Diet Turbos – How Low Lag is No Lag?

Interesting | November 12th, 2009 by 35

A very popular food product on the market is Activia Fat Free Yogurt. Lauded for it’s nutritional benefits, many health conscious patrons choose it because …

A very popular food product on the market is Activia Fat Free Yogurt. Lauded for it’s nutritional benefits, many health conscious patrons choose it because it is “Fat Free.”

In actuality, Activia Fat Free yogurt contains 0.4g of fat per 100g serving. You astutely point out that this is a contradiction, but according to legal guidelines, any product with less than 0.5g of fat per serving can be entitled, “Fat Free.”

Certain products or machines have intrinsic details or properties that cannot be removed. These details can be minimized or optimized, but never completely removed by definition. Dairy products will always contain a measure of fat content. Turbos will always contain a measure of lag. We know all about yogurt now, but how little Turbo Lag constitutes ‘No Lag?’

BMWtwin turbo diesel 655x193In it’s simplistic form, a turbo uses discharged exhaust gases to pressurize intake gases, thus producing more power. There is tremendous energy in the flow of expelled exhaust gas leaving the engine. This is where the turbo kicks in – pun intended.

For the sake of brevity, this article will not delve deeply into the technical makeup of a turbo, but we will consider the basics as they relate to turbo lag.


During the ‘Exaust Phase’ of a four cycle engine, the exhaust valves open and the piston forces the combusted fuel/air mixture out of the cylinder with a plunging movement. Since at the same time as this cylinder is being plunged of exhaust gases, another cylinder on the crankshaft is in it’s power stroke, this exhaust gas is expelled with the full force of the engine’s power. The inertial energy of the flywheel also plays into this. The force of expelled exhaust gases is what “spools” or spins the turbo.

A turbo essentially has two halves, an exhaust turbine and a compressor turbine. The exhaust gas always acts upon the turbo first. The exhaust turbine translates the motion and pressure of the fluid hot gases into rotational inertia. The subsequently spinning half-shaft is connected to the drive shaft of the intake compressor, and this in turn compresses the intake gases from below atmospheric pressure to well above.


The time required for the exhaust flow to overcome the rotational inertia of the turbine rotor and subsequently produce boost at a given rpm threshold is perceived as a ‘delay’ in power – this is turbo lag. This factor cannot be completely eliminated, but can be reduced and optimized. (For example, “E-Turbos” use electric motors to spool the turbos from idle up to the boost threshold at a consequence of increased weight and complexity).


Fluid dynamics also come into play as gases have a property of ‘elasticity’ when they are compressed or vacuumed. As a gas is compressed there is a factor of time – it cannot be compressed instantly. This time loss further plays into turbo lag, though it is marginal. This factor cannot under any circumstances be eliminated, though it can be minimized through intelligent engineering. Note the innovative placement of the turbos within the ‘V’ banks of the new S63 ///M engine on the X5/6 M vehicles, thus significantly reducing the distance between the exhaust valves and the exhaust turbine. This virtually eliminates the time factor in the compression of gas over distance, greatly reducing turbo lag. The layout also compliments the engine’s highly compact dimensions. The patented exhaust manifold routing two cylinders from each bank together into the turbo also plays an inventive role in nearly eliminating lag through consistent pulse pressures to the turbo, even at idle. The result, virtually no turbo lag from idle to redline, under any sudden throttle load.


Is there a relevant advantage to turbocharged performance engines over naturally aspirated performance engines?

In a Normally Aspirated engine, the intake air is ‘sucked’ through the intake manifold, piping and air-filter, and this vacuum results in negative pressure (below atmospheric pressure of 14.7psi @ sea level) in the intake manifold. Boost pressure can vary greatly, but for simplicity of illustration we will say that a turbo is compressing air into the engine at 10 pounds per square inch. Atmospheric pressure is a constant at 14.7psi @ sea level. Therefore we add our boost pressure to 14.7psi (or 1 atm) and arrive at an intake manifold pressure of approximately 24.7psi (subtract parasitic vacuum losses through the intake system of a few psi, varying by car). The combined boost plus atmospheric pressure reading from inside the intake manifold would then be written 24.7psia (psi absolute). The boost measurement itself would be written as 10psig (psi gauge).


As you can see from the above illustration, a turbocharged engine with approximately 10psi boost has almost twice the intake pressure of a NA engine. Naturally, this plays into the engine’s development of power and efficiency.

