Inside BMW’s QUAD TURBO Diesel Engine

In the world of diesel engines, we’ve seen a lot of interesting changes and trends throughout the years.

But BMW took it a step further by introducing one of the weirdest diesel engines you’ll ever hear of: a 3.0L inline-six diesel with not just one, not two, not three, but four turbos.

So, today let’s take a dive into the BMW B57 quad-turbo engine and see what it’s all about.

Where Did it Come From?

So the first question is, how on earth have I never heard of this thing? Why aren’t more people talking about such a wild and interesting engine? And well, there are a few answers to this.

Part of the problem is that this engine is very new. As a matter of fact, the entire BMW B57 diesel engine platform wasn’t introduced until 2015, just a short eight years ago.

Man, those were simpler times, but not for BMW. You see, they were busy developing a host of different modular engines.

Remember the B58? The engine found under the hood of the MK5 Toyota Supra. Well, that engine is part of a family that includes the B38, B48, and B58 gasoline engines.

But not just that, this modular engine family also includes their diesel B37, B47, and B57 engines.

All of them share the exact same cylinder displacement of 500cc, which is what partially makes them a “modular” engine family.

And in typical fashion, the biggest engine of the family is the one that got the coolest variants, which takes us to the B57D30S0 engine we’re looking at today.

God, I hate BMW’s engine naming convention. It’s clear and concise, but man, that is one obnoxiously long name.

Anyways, BMW developed this particular engine to use in their largest sedans, the G11 750 and the G30 M550, as well as their X5, X6, and X7 SUVs.

And part of the reason they decided to develop something with four turbos was as a response to Audi’s SQ7 TDI, which had a tri-turbo system of sorts, thanks to its use of an electronically driven compressor wheel.

Why Bother With Four Turbos?

But the thing is, the B57 diesel engine family includes engines that use one and two turbos. So why bother with four turbos? What exactly is the point of this? Power, response, torque, fuel economy? Well, it’s a little bit of everything.

On the bottom end of this quad-turbo beast of an engine, it’s much like other B57 diesel engines, with an aluminum cylinder block and individual cylinder displacement of 500cc, just like the other engines in their modular engine family.

The top of the engine features an aluminum cylinder with four valves per cylinder. So, on the surface, it looks much like their other BMW B-series diesel engines because it really is much like their other engines.

Okay, so if everything else is fairly similar, then let’s take a look at the turbo system. How exactly do you get four turbochargers to work with a 3.0L six-cylinder engine?

Four turbos powered by six cylinders? That would be a turbo for every 1.5 cylinders. It doesn’t really make sense.

How it Works

Well, the easiest way to think of this turbo system is as a dual multi-stage system with two low-pressure turbochargers and two high-pressure turbochargers.

The whole point behind using multiple turbos is to mimic the high-end performance of a large single turbo, but with improved transient response.

I know a lot of you guys out there think big single turbo engines with a really high boost threshold and poor throttle response are cool because seeing a big turbo car spool up is always cool, but trust me, daily driving a turbo car with poor throttle response sucks.

This is part of the reason that Ford uses ultra-small turbochargers with all of their EcoBoost engines.

Small turbos will always have better transient response than large turbos. It’s just simple physics.

So, with two low-pressure turbos and two high-pressure turbos, the quad-turbo B57 should theoretically have an incredible throttle response and just about zero lag.

What’s interesting, though, is that the two smaller turbos and one of the larger turbos are always active, while the second large turbo doesn’t come into play until you really put your foot on the gas.

And on top of that, the two smaller turbochargers are variable geometry units to even further improve throttle response.

And the big benefit worth talking about is how much power this engine produces, around 394hp and 561lb-ft of torque, while also delivering a whopping 41mpg.

Typically with diesel engines, you can either have really good power or really good fuel mileage. It’s relatively hard to do both with modern diesel engines that have to also deal with the emissions systems choking them up.

That’s not always the case, but these power levels mixed with that kind of fuel mileage while still dealing with all the euro emissions systems are quite impressive.

Lacking Aftermarket Performance

Was it a little odd, however, that this ridiculous quad turbo system doesn’t translate over to impressive aftermarket performance?

And what I mean by this is that you might expect this type of system to be capable of something like 500hp and 1000lb-ft of torque.

