Mike_TX

Well ..... to take your last point first, remember the engine in the 335 is a 3.0 liter. The Bimmer 3.0-liter is bulletproof and is ideal for turbo duty because a smaller-displacement engine responds better to a turbo (less lag, generally better tractability). And they used two small turbos for the same reason - virtually no turbo lag, since the small turbos spool up quickly.

So if they can make a 3.0-liter churn out 306 silky horsepower, it may be a better solution than a naturally-aspirated 3.5-liter doing the same. And certainly more "elegant".

All you need do is read a few reviews to see the IS isn't going to handle like a 3-series, whether the IS has the Sport Pkg. or not. It's a good car, but the electric power steering and general isolation make the IS far less a driver's car than the Bimmer. After all, the 3-series has been considered "almost-perfect" for decades, and for good reason.

Now, that's not to say the IS is a bad car or a bad buy. It's a fun car, with an abundance of power and reasonable handling. Just don't put it up against the BMW and expect to win ... that's just not reasonable.
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lobuxracer

iTrader: (2)

There is no advantage to two turbos unless they are operated sequentially. That's a myth that just won't die, but it's not possible to heat up two turbos and have them respond better than one. I'm disappointed in the number of new turbo applications for gasoline engines for a lot of reasons. Don't get me wrong, I own a car with twin turbos, and it's certainly got a reputation for power, but the reputation has been earned with good singles, not twins.

2Q2B_slow

The only thing I regret is paying too much for the lease of my IS, other than that I love the car. But I do have to say that it is kind of hard for me to see other cars around me on the road since I am too use to my old RX. (I am already getting use to the car though).

4TehNguyen

iTrader: (1)

yes the lag is pretty non existant but it suffers in high rpms as seen in the dyno in car chat, it cant flow enough air to maintain boost in higher rpms so its losing torque after 4k rpm all the way to 7k rpm. Great for city driving but not so good on a track. Once modders start putting on larger turbos going to see that max torque curve shift to the right towards the midrange where it should be trackwise, little less friendly for city, and the high rpm torque shouldnt drop off as bad (going to be a beast when this happens, it will beat M3s)

Mike_TX

In the 335, one turbo feeds 3 cylinders and the other feeds the other 3.
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lobuxracer

iTrader: (2)

Still pointless. Surface area per unit of heat is still more with the twins so more losses than gains. And we've lost scavenging from one bank to the other. Even more pointless. But hey, anything can be made to work with a big enough hammer or in the case of turbos, small enough wastegate.

Mike_TX

I guess I'm not that smart ... it seems the greater surface area of the turbo per unit of heat would be advantageous, since we're not working with heat here, but with flow (exhaust and intake). And to the extent there is a larger surface area with 2 turbos, that means they are a more effective dissipator of heat (which is normally good on the intake side, and not a bad thing on the exhaust side, either). :shrug:

Uhhh ... there's only one bank on a straight six. I guess you're taking each set of 3 cylinders fed by its respective turbo to be a "bank" (?) I suspect neither the intake nor exhaust scavenging is really a big issue for most observers, since they're extracting a lot of bhp from 3.0 liters as it is - well over 110bhp/L, depending on the actual output. Not that it couldn't be better, I suppose, but it's hardly shabby.

I think the way it chased that M3 is good evidence the twin turbos, and the torque fall-off, and all the other stuff is pretty much moot. This is an engine to be reckoned with, for sure.

dmz

You for got something called turbo lag. 2 small turbo have less lag than 1 big turbo. Just look at those big *** single turbo supras, They make huge power at high rpm, nut none of them are practical for daily driving due to the turbo lag at low rpm.

lobuxracer

iTrader: (2)

No they don't. The physics don't support this, and testing in the real world doesn't either. Twins spooling up faster is a myth.

If you compare apples to apples - twins with compressor maps that look similar to a single turbo, not some huge hairdryer turbo, you'll see there is no improvement at all. Turbos spool from 1k rpm to 100k rpm in much less than a second pretty much regardless of size if the exhaust heat is adequate.

lobuxracer

iTrader: (2)

Dissipating heat is the last thing you want to do. Heat drives the turbos, not flow. If flow were the driver, that big diesel truck next to you on the freeway would not spool up when the driver steps on the fuel. Diesels do not restrict airflow at all, they restrict fuel only, so they have full airflow on every single cycle. The turbine doesn't spool until you add waste heat and your whole PV=nrT thing takes off because T just shot through the roof.

