What is 'Turbo Lag'?

Let’s set the record straight on a commonly confused phenomenon

4y ago
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As manufacturers try to develop more power from their engines while emitting less, turbocharging is becoming all the more common. Some see turbochargers as impure, and serve only to muffle exhaust notes and ruin power delivery. Others however, see turbochargers as the ultimate way to drastically improve engine performance. There are pros and cons to both forced induction and free breathing motors, but that’s not what this article is about; I want to talk to you about ‘turbo lag’.

The definition of turbo lag seems shrouded in confusion and ambiguity, and even as an engineer who is exposed to world-leading research papers in the area, I often get confused about what people are talking about. The term 'turbo lag' is often used when talking about two separate phenomena relating to turbochargers; boost threshold and transient response delay. So, here I present the definitive terminology that car enthusiasts should use when talking turbo.

BOOST THRESHOLD_

The main way in which people use the term is in relation to when the engine comes ‘on boost’. Turbochargers force more air into an engine than a naturally aspirated power unit by harnessing exhaust gas flow. Exhaust gasses spin a turbine on the hot side of the turbo, which is connected to a compressor wheel on the cold side. This means that meaningful boost is only realised when there is enough exhaust gas flow