[cstavaru]

I just saw this line in the new N55-based BMW 5-series review from Automobile Magazine:

"we sampled a 535i, powered by BMW’s new N55 single-turbo inline six-cylinder. The new engine is rated at the same 300 hp and 300 lb-ft of torque as the outgoing twin-turbo unit, but it doesn’t have quite the same punch at the low end."

http://www.automobilemag.com/reviews...ew_engine.html

Well, now we know why they still use the twin-turbo in the more sports-oriented model...

Go N54 !

[stressdoc]

The differences between the N55 and N54 in the 535i are determined by software specs, not inherent differences in the performance potential of the bi-turbo (N54) versus twin-scroll+valvetronic (N55) designs.

Believe me, there are variants of the N55 in R&D that smoke the current N54 & N54B30.

[cstavaru]

Originally Posted by stressdoc

The differences between the N55 and N54 in the 535i are determined by software specs, not inherent differences in the performance potential of the bi-turbo (N54) versus twin-scroll+valvetronic (N55) designs.

Believe me, there are variants of the N55 in R&D that smoke the current N54 & N54B30.

If you ask me, I would say a single bigger turbo can never spool fast enough for low-end torque. A single turbo can be equal, but can never be better than 2 turbos for low-end. Valvetronic does not help much either, because at low RPMs the air volume is low so a throttle plate in the air flow instead of valvetronic doesn't really slow things down. This is where the lack of low-end punch comes from.

Of course, software can bring more additional "punch" but so is the software from GIAC, Evotech, etc. The question is, can it bring the additional punch while keeping the targeted BMW parameters ? If it did (with the current hardware), they would have released it at least in the heavier models like the 535i which would really benefit from more torque.

When I will see an M3 with single turbo and Valveronic I will give the props to N55, but I really believe the next M3 will have a twin-turbo without valvetronic in order to provide low-end torque and a high-RPM redline (where the Valvetronic has problems).

[cotmfk]

Lag can be fun, though... like when all of a sudden 30 psi hits!

But, for everyday driving, not so much. Two twin scroll turbos as an upgrade option to the N54 might be something to look at. Two bigger turbos, but twin scroll to try and help alleviate low end lag.

[Sniz]

I would expect the n55 to have a little less low end grunt, but be surperior up top where it really helps for performance reasons.

a n54 powered vehicle can certainly pull my M3 on low end rpm pulls, but once i'm spooled, good bye.

same idea but the n55 wont be as laggy as a larger turbo for many reasons, twin scroll and valvetronic being 2 of the main ones.

[stressdoc]

Originally Posted by Sniz

I would expect the n55 to have a little less low end grunt, but be surperior up top where it really helps for performance reasons.

a n54 powered vehicle can certainly pull my M3 on low end rpm pulls, but once i'm spooled, good bye.

same idea but the n55 wont be as laggy as a larger turbo for many reasons, twin scroll and valvetronic being 2 of the main ones.

Avoidance of lag has always been a top priority for BMW for these bread&butter cars. Twin scroll is another way of getting boost at low rpm/pressure -- a small impeller with little inertia, and then a big one for serious boost (like the turbo in the diesel). The twin turbo design did much the same -- little turbos spin quickly, two turbos push more air.

Edit: misuse of term "impeller", see comments below.

[Tee212]

always trade offs

[cstavaru]

Originally Posted by stressdoc

Twin scroll is another way of getting boost at low rpm/pressure -- a small impeller with little inertia, and then a big one for serious boost (like the turbo in the diesel).

Sorry but this is not true. Twin-scroll just means that exhaust gases (from the six cylinders in this case) are split in two separate streams of flow (one flow from each 3-cylinder bank) which enter the turbine separately and spin the same impeller which has two sets of fins, one for each flow. The advantage is the higher pressure in each flow and the alternating exhaust pulses in each flow path (after a cilinder fires the exhaused gas from that cylinder creates and exhaust "pulse") which create the high turbine RPM faster than a single flow.

[bimmerboy]

Originally Posted by stressdoc

Twin scroll is another way of getting boost at low rpm/pressure -- a small impeller with little inertia, and then a big one for serious boost (like the turbo in the diesel)...

Sorry but you are wrong.
Read here from BMW: Twin Scroll Turbo System Explained

Twin-scroll turbo system design addresses many of the shortcomings of single-scroll turbo systems by separating those cylinders whose exhaust gas pulses interfere with each other. Similar in concept to pairing cylinders on race headers for normally aspirated engines, twin-scroll design pairs cylinders to one side of the turbine inlet such that the kinetic energy from the exhaust gases is recovered more efficiently by the turbine. For example, if a four-cylinder engine’s firing sequence is 1-3-4-2, cylinder 1 is ending its expansion stroke and opening its exhaust valves while cylinder 2 still has its exhaust valves open (while in its overlap period, where both the intake and exhaust valves are partially open at the same time). In a single-scroll or undivided manifold, the exhaust gas pressure pulse from cylinder 1 is therefore going to interfere with cylinder 2’s ability to expel its exhaust gases, rather than delivering it undisturbed to the turbo’s turbine the way a twin-scroll system allows.
The result of the superior scavenging effect from a twin-scroll design is better pressure distribution in the exhaust ports and more efficient delivery of exhaust gas energy to the turbocharger’s turbine. This in turn allows greater valve overlap, resulting in an improved quality and quantity of the air charge entering each cylinder. In fact, with more valve overlap, the scavenging effect of the exhaust flow can literally draw more air in on the intake side while drawing out the last of the low-pressure exhaust gases, helping pack each cylinder with a denser and purer air charge. And as we all know, a denser and purer air charge means stronger combustion and more power, and more power is good!
But the benefits of twin-scroll design don’t end there. With its greater volumetric efficiency and stronger scavenging effect, higher ignition delay can be used, which helps keep peak temperature in the cylinders down. Since cooler cylinder temperatures and lower exhaust gas temperatures allows for a leaner air/fuel ratio, twin-scroll turbo design has been shown to increase turbine efficiency by 7-8 percent and result in fuel efficiency improvements as high as 5 percent.