You might think this, especially if you’re used to American truck diesel engines, which can produce ridiculous amounts of power with just simple tuning, but unfortunately, it isn’t quite the same for the BMW B57 diesel.

While it does have four turbochargers, the thing is, they’re all fairly small turbos, which means peak power isn’t all that great, even with tuning. But there are also other limiting factors that we have to consider.

Here we are looking at G-Power, which is a German tuning company that got their hands on a 750D with this engine, and they turned it up, but the results were less than you might expect, at 460hp and 634lb-ft of torque.

Now that’s not all that bad because you have to remember this is only a 3-liter we’re talking about here.

It’s not like a big 6.7L Cummins, so when you account for the displacement, the aftermarket power figures aren’t as bad, but still kind of underwhelming.

And compared to stock, we were talking about a power increase in the 10% to 15% range. Again, that’s not bad, but frankly underwhelming for such a wild turbo system.

But part of the problem is the transmission behind this engine simply not being able to handle the low RPM torque. At least, according to G-Power.

But, that increased power figure combined with the 750D’s all-wheel drive system equals out to a zero to 60 time of 4.3 seconds, which is roughly equal to the M3’s from the same timeframe.

Granted, the M3 is rear-wheel-drive, and the 750D is all-wheel-drive, but still. Going zero to 60 that fast in a luxurious sedan that’s borderline the size of a limo that can get up to 41mph is pretty impressive.

Of course, one thing this quad-turbo diesel does very well is deliver power throughout the entire rev range.

What do I mean by this?

Well, peak torque stays pretty consistent from 2,000RPM to 4,000RPM, with peak power coming in around 5,000RPM.

That means right off the line, this engine pulls hard and just keeps pulling. It doesn’t have much of a “powerband”. It’s just power everywhere past 2k RPM.

And another interesting feature you’ll find on BMW B57 diesel engines, is the mild hybrid system, thanks to the use of a 48-volt integrated starter generator.

Again, this just simply helps to improve fuel economy, but with how weak this particular hybrid system is, it’s borderline a marketing gimmick.

Other things worth noting are the piezoelectric injectors that deliver up to 10 injections per engine cycle to further help with power and efficiency.

The Euro 6D Curse

Then there’s all the bad stuff you’ll see on every modern diesel emissions equipment. Now typically in diesel engines, the government-enforced emissions components are the least reliable part of the engine, which only gets worse as emissions restrictions get stricter every year.

And to put it simply, it has everything you would expect.

This includes a big exhaust gas recirculation system, a diesel particulate filter, a diesel oxidation catalyst, and selective catalyst reduction. And all of this is required to meet the very strict EURO 6d standard.

But, that EURO 6d standard is also the exact same thing that forced BMW to kill this engine off after just a few short since its introduction.

The specific standard this engine failed to meet is the 95 grams of CO2 per kilometer standard, BMW’s quad-turbo B57 diesel outputs between 154 and 163 grams depending on the model of the car it’s equipped in.

Now that might not sound like a huge difference numerically, but it’s a number that is nearly impossible for a production engine like the quad-turbo B57 to reach.

Of course, it’s possible to reach that number, but that doesn’t mean it’s a wise business decision.

And unfortunately, BMW isn’t the other one that wasn’t able to meet this new ridiculously tough standard, as other companies, such as Volkswagen, had to also discontinue their larger diesel engines for the exact same reason.

And then other companies, like Volvo, just threw the towel on small road car diesel engines entirely because of how tough the new standards are.

And when you consider the extra cost of this engine using twice or four times as many turbos are many other popular diesel engines, it all starts to make sense why BMW had to kill it.

It was complicated. It was expensive. And it wasn’t going to be able to meet the upcoming emissions standards, which is really unfortunate because the quad-turbo B57 offered a blend of performance, emissions, and fuel mileage that we haven’t really seen in a diesel car engine.

Yes, other great automotive diesel engines make loads of power and are super reliable, but few of them can do that while offering the B57’s fuel mileage and emissions output.

Summary

So, what do I think?

Well, it’s certainly an interesting engine, and it’s really cool to see a whopping four turbos on a little 3.0L engine, but the lack of aftermarket and frankly underwhelming performance with that advanced turbocharger setup leads me to believe this engine will never go anywhere in terms of the aftermarket performance world.