BMW F1 Turbo 1500cc 900 hp

BMW has long utilized turbo charging in the design of their engines. In 1987 their 1.5L 4 cylinder turbo F1 engine supplied to Benetton was producing around 1,300 bhp in qualifying trim at 79.8psig. This turbo engine redlined at 11,500rpm! There has been some concern from the masses that BMW ///M has lost its way and turbo ///M cars will not satisfy as they will not be able to rev freely and communicate as purely with the driver. Rest assured, ///M will continue to deliver world benchmark engines, it’s in their heritage and their very DNA.

BMW F1 Turbo 900 hp

Lest we forget the BMW 2002 Turbo. Launched at the Frankfurt Auto Show of 1973, it was BMW’s first production turbo sports car. Producing 170hp at 5,800rpm from a 2.0L 4 cylinder and weighing only 2,381 lb, the 2002 was a legend of the time. Rocketing to 60mph in only 7.5 seconds, the 2002 Turbo was a true visionary of the time, and is a worthy ancestor to turbo sports cars of the modern era.


No discernible lag.

Discernment is a drivers scalpel. Trained and steady hands will guide a car on the limit of adhesion with the road surface – but his machinery must measure up. Long praised for it’s unmatched steering feel and precision, BMW has set benchmarks in the manufacturing of precision sports cars and sporting cars. But one driver input that cannot be overlooked is throttle response. Throttle response must be linear, progressive, free of undulations or sudden peaks. To a trained hand, the throttle plays a large role in harmony with steering wheel input, in controlling the car’s path. Throttle is in fact an input of steering while driving at the limit. Here is where turbo lag can kerfuffle even the best and brightest of drivers, catching one off-guard with a sudden surge in the power band. To this end, BMW’s new generation of turbocharged engines succeed admirably, and they continue to close in on perfection.

Yes, to a discerning driver, BMW’s new turbo engines are free of lag. And what better standard of excellence is there? BMW has created the Diet Turbo. Yogurt anyone?


35 responses to “Diet Turbos – How Low Lag is No Lag?”

  1. Artmic says:

    i hate the lag i have on the 335 coupe right … i can still feel it, at least 1 second before it kicks in under certain driving conditions. (Not from a complete standstill, but while driving)

    • Doug says:

      This is even with the ECU update (if it applies to yours) ?

      I don’t think it “kicks in”, it’s more of a swell that peaks after a second or so, but there’s still a lot of immediate power without that.

  2. Truly fantastic article, both from a mechanic’s and enthusiast point of view.

  3. Misha N says:

    Great read, thanks!

  4. Auday says:

    nice article! But really you think that throttle response is the same? I haven’t driven the X-M cars, but the 335i is not a responsive engine, it’s not that bad but compared to it’s closest match from an M-div the S54, it’s not even close.

    A technical question for you Shawn, why don’t they connect the turbo shaft with the crank? something like a hybrid between Turbo and Super Charge where you have the responsiveness of the SC and at the same time utilize the exhaust wasted energy. In this case the turbo performance will be deterministic in time, i.e. regardless of the previous state of the turbo, at any given time the turbo charge is a function of the engine RPM. They could also have a variable gear ratio connecting the two shafts that changes linearly over the RPM range to avoid negative energy transferred from the crank to the exhaust at lower RPMs.

    • Doug says:

      You mean a supercharger. I think that a supercharger is less efficient, enough so that it limits the actual gains. There was a recent quote that a particular 700hp supercharged engine (a merc) used 70hp just turning the “kompressor”.

      But I t basically agree, supercharging is a much better solution for responsiveness and linearity.

      Also, for responsiveness, Porsche uses a design (or did) with an oversized intercooler that provided a reservoir of readily available compressed air for instantaneous power peaks well above the engines ratings.

    • Doug says:

      sorry I was redundant… I misunderstood and started writing before finishing reading your post :/.

      That’s an interesting idea. You’d probably need some dynamic optimization to the flow, like… those variable vanes. Actually, maybe you could have a supercharge and then an electric turbine to collect exhaust energy at an optimum turbine+generator speed?

      • auday says:

        or maybe if the carhas a hybrid or KERS solution then just use the exhaust to charge the batterys.
        Yeah I think using variable vanes is better than have a complicated dynamic gear.

  5. Parker says:

    Nice, one of my favorite articles from this website yet!

    • Horatiu B. says:

      Thanks Parker! Shawn has so much passion for BMW and cars that you can see in his writing. Thanks for the kind words, we’re trying to get better every day, even though sometimes we make mistakes

  6. Babken says:

    335i has a turbo lag? Guys, you seem to have a problem with your perception organs.

    • Artmic says:

      Yes the 335 has turbo lag, at least for me. Not always, but if you are driving at least 30mph, and step on the gas pedal super hard, there is at least 1 maybe 1.5 seconds wait time before the car actually does anything. The RPM needles goes high, but nothing happens and then the car moves after some lag.