Just tired of people spreading mis-information....

[GreenPlease]

[quot]But, for everyday driving, not so much. Two twin scroll turbos as an upgrade option to the N54 might be something to look at. Two bigger turbos, but twin scroll to try and help alleviate low end lag.[/quot]

two twin scrolls on any six cylinder engine makes zero sense. It would actually be significantly less efficient. The whole point of a twin scroll turbo is to seperate the pulses of exhaust gas.

[DJKhan85]

^ aren't they putting dual twin scrolls in the next M3 engine(I6)? Is that just forum banter at this point?

[cstavaru]

Originally Posted by DJKhan85

^ aren't they putting dual twin scrolls in the next M3 engine(I6)? Is that just forum banter at this point?

No way. For dual twin-scroll you would need to create 4 "equal" streams of flow but you have 6 cylinders...so you can't

It will either be a twin-turbo I6 (my guess is 3.2-3.6 liters) or a twin-scroll, twin-turbo V8.

[stressdoc]

You guys are right of course -- the turbo in the N55 is not a sequential design, nor an active variable geometry design. I should always double check myself whenever I use the word "impeller" because I use it inappropriately to refer to the turbine fan. My point is simply that the turbine fans in the N55 are designed for low threshold, reduced lag, and high power. I don't understand the nuances of how the blades are designed differently to take advantage of the separated manifold.

Pretty good summary here:
http://kilometermagazine.com/artman2...Turbo_Six.html

But I want to see a better cutaway than what BMW provided in the Press package.

Any advantages to a manifold/header divided 6 ways, with separate inputs from each cylinder to the turbo? Complicated mess.

[supracg]

If the n55 can create more power at the high rpms, why does the iS essentially have the n54 engine?

[TwinTurboXI]

Originally Posted by Nick.CBR

Please expand on this. That's a pretty wild statement to make without citing your sources.

Nick

+1

[stressdoc]

What is so "wild" about BMW R&D having multiple engine versions? They always do. Most obviously never make it to market. Second point, if you have people that you know inside BMW you don't name them. Third point, I don't write about stuff here unless it's already out, in this case Michael (Mapezzul) discussed N55 variants on his blog.

[cstavaru]

Originally Posted by stressdoc

What is so "wild" about BMW R&D having multiple engine versions? They always do. Most obviously never make it to market. Second point, if you have people that you know inside BMW you don't name them. Third point, I don't write about stuff here unless it's already out, in this case Michael (Mapezzul) discussed N55 variants on his blog.

What's the blog address of Mapezzul ? He seems very knowledgeable...

[JakeN5]

lag or no lag, I'd still go for the N55 just because it's a newer toy, then patiently wait for the tuners to do their tricks. On top of that, the possibility that the N55 may be a more dependable unit more than makes up for its shortcomings.

[cn555ic]

Originally Posted by JakeN5

lag or no lag, I'd still go for the N55 just because it's a newer toy, then patiently wait for the tuners to do their tricks. On top of that, the possibility that the N55 may be a more dependable unit more than makes up for its shortcomings.

Well everything that comes out seems to be the best thing out, we will wait and see what the N55 platform brings, but I have a feeling the N54 will be a special engine that enthusiast will be looking for a long time after its no longer in production...If it wasn't anything special why would they even offer it in the 335is model, knowing that the enthusiast would pay top dollar for a N54 engine..

[scheherazade]

Originally Posted by supracg

If the n55 can create more power at the high rpms, why does the iS essentially have the n54 engine?

for the average joe, 'twin turbo' sounds better than 'turbo'.

how many people do you know that would drive a more laggy vs less laggy car, and say the more laggy car is more sporty...

hardly anyone spends any time over 6000 rpm. And the car isn't marketed towards the serious performance crowd (more like the 'ooh, sporty sounds nice!' crowd).

-scheherazade

[5soko]

The newest isnt always the greatest. That goes for both tuning potential greatest and daily driving consumer greatness.

The N55 seems to be and have less tuning potential than the N54 because of the valvetronic, and of course it going to come with some tough anti-tuner hardware/software mix that will most likely keep tuning power levels at bay.

For everyday driving, the N55 has the stuff to make it really good. If and how much better than the N54 we will have to see, tq coming in 500rpm early is going to be hard for alot of people to see a difference in there dail driving, so we will have to see, i expect to test drive one as soon as it comes out at my local dealer.

Tuning wise, the N54 is going to be the better choice. since there is more R&D which is 3 years into it if not longer which is major.

All we can do is speculate but tuning wise things are really leaning on the N54 side.
Daily driver and out of the box wise, the N55 has the right stuff but if it will be better we will have to see.

[Spool]

you guys are complaining about a little turbo lag...my shit spools at 5000 RPM...LOL