I was at the dealer Friday (new steering pump) and had a conversation about extended warranties. (They're going to send me theirs, and I'll report back. It looks like a good deal, at first blush).

As part of the conversation I mentioned the concern that's been discussed here and on the A6 forum about getting both turbos replaced (per Audi specs) should one go.

He nodded wisely and said something like "oh, yes, that's because it's a two-stage system." This was not a service tech, though.

Now, I had this naive idea that the two little turbos in the 2.7T were sort of assigned in parallel to the two banks of 3 cylinders. This "stage" comment, though, implies that they're in-line, first one pumps it up a little and second one pumps it up more and feeds all 6 cylinders.

Anybody know which it really is? HPH

 

Nope one turbo per bank of cylinders feeding a common plenum. Not staged, But more like series, each adds the same amount to common intake.

 

being staged per se (one feeding the other), that they both feed all six cylinders, only one is doing it alone at low RPM while they're both pumping air at higher RPM.

 

One turbo per bank of 3 cylinders... in parallel.

 

It is NOT a sequential (series) system, but rather a parallel system.

The intake out of the MAF is split by the Y pipe on the top of the engine. This feeds each of the two matched (very important) turbos. The turbos are driven off of the exhaust from their respective bank of cylinders, compressing the air which flows back up to the infamous Throttle Body Boot, through the throttle body and into the intake manifold where each cylinder is fed.

The layout of a V6 does not lend itself to a sequential setup mostly because of the plumbing. The only production cars that I believe have had sequential systems are the last generation Toyota Supra (inline 6) and the last RX7 turbo (rotary). In a sequential system, one large turbo feeds the second small turbo. The advantage is that you get decent response from the second turbo, which pulls through the large first stage turbo at low RPMs, as well as the better top end provided by the high flow characteristics of the large first stage. Because the first stage is feeding compressed air into the second stage small turbo, the second stage turbo will not overspin.

With a parallel system, you are limited by the sum of the flow provided by the two small turbo, which must be sized to provide quick spool up at the bottom end and reasonable flow at the top end. That is the trade off, turbo lag vs. top end performance.

HTH!

 

"those twin turbos really put some whup-*** in that 2.7!" Stated with a straight face -- he's not a very technical guy. Anyway, that made me chuckle.

 

First the dual turbo set-up on the 2.7T petrol

<img src="http://pictureposter.audiworld.com/42103/2.7t-01.jpg">

Second, the single turbo set-up on the 2.5tdi diesel

<img src="http://pictureposter.audiworld.com/42103/2.5tdi-01.jpg">

Someone else can explain them !!!

Just my €0.02.

 

On the petrol engine, you can clearly see that on one bank of 3 cylinders, that the exhaust gasses drive the turbocharger who's impeller draws ambient air from the primary air intake, through the air filter, sends it on it's merry way through one of the intercoolers, and on to the inlet manifold of the same 3 cylinders, monitored by a pressure valve/wastegate who should blow-off excess pressure back to the primary air intake, seemingly totally independant of what's going on over on the other side of the engine, which should be doing basically the same, totally independantly.

There are a lot of do-daas around the turbocharger associated with exhaust gas recirculation, which is needed to comply with emission standards, but not essential to the running of the engine.
It would probably be better without it,
but not for the environment.

I'll shut up now.

Best wishes.