I could go into a long dissertation about heat management but basically if they've designed it with twin turbos its more than likely a packaging issue. It surely isn't performance that is driving this decision. I've never seen properly sized twins out spool or out perform an equivalently sized single, and I've looked at a lot of them.

AFA the two banks - engineers see any six cylinder as either three parallel twins or two triples. From an exhaust perspective, you are way ahead to set up the engine as two triples. A 6 - 2 - 1 exhaust makes great power because the power pulses are 240 degrees apart in both banks and the cylinders do a wonderful scavenging job on each other. In the three twins model 6 - 3 - 1 the power pulses are 360 degrees apart, so you have more opportunity for the exhaust to lose heat and velocity.

I'll bet BMW are just using log manifolds so most of this is moot anyway. Especially if the top end is falling off rapidly. A proper header would probably net a huge return but be a warranty and packaging nightmare.

AFA this being an engine to be reckoned with - maybe. If it weighs less than 450 lbs, then it's OK for sure. If it weighs more, it's a pig. If it weighs in between 390 and 420, it'll be awesome.

Mike_TX

Wow, Lobux - you gotta be kiddin'. There is more mass to the impeller in a big turbo than a small one, and I don't know about your physics, but mine tells me a small mass can be put into motion more quickly - and with less energy - than a large one.

So give us a break here. BMW's engineers felt two smaller turbos would be more efficient, and I'm sure they tested it thoroughly. I tend to believe them more than you, I'm afraid ...

lobuxracer

iTrader: (2)

Whatever. I've only been doing this for 25 years. Think what you like.

Mike_TX

Heat never turned a single turbo vane. If it did, you could apply a blowtroch to one and it would spin like crazy. Heat does contribute to the expansion of the exhaust gases, applying more force to the impeller, but without flow, you'd have nothing but a very hot - and stationary - turbo.

Whether you talk about diesels or gasoline engines (or turbojets), it's the exhaust that drives the turbo. The driver gives it fuel, which generates exhaust flow from its combustion, which turns the turbo, which boosts the air pressure on the intake side.

Heat is an enemy to turbos, since they need a lot of lubrication, and oil can absolutely cook to carbon at temperatures encountered in the exhaust stream where the turbo lives. It also fatigues the turbo's vanes and shaft and bearings and erodes their surfaces, so controlling heat is important.

I'm sure you've had a lot of experience, but I can't live with your science. I doubt I'll change your mind, so I guess we just disagree.
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Dabigman

BMW engineers have to balance performance, reliability, and packaging. That doesn't mean that Lobux is wrong, just that they can't have everything. Has BMW ever stated why they didn't go with sequential turbos? My guess is that it was too far out of line with one of the categories in my first sentence.

lobuxracer

iTrader: (2)

Actually if you put a blowtorch to a turbine it will spin for the same reason it does with exhaust gases. Bad example.

Heat is not the enemy of spool. Heat is the enemy of the materials used to make the turbo. Why do you think Inconel and ceramics are so common in serious turbines? Why to they sell wraps for turbine housings? Read the first line of the link - increase hp and reduce turbo lag. They improve spool by retaining heat.

Sure you'll cook the oil if the flow is inadequate and the center cartridge overheats. But you'll do the same thing in any engine part where oil flow is restricted and temperatures exceed 400F. Any oil will coke under the right conditions.

In the world of airflow, kinetic energy is heat. Kinetic energy in a gas is what drives a turbine. Gas molecules striking the surface. True for ALL turbines large and small, and based on Newton's laws of equal and opposite forces in actions and reactions. I get it just fine. I think about gas flow differently because I've been exposed to high vacuum systems and cylinder head porting. They both force you to think like an air molecule if you ever hope to achieve anything.

We can agree to disagree about how we interpret the science around us, but it doesn't change WHY BMW chose to do what they did. And I'd still bet a lot of money it has more to do with packaging than anything.