      If i’m doing a 0 to 60 run, it does not have that turbo lag though, at least that i can notice.

  7. John Hietter says:

    They should find a way to store the effects of the turbo so throttle response will be instant instead of taking in more air after you punch the throttle.

  8. Doug says:

    this is great stuff. I’d love to see more such engineering articles on the site.

  9. Shawn says:

    Auday, Artmic,

    My current daily driver is an E46 M3, and I have become accustomed to some of the sharpest throttle response offered by any engine. While I have not logged too many miles on the N54, I did a track day a while back with a 135i. I drove it back to back with the non-turbo 128i and was very impressed by the throttle response. No, it’s not as sharp as the S54, but then, few NA engines are.

    What’s more, the N54 is not BMW’s premier I-6 turbo engine (though it did win International Engine of the Year Award in 2007 and 2008). To even further reduce lag, BMW has introduced the N55 engine (so far used on the 535i GT) which utilizes a single, twin scroll turbo with a unique exhaust manifold separating 2 groups of 3 cylinders into the turbo. This is the first turbo engine to also use Direct Injection and VALVETRONIC concurrently. The VALVETRONIC was revamped with faster adjustment (the new adjustor has 1/10th the inertia factor as the previous model) for even more direct throttle response.

    The new S63 on the X5/6 ///M is the world’s Holy Grail of Turbo engines, and I can’t wait to get behind the wheel of one.

    Turbocharged engines will most likely never have exactly the same drive characterists as NA engines – they are inherently different. That is also to say that Diesel engines will never have the same drive characteristics as galsoline engines. But that does not detract from the character of a diesel engine – in the hands of Audi, Diesel power has dominated LeMans for several years! BMW’s performance diesels are full of artful engineering that overflows with character and power. The new turbo engines are equally as impressive and continue to get better.


    To your question; a device connecting the crankshaft directly to an intake compressor is by definition called a supercharger. Some superchargers are driven by belts, gears or chains. Superchargers have high parasitic energy losses in the range of +/- 20% engine output. For this reason they are relatively inefficient in comparison to a turbo and they lack the full power potential of a turbo. As for your mention of a hybrid system utilizing both a turbo and supercharger, Audi has developed such an engine and it’s been coined a “Twincharger.” While it has various benefits as you mentioned, BMW has gone about the same mission in a different way, using a more holistic, efficient design. There are many benifits to this approach including weight reduction. In fact, the N55 is nearly 9 lb lighter than the N54.

    Hope that answers your question.

    • Ray says:


      I think Auday was not referring to using the crank to drive some sort of compressor like the supercharger, but directly driving the crank with exhaust gases. I mean, a turbo forces more air in, but more air also requires more fuel, so the turbo still doesn’t utilize all of the energy released from the combustion event by far.

      However, what Auday suggested could increase efficiency and power at the same time, because it’s basically utilizing a source of energy that would’ve otherwise been expelled out of the tailpipe in most other systems.

      And even though there are systems in development that strive to exhaust gases to generate electricity (perhaps to directly drive other accessories as well?), I would still personally favor a system that uses the energy from combustion more efficiently to DIRECTLY produce propulsion.

      I’m sure there are complications and reasons why this hasn’t been done; there’s the potential of excessive back-pressure, the complexity of synchronizing the force that the exhaust gas exerts on the crank to what’s going on in the combustion chamber, but I personally think that idea is worth expanding upon.

      After all, it’s much better than using a turbo to run a richer air/fuel mixture, then sending the still fuel-rich exhaust to the cat to have it burned up there wastefully.

      So kudos to you, Auday, for mentioning such an idea, even if it was just out some sort of playful imagination or curiosity!

    • auday says:

      @Shawn, the S54 is more responsive because that’s what it’s designed for with independent throttle bodies and high revving intake geometry, light pistons …etc, and thats what M engines should be,… BMW will never achieve the the same thing with Turbos.
      The problem with turbo is not just the lag, it’s also the non deterministic characteristic of the engine,… in the NA engine at any given RPM and throttle the torque is almost fixed (especially with a short open intake like the S54’s), in Turbo on the other hand the engine torque is also dependent on the RPM of the turbo shaft and that depends on the previous state of the engine so the torque will take time to adjust while the turbo shaft speed is adjusting to the new changes and settle to a certain speed.

      Thats why I suggested hooking the turbo shaft to the crank shaft to make it still utilize the exhaust energy (unlike SC) yet be in sync with the engine (which I’m not sure would work because of the negative energy where the engine could accelerate the exhaust gases out rather than use their energy, but probably there is a way to do it).
      But even with that, building up the pressure difference will take time, so it will never be the same as NA. It looks to me that Turbo engineers are trying so hard and adding all that over-engineered technology and complexity to achieve what the M Naturally Aspirated engines achieve naturally, pun intended.

