Sulu

It seems that you may be confusing twin-turbocharging and twin-scroll turbocharging; they are different.

Twin-turbocharging uses 2 turbochargers in sequence, a smaller one for lower engine speeds (to reduce lag) and a larger one for higher engine speeds. The smaller one may feed the larger one or the larger may be used alone at higher engine speeds.

The twin-scroll turbocharger is a divided turbocharger with a 2-scroll turbine and 2 inlets, each feeding a different scroll of the turbine. The twin-scroll turbocharger is designed to take advantage of the equal-length exhaust manifold (e.g. the 4-into-2 exhaust manifold -- I do not know if there are twin-scroll turbochargers on 6- or 8-cylinder engines). The equal-length exhaust manifold prevents the different exhaust pulses (due to the firing order of the cylinders) from interfering with the exhaust of other cylinders.

Each scroll of the turbocharger is fed by a different pair of cylinders, taking advantage of the different exhaust pulses, ensuring that the turbocharger is better able to take advantage of the otherwise-wasted exhaust.

peteharvey

Back in the mid-1990's, I used to test drive my friend's Saab 900/9000 and 9-3/9-5 "low pressure" turbo.
It was a great alternative to a V6.
Then Saab collapsed.
Many years later, roughly 2010 onwards etc, Benz had a similar 2.0L four cylinder low pressure turbo.
In 2014, I tested the Benz C300 2.0L low pressure turbo.
I thought it was great too.
So instant.
So powerful.
Yet decently smooth and quiet.
A real alternative to a six.

I thought the BMW 3 Series 2.0L turbos had peakier torque curves, with more of a flat spot at low throttle openings, and a big kick as the turbo spooled in.

Then the IS200t.
I thought it was quite similar to the Benz 2.0 turbos.
Reasonably quick response, and plenty of punch down low, and decently smooth and quiet.

I have test driven BMW 3.0 in-line six twin turbos for many years, and when I nail the throttle, there is a flat spot down low, followed by a turbo boom as the turbine spools up.
Several years ago, I tested the E400 V6 Twin Turbo a few times, and like the BMW 3.0 TT, the Benz had a flat spot down the bottom, followed by a turbo boom.

Thus I really liked the more instantaneous torque of the Saab low pressure turbos, and the Benz 2.0 turbo, and IS200t.

Now that I have had a good experience with my wife's IS200t, I actually don't like the 2.0 turbos that much anymore.
The problem is especially at very low throttle openings, pottering around town at 30-40 mph.
I can really hear the four cylinder drone, which is no match for the smooth cat-like purr of Lexus' 60 degree V6's.
At very small throttle openings, the IS200t feels lethargic for torque/power.
At slightly wider throttle openings, it is still weak, and I can tell that the turbo has not spooled up.
On wider throttle openings, then there's plenty of power, and that four cylinder drone is gone - replaced with a much smoother sounding hush.

Overall, I now think that these 2.0L turbos are okay, and great for wide throttle performance and economy.
However, they lack true cat-like purring smoothness, and they also lack instant throttle response, with a nice and flat torque curve.
I actually prefer my wife's old 2006 and old 2014 IS250's more, especially for their smoothness, throttle response and flatter torque curves, despite losing maximum power and fuel economy to the 2.0 turbos.
However, I do drive very sedately, while others will drive much more aggressively than me, hence the 2.0 turbo could be for them.
Thus, both atmos V6 and 2.0 turbos are good, but in different ways.

Ditto this forthcoming 5LS 3.5 TT.
I suspect the old atmos V8's will still have superior smoothness, throttle response, and a flatter torque curve at very small throttle openings at 30-40 mph around town.
While the new 3.5 TT will have more max power with 0-60 in just 4.5 seconds, and better fuel economy.
Once again, both engines are great, but in different ways.

The hybrids like the 450h and 600h have their good and bad points too.
Great flat torque curve, but compromised trunk space, and plenty of excess weight impeding their handling.

Meanwhile the 300h 2.5L four cylinder hybrid has typical poor four cylinder NVH characteristics, and it is extremely weak in performance.
The 2.0 turbos are much smoother and far more powerful than Lexus' 2.5L four cylinder gasoline-electric hybrids.
The 300h needs much bigger capacity batteries, and much bigger electric motors, or better still, team the much more powerful and much smoother 2.0L Turbo to batteries and electric motors.

Me? I would take the 2.0 turbo over the 300h hybrid any day.
However, someone who is very economy and environmentally minded would take the 300h hybrid and live with the compromised trunk space, compromised performance, compromised handling, and compromised noise/vibration/harshness.

There are no perfect engines.
Each engine has its set of pros and cons...
.

