non symmetrical Turbos

[DamoM]

I start this thread by saying that i ma happy to be blown out of the water but it ocured to me that with all this talk about RS6/RS4 hybrids vs. RS6 stage 1 and 2 or straight K04s or even K16s, in every case there is lilkely to be a trade off of responsiveness (read lag) against top end power.

As the RS4/S4 have twin turbos, why can you not fit unequal sized turbos, one ot 'help' low down response and one of kind of take over when the requisite revs are reached to enable sufficient spooling.

Two reasons that I can think 'might' be issues are firstly that the turbos in isolation would not be enough to provide the necessary oomph either at the low or high end of the rev range and so the whole point is lost, secondly that as they effectivly spill into the same plenum (this could be resolved with some welding though) that the small turbo would be providing back pressure that stops the large turbo from spooling and vice versa.

Anyway, as I said, just thought I would throw it in the mixer but don't crucify me if I have missed any basically obvious points!!!

[simple1]

To answer your question, you need to look at the configuration of the engine, with two turbos in V config, your thoughts wont work, this only works on a straight cylinder config, or you would have one side bank working harder than the other. To over come what you describe the new generation twin scroll turbos would be nore suited.

[derdle]

I recall from the 80's or 90's that you could buy/fit "twin entry" turbos which were effectively 2 turbos in one (iirc) and this gave the benefits of a quick spoolup of a small turbo before the really big boost of a larger turbo kicked in. Not sure if Saab experimented with these at all.

Paul

[DamoM]

To answer your question, you need to look at the configuration of the engine, with two turbos in V config, your thoughts wont work, this only works on a straight cylinder config, or you would have one side bank working harder than the other. To over come what you describe the new generation twin scroll turbos would be nore suited.

But don't they end up boosting in to the same plenum chamber anyway, the that each bank still gets equal boost? It is only the exhaust that works differently in terms of spooling but I don't think that is an issue.

[DamoM]

I recall from the 80's or 90's that you could buy/fit "twin entry" turbos which were effectively 2 turbos in one (iirc) and this gave the benefits of a quick spoolup of a small turbo before the really big boost of a larger turbo kicked in. Not sure if Saab experimented with these at all.

Paul

Yeah, I remember something about those. I'm sure they were on a Mazda RX7 that I had a few years ago. That had very little lag but pulled strongly at high revs.

[simple1]

I recall from the 80's or 90's that you could buy/fit "twin entry" turbos which were effectively 2 turbos in one (iirc) and this gave the benefits of a quick spoolup of a small turbo before the really big boost of a larger turbo kicked in. Not sure if Saab experimented with these at all.

Paul

Yeah, I remember something about those. I'm sure they were on a Mazda RX7 that I had a few years ago. That had very little lag but pulled strongly at high revs.

Saab and Porsche been at it for years, not sure who came up with the idea, probably Garrett but for all intensicve purposes, they were the first twin scroll turbos. Today they have smaller exhaust and bigger exhaust vains, one to turn at lower revs and spool up, the other to provide boost at higher revs.

[shineydave]

I can see where you're coming from as the Mitsubishi GTO has a V6 with a smaller turbo to give initial boost and a larger one that takes over when the smaller one has got things up and running, it'd be interesting to see exactly how they are configured in the exhaust tract though and exactly how they are controlled

[dummi]

with a small turbo in the equation you are always sacrificing top end, as audi have designed the engine bay its very tight, the only way i can think of to keep both banks under the same stress is using a variable vein like simple1 says, or run some complicated manifold that would switch exhaust gas from running through the small turbo to the large turbo, then you could run a big mo fo turbo and a little turbo to get into its power band. doesn't the 997 have variable vein...

the turbo feeding turbo idea is also an interesting one do your calc's and u could produce something interesting

[searider]

Can't see why it would be difficult. You don't need to worry about what goes on in the exhaust side - each turbo just uses the exhaust gas from its own bank. IIRC the saab V6 turbo in the early 9-5 only had one turbo working purely on the exhaust from one bank. The intake obviously went to the plenum and fed both banks.