      @Ray thanks for the nice words man, as you said it’s nothing more than playful imagination.

  10. 100$ GUY says:

    Yeah, lovely reading…..keep it coming.

    But didnt get why just of a sudden the author says:
    Yes, to a discerning driver, BMW’s new turbo engines are free of lag.

    Says who?
    Compared to what?
    By which standards?

  11. Shawn says:

    Ray, Auday,

    This article was written about relevant, current technologies and focused on BMW’s current design direction with turbocharged engines.

    Again, as far as the responsiveness of turbo engines vs NA, I’ll quote myself from a previous comment, “Turbocharged engines will most likely never have exactly the same drive characteristics as NA engines – they are inherently different. That is also to say that Diesel engines will never have the same drive characteristics as gasoline engines. But that does not detract from the character of a diesel engine – in the hands of Audi, Diesel power has dominated LeMans for several years! BMW’s performance diesels are full of artful engineering that overflows with character and power. The new turbo engines are equally as impressive and continue to get better.”

    Further, I would reserve judgement until a stint behind the wheel of the S63 engine. ///M has not forgotten their mandate in terms of engine character and driveability/responsiveness, nor will they ever. But again, they are looking at a more holistic approach with efficiency becoming a prominent factor.

    Auday, you mentioned “hooking the turbo shaft to the crank shaft to make it still utilize the exhaust energy (unlike SC) yet be in sync with the engine” to derive some improvements in responsiveness.

    As soon as you’ve directly linked the crankshaft with a compressor, you have what is coined a “supercharger,” and along with that come all of it’s inherent weaknesses – namely a sacrifice in efficiency. Again, VW has already designed a similar model to what you have described (compressor linked directly to crankshaft for responsiveness, with turbo still gathering exhaust gas), and it’s called a “Twincharger.” As was already mentioned, it takes a lot of energy off the crank shaft to power the compressor, and while you would achieve improvements in responsiveness as you mentioned, the parasitic energy losses would overshadow the gains vs pure turbocharging – a very efficient technology.

    Ray, you mentioned “it’s much better than using a turbo to run a richer air/fuel mixture, then sending the still fuel-rich exhaust to the cat to have it burned up there wastefully.”

    Modern turbocharged engines are incredibly efficient, this is a fundamental reason why BMW has chosen to develop turbos in their “Efficient Dynamics” engineering approach. There is no unburned fuel exiting the engine in the exhaust what-so-ever. In fact, the N55 engine has received an ULEV II rating!

    Ray and Auday, it sounds like both of you have a potential career in engineering (perhaps you already are?). You’re right, in modern times engines are being designed to extract every joule of energy out of every drop of fuel. There is a saying that, “necessity is the mother of invention” and there’s no doubt that it’s your type of creative thinking that drives new technologies forward – we must think outside the box to keep improving. You also mentioned “directly driving the crank with exhaust gases” and that sounds like a brilliant idea if that energy could be mechanically harnessed back into rotational inertia in a controllable, usable fashion.

    A really great site to check out is The SAE are constantly exploring inventive engine design solutions and it sounds like the right place for your great ideas! I’m not sure where you live, but it would be worth a trip to visit their annual convention in Detroit, April 13-15 2010 at Cobo Hall – it’ll blow your mind! In my humble, limited knowledge, I’m not too sure about how to apply your ideas – but I’m sure at the show you could bump into someone who does. Or maybe you’ll walk by an exhibit where someone has a working model of exactly your idea (like usual) and you’ll just have to compliment it and go cry in the bathroom. lol

  12. Auday says:

    Right, they will always have different characteristic, but that characteristic is part of the essence and definition of M brand, it doesn’t matter if it wins races or scores the best time on the Ring if it doesn’t have the same feeling that makes the M cars one of the most fun brands to drive. But as you said lets leave this till we try one of the new ones (hopefully not on the next M3 though).

    The negative energy transferred from the engine to the turbo was my concern and I emphasized this in my post, so definitely we dont want the crank to run the compressor like SC but rather keep the turbo in sync with engine (at a given ratio). Maybe one approach could be using a speed-difference-sensitive magnetic clutch connected to the rank that accelerates/decelerates the turbo immediately when it senses any lag in rotation speed within milliseconds.
    Anyway I’m sure they thought of all ideas and rejected them for good reasons, so yeah wont be crying in the bathroom :)

    Thanks for the link I’ll check it out… unfortunately I live in Vancouver, but that sounds like a place I would love to be!

    thanks for the detailed reply

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