TangoRed

You mean C250- and that was a 1.8L I4. And I'm actually shocked you felt that way, as I felt that model had quite palpable turbo lag. The C300 was a 3.0L V6 or 3.5L V6 in the last generation C-class depending on the year.

peteharvey

So at first on test drive I thought the modern day four cylinder turbos from C Class & IS were really good, but only after true ownership did I realize the lag & harshness at very narrow throttle openings around town at 30-40 mph.
.

Rhambler

The "twin" turbos in this car's case, I think, is just two single scroll turbos, one on each cylinder bank versus two sequential turbos.

This makes lag even worse.

A straight six might make better use of "twin" turbos, sequential, like Sulu mentioned versus a V6 due to plumbing. And there are cars with twin or two non-sequential twin-scroll turbos. I think Porsche does this.

But twin sequential and twin scroll both help lag, and I think the latter does better. Twin turbos with two separate turbos, non-sequential is akin to a 4 cylinder with a single, single-scroll turbo like the 2.0 liter. And guess what, lots of lag.

The LS 500 will probably have lots of lag. No amount of gearing or anything else will help much imo. Just the laws of physics at work.

peteharvey

Yes, nothing special about a twin turbo - just a single turbo per bank.
Eg a quad cam is really only a twin cam per bank.
25 years ago, Mazda made an RX-7 coupe with twin sequential turbos.

So far, I've never driven a turbo six with an atmos-like torque curve.
IMO, BMW's 535i & Benz E400 just don't have flat torque curves, especially at small throttle openings.
However each to their own.
I'm an atmos man, but someone else may love turbos...

mmarshall

Yes, it may be two different methods, but the overall effect is the same (which is what I was getting at)....one level of boost available at low engine speeds, and another as RPM builds. Either way, it helps eliminate the traditional turbo lag.

I remember how bad the lag could be on some of the early turbos. Test-driving a Dodge Daytona with a turbo, back in the early-to-mid 1980s (don't remember exactly which year...maybe 1984 or 85), you pressed the pedal down and the car didn't respond at all for several seconds......then, boom, it all came on at once, pressing you back into your seat. It could be hazardous if you had slow-moving traffic in front of you, and it came on at the wrong moment.

peteharvey

The original 535i Turbo used twin parallel turbos, while the new 540i Turbo uses a single twin scroll turbo.
Over the years, I've test driven the old 535i Turbo many times, and I have not liked their "peaky" torque curves.
Apparently, the 540i Turbo has the same peak power, but improved economy, and supposedly superior throttle response and low rpm torque.
I will have to test the newer 540i single twin scroll turbos; if I'm not wrong, it will be much of the same, hence Benz is coming out with an in-line six with "electric" turbos!
If the forthcoming 5LS has electric turbos, then that could really be something, and it would be nice to see them lead, rather than follow...

Rhambler

That would be neat but frankly they would have mentioned that imo if those turbos were indeed electric since that is a big deal. I think they're just conventional.

I studied the engine cutaway photo quite a bit and all of them, at least the close ones, had the turbo cut out or not shown, just the plumbing to it from the exhaust manifold.

However, I did notice electric wastegates on the cylinder head in one photo versus your typical pneumatic wastegate. Not sure of the benefit other than more precise control of over pressure and exhaust maybe?

Sulu

The newly-announced Mercedes-Benz engines with 48-volt electrical system will come with a mild-hybrid system for low-end power and economy, and also have an electric turbocharger for high-end power. I read that VW will be offering a similar system in 2020.

Toys4RJill

Poking around at Autoweek, I am glad Lexus did not remove all the covers in the engine compartment

coolsaber

covers are here to stay, thermal efficiency

TangoRed

Where did you get that from? They are used primarily for noise reduction and presentation.

e325rkh

Regarding power and fuel economy. Unsure if this is also your experience. The German (and some American) manufacturers under-rate the engine performance, usually beating the rating power and performance. In my experience, the Lexus models I owned seem to over-rate the performance, never matching manufacturer claims, however, they remain smooth and get better than expected fuel economy. I expect if this is done, it is because at the end of the day the typical target Lexus buyer (or most of us) does not need that power and just like playing the numbers game and would rather have great fuel economy.

Regarding the G90, I drove both the V-6 and V-8 versions. The V-6 does feel more powerful than the V-8, I suspect owing to the higher torque and lower RPM's, similar to what I expect from the TTV6 in the LS. I also own a turbo diesel, which does both, powerful and fuel efficient (of course cheating was involved). If you really want the best of all worlds, try the TTV8's from Audi/Mercedes/BMW. I am sure one will be on the to-do list at Toyota/Lexus.

e325rkh

I wish more of the luxury manufacturers did this. You can easily remove and prefer the choice.