On the intake side a simple non return valve somewhere on the outlet from the small turbo would prevent the boost going the wrong way once the big one is up and running.

Similar but different are supercharged and turbocharged marine diesels from Volvo Penta KAD32 etc. They have a belt driven supercharger on an electromagnetic clutch. This kicks in at low revs. Once the turbo has spooled up and is producing more boost than the supercharger it takes over and the supercharger is switched out. There is a non-return valve in there somewhere.

Not sure how it could all fit under the bonnet of an A4 though!

[dummi]

supercharger turbo is a good one, thats what they're doing on the 1.4 golf gti.

the problem running different of each bank is the back pressure from the turbo on a balanced reciprocating engine you can forget high rpm fun

[philipwalker]

I think Subaru run sequential turbos on the older Legacy GT. From memory the change over is not that smooth.
Think the boost direction is controled by a reed valve arrangment in the inlet track.

[DamoM]

are there any of the experts that would be able to give their er expert opinion on this one?

[dodgydave]

I start this thread by saying that i ma happy to be blown out of the water but it ocured to me that with all this talk about RS6/RS4 hybrids vs. RS6 stage 1 and 2 or straight K04s or even K16s, in every case there is lilkely to be a trade off of responsiveness (read lag) against top end power.

As the RS4/S4 have twin turbos, why can you not fit unequal sized turbos, one ot 'help' low down response and one of kind of take over when the requisite revs are reached to enable sufficient spooling.

Two reasons that I can think 'might' be issues are firstly that the turbos in isolation would not be enough to provide the necessary oomph either at the low or high end of the rev range and so the whole point is lost, secondly that as they effectivly spill into the same plenum (this could be resolved with some welding though) that the small turbo would be providing back pressure that stops the large turbo from spooling and vice versa.

Anyway, as I said, just thought I would throw it in the mixer but don't crucify me if I have missed any basically obvious points!!!

Short answer: In a twin arrangement you need to have equal back pressure between the banks hence equal turbos.

Only way to run sequential would be to run both banks into one exhaust manifold and then go with a sequential arrangement with a switch over. I doubt there is any space under the bonnet for all this complicated plumbing.

VTG is a nice idea but as tuning on the 997 shows quite limited for high power outputs. That's why tuners mount old skool waste gated turbos on highly powered 997s.

Dave

[S2tuner]

I start this thread by saying that i ma happy to be blown out of the water but it ocured to me that with all this talk about RS6/RS4 hybrids vs. RS6 stage 1 and 2 or straight K04s or even K16s, in every case there is lilkely to be a trade off of responsiveness (read lag) against top end power.

As the RS4/S4 have twin turbos, why can you not fit unequal sized turbos, one ot 'help' low down response and one of kind of take over when the requisite revs are reached to enable sufficient spooling.

Two reasons that I can think 'might' be issues are firstly that the turbos in isolation would not be enough to provide the necessary oomph either at the low or high end of the rev range and so the whole point is lost, secondly that as they effectivly spill into the same plenum (this could be resolved with some welding though) that the small turbo would be providing back pressure that stops the large turbo from spooling and vice versa.

Anyway, as I said, just thought I would throw it in the mixer but don't crucify me if I have missed any basically obvious points!!!

Short answer: In a twin arrangement you need to have equal back pressure between the banks hence equal turbos.

Only way to run sequential would be to run both banks into one exhaust manifold and then go with a sequential arrangement with a switch over. I doubt there is any space under the bonnet for all this complicated plumbing.

VTG is a nice idea but as tuning on the 997 shows quite limited for high power outputs. That's why tuners mount old skool waste gated turbos on highly powered 997s.

Dave

I'd like to see high powered 997s running 997 engines and ECUs in the first place, highest is 630. Sportec SPR-1 has a 996 engine. IMHO you're going into too much of an unknown territory there Dave.

[s4woody]

IMHO you're going into too much of an unknown territory there Dave.

unknown for who..him or you..dave have you been upsetting people..?