engine as a result of the fine engineering tolerances and the output yield but very good at generating lots of boost early on. The SLR uses a screw-type blower.

Centrifugal-type; these blowers are kind of like turbochargers chopped in half. In this case instead of having an exhaust impeller being driven by exhaust gas, a belt driven pulley connects to the compressor turbine. Very efficient, lightweight and compact units, well suited to smaller engines. Employed in the previous supercharged Mini Cooper S.

The overriding theory though is that superchargers are basically air pumps that consume an engine's power by virtue of the fact that: a) they have a mechanical and frictional load on the engine by virtue of the rotating components and their (often) considerable mass.
b) because they are used as compressors, the work performed to compress this air is a resistance to the engine and as a consequence saps power.

In the end though, superchargers are widely acknowledged as the simplest way of gaining dramatic increases in torque and power from an engine - but as we can see, you can't get something for nothing.

[warot]

Anyone else think that Martin deserves his own sub-forum? That was a serious question btw.

I've read car engineering books (hell one of them by Mr.Carroll himself) and they are nowhere as easy to understand as what Martin just wrote.

This is a huge huge boost to GCZ. If it's at all possible, I would love to see a subforum where Martin just kind of blogs about technical car stuff that he chooses, maybe once a week or something.

I remember one time I asked you why it's not possible to have an inline 12 engine hehe

Oh back on topic... this car looks ok... more interested in engine. That's about it

[PanterroR]

Originally Posted by warot

Anyone else think that Martin deserves his own sub-forum? That was a serious question btw.

I've read car engineering books (hell one of them by Mr.Carroll himself) and they are nowhere as easy to understand as what Martin just wrote.

This is a huge huge boost to GCZ. If it's at all possible, I would love to see a subforum where Martin just kind of blogs about technical car stuff that he chooses, maybe once a week or something.

That's a very plausible idea and i personally thank you for that. I'd like that too, but we need to ask Martin first and if he accepts, i don't think Mark (admin) would hesitate to do so.

warot, thanks a million!

[ACE]

Originally Posted by martinbo

You'll notice that this new supercharged 3.0 TFSI engine delivers maximum torque at 2200 rpm - higher than that of the 335i (+- 1500 rpm iirc) - and this is directly as a result of the supercharger rpm having to reach a certain speed in order for the engine to produce the desired amount of torque. So, in effect, it is the 335i's turbo engine that is more responsive, hence the driver's perception of lag is significantly diminished.

So in such cases we can see that turbocharging technology has come a long way (small, twin-turbo'd configurations; twin-scroll turbos) in the interests of reducing lag almost entirely. But, as long as turbo's are spun by exhaust gases - there will always be some small measure of lag.

I am sorry I am going to have to disagree with you here, Martin, despite your obvious advanced engineering knowledge. Comparing this engine to the 335i (N54) and then hypothesizing that twin turbos (as in the 335i) are more responsive goes against what Audi engineers have found in real world testing. See page 1 of this thread: "extensive testing on the big V6 has proven the superiority of mechanical charging. In conjunction with direct injection it is far superior to twin turbochargers, both in terms of packaging and of starting performance and responsiveness".

This supercharger increases peak torque / power about twice as much as the turbos in the 335i.

Anyway, who buys an S4 or 335i to drive it at engine speeds below 2200rpm? 80-120km/h times are 4.4sec (4th gear) vs 5.3sec for the 335i. The fuel consumption figures are about the same too. That's impressive.

I am going to wait for the independent road tests and my own driving experience before I decide which is better. No offence intended, but until then, I am going to take the Audi engineers side on this one.

[Just_me]

Originally Posted by ACE

I am going to wait for the independent road tests and my own driving experience before I decide which is better. No offence intended, but until then, I am going to take the Audi engineers side on this one.

Me too , but I dont take anyone's side just yet

[martinbo]

Originally Posted by ACE

I am sorry I am going to have to disagree with you here, Martin, ...

I am going to wait for the independent road tests and my own driving experience before I decide which is better. No offence intended, but until then, I am going to take the Audi engineers side on this one.

None taken in the slightest ACE. This is afterall a forum where debate is the essence of learning.

I must first point out the following:

Originally Posted by me

In short, the quickest way to answer your question is to say this:
No. Supercharged engines do not suffer from lag in the way that a turbocharged engine does.
~~~
So in such cases we can see that turbocharging technology has come a long way (small, twin-turbo'd configurations; twin-scroll turbos) in the interests of reducing lag almost entirely. But, as long as turbo's are spun by exhaust gases - there will always be some small measure of lag.

I most certainly acknowledge that supercharging - especially employing the latest roots-type technology - is more throttle responsive than turbocharging.

However, what I am pointing out is that modern turbocharging has come a long way and that the engine response of turbo'd cars like the MINI Coopers S and, more pertinently, the 335i is massively impressive. I have years upon years of personal acquaintance with turbo'd engines and in my experience the throttle response of the 335i is unbelievable. Surely not better than a new S4 at 2200 rpm and beyond but darn close.

Also, the peak torque of the S4's engine is telling: there is a period of time when the engine is producing less than its peak torque as the supercharger spools to the optimum rpm from low revs. Just like a naturally aspirated car.

What I'm trying to convey is this concept of responsiveness. There are two trains of thought here; the first is where is peak torque created in the rev range - in this case the N54 is superior - and the next is the perception of throttle response i.e. from when the throttle is closed, then fully opened and the time elapse perceived by the driver between this and the engine "reacting". In the case of the latter the supercharged engine is, without question, more responsive. The problem encountered is that torque figures are always quoted at max throttle openings and this skews the impression of torque availability and hence, engine responsiveness.

Also, we've now seen that the N54 engine is easily good for 245 kW and 440 Nm in the new F01 740i so the output figures are clearly very carefully controlled by electronics and not actual mechanical physics.

It is interesting to note that the Audi engineers (read marketing dept ) have made the comment around the vast superiority of mechanical charging vs turbocharging whilst all the while:
- employing turbocharging in many of their other vehicles
- having turbocharging as a tradition in their stable
- just about every other manufacturer eschewing supercharging for its drawbacks and moving toward turbocharging.

A case of sour grapes that their long time rivals have achieved such considerable success with turbocharging? I'm speculating. But it is fun!

But ja, can't really disagree with you.

[dr Dunkel]

Originally Posted by Just_me

Me too , but I dont take anyone's side just yet

I'm with you on this one. Audi staff will of course say they have done a better job. Everything else would be out of the question. The real world will have to tell...