[Socom]

Any idea why BMW went with the twin turbo instead of the sequential turbo in the 335i?

[E90Fleet]

Apparently partly had to do with turbo lag control (535d stilll has some lag) and turbo heat control ( remember the diesel engines that have the sequential setup have a much lower exhaust gas temperature than the petrol engines )

The high exhaust gas temperature is also why hardly any turbo petrol engines have variable vane turbos. Only main one that has it is the new Porsche 911 Turbo, and it uses very advanced, very expesive materials for its turbos.
Most new diesel turbos on the other hand have the variable vane system.


Some say they also didnt want the power too high, else the non-turbo M3 would have problems

[poldim]

I remember wiki had a little thing on the VGT when I researched about porsches work.

EDIT: found it: http://en.wikipedia.org/wiki/Variabl...y_turbocharger

[Insider]

Originally Posted by Socom

Any idea why BMW went with the twin turbo instead of the sequential turbo in the 335i?

A sequential turbo would have meant one small turbo and one large turbo. The large turbo would produce a lot of boost, and therfore a lot of power. BMW did not want a lot of power. They wanted around 300hp, and this set-up gets them there perfectly.

[KL2DC]

Also...BMW needed to maintain their legendary I-6's feel i.e. linear power delivery/flexibility. I think they put a lot of effort to replicate the feel of a large NA engine with the N54 setup. Great job!!

[voltron1011]

Remember when Dodge had a VNT turbo back in the early '90's? There was very little lag out of the turbo, however the system was total crap with regards to quality, because of what Fleet just explained. I think it was only around for a year or two, before Dodge shit-canned the idea.

[2JZfan]

A sequential turbo system does not necessarily mean that one turbo is larger than the other. The twin turbo Supra is a sequential setup, but both turbos are the same size. At first all exhaust is directed through only one turbo while the other turbo is blocked off. This first turbo therefore spools almost instantly. About the time that this single turbo would run out of airflow to support X psi of boost, the second turbo gets to share in the exhaust and thus it spools too. Of course you also have to block off the intake side of the second turbo during stage 1 because otherwise the boost from turbo #1 would blow backwards through turbo #2 and do you no good. This system is far more complex than a standard "twin turbo setup" and its only real upside is quicker spool. With today's modern turbos, you get X hp with much quicker spool than the turbo that would have been required to make X hp 5 years ago. Therefore, BMW was able to size two twins that would run in parallel and yet still have the desired spool and power characteristics, without all the extra cost and complexity of a sequential system.

And in case anyone is wondering "why twins at all?", two turbos that together can produce X hp will always outspool one single turbo that can produce that same amount of hp if everything else is equal (ie. both turbos have the same caliber of aerodynamics, both are the same type of bearing system, etc.) This is because the majority of the spool time is due to the delay in accelerating the wheels in the turbo, and the moment of inertia of a disc increases polynomially with diameter. In other words, it takes way more energy to accelerate one "big" compressor wheel than it does to accelerate two "small" compressor wheels. BMW seems to put a heavy emphasis on achieving zero lag and this is undoubtedly why they chose twins over a single when obviously there are dozens of single turbo choices that could have supported this power level at a cheaper price point.

[stressdoc]

I bought into the standard line from BMW R&D that small twins provide quick, even spool up. Easier to generate the early and flat power curve that they wanted for the 335. I think there are some interesting new engine management opportunities for turbos with the piezo DI system.

[Insider]

Originally Posted by 2JZfan

A sequential turbo system does not necessarily mean that one turbo is larger than the other. The twin turbo Supra is a sequential setup, but both turbos are the same size. At first all exhaust is directed through only one turbo while the other turbo is blocked off. This first turbo therefore spools almost instantly. About the time that this single turbo would run out of airflow to support X psi of boost, the second turbo gets to share in the exhaust and thus it spools too. Of course you also have to block off the intake side of the second turbo during stage 1 because otherwise the boost from turbo #1 would blow backwards through turbo #2 and do you no good. This system is far more complex than a standard "twin turbo setup" and its only real upside is quicker spool. With today's modern turbos, you get X hp with much quicker spool than the turbo that would have been required to make X hp 5 years ago. Therefore, BMW was able to size two twins that would run in parallel and yet still have the desired spool and power characteristics, without all the extra cost and complexity of a sequential system.

And in case anyone is wondering "why twins at all?", two turbos that together can produce X hp will always outspool one single turbo that can produce that same amount of hp if everything else is equal (ie. both turbos have the same caliber of aerodynamics, both are the same type of bearing system, etc.) This is because the majority of the spool time is due to the delay in accelerating the wheels in the turbo, and the moment of inertia of a disc increases polynomially with diameter. In other words, it takes way more energy to accelerate one "big" compressor wheel than it does to accelerate two "small" compressor wheels. BMW seems to put a heavy emphasis on achieving zero lag and this is undoubtedly why they chose twins over a single when obviously there are dozens of single turbo choices that could have supported this power level at a cheaper price point.

That's not correct. The Supra Turbo has two different size turbos. A small one, then the bigger one kicks in later on in the RPM band.

[txusa03]

there is an article on the new turbo engine in the last bmw magazine (or the 335i brochure) that provides lots of interesting technical discussion and why they did what they did.

[2JZfan]

That's not correct. The Supra Turbo has two different size turbos. A small one, then the bigger one kicks in later on in the RPM band.

You are COMPLETELY wrong. I know this car inside and out. Not only do I personally own a Supra, I also own a company that builds custom parts for the MKIV. Both turbos are the same size, this is a fact. I happen to have about 10 take-off units in my shop so I have seen this first hand (not to mention that it's widely documented that the car uses two of the exact same Hitachi turbos). You would think a person would do a simple google before going out of their way to make themselves look like an ass.

[Borti]

You cannot have a car with different sized turbo-chargers. It would be putting uneven pressure on the cylinders.

[shiv@vishnu]

Originally Posted by Borti

You cannot have a car with different sized turbo-chargers. It would be putting uneven pressure on the cylinders.

Tell that to Saab who, IIRC, have used asymetical turbo systems on their 6 cylinders before. That means that the turbo was powered by 3 out of the 6 cyliders. Crazy!

-shiv

edited: it was a 6 cyl not a 5 cyl.

[Last E92 M3]

Originally Posted by shiv@vishnu

Tell that to Saab who, IIRC, have used asymetical turbo systems on their 5 cylinders before. That means that the turbo was powered by 3 out of the 5 cyliders. Crazy!

-shiv

never NEVER heard of such thing...
hows that work?

[shiv@vishnu]

Originally Posted by Last E92 M3

never NEVER heard of such thing...
hows that work?

It was a v6. The exhaust from the front bank fed a light pressure turbo. The turbo fed the entire engine. It was pretty neat. I suspect it was more for packaging purposes than anything else.

Shiv

[Borti]

Originally Posted by shiv@vishnu

It was a v6. The exhaust from the front bank fed a light pressure turbo. The turbo fed the entire engine. It was pretty neat. I suspect it was more for packaging purposes than anything else.

Shiv

I am a little confused. Could you elaborate a little further? Were both turbo's applying pressure to the entire engine?

Thanks Shiv.

[tommithy]

From what I can tell via google the Saab 9-5 used an asymmetrical turbo design. The used a Garret G17 turbo hooked up to one bank of cylinders on their V6 (which is actually a GM sourced engine) that fed the intake thus turbocharging the engine. The G17 is a very lightweight (by design standards of the day) turbo that didn't take much exhaust to spool up.

The design in general was all about getting low end power and torque in an economical sense. Saab saved money by using a small turbo and got the power they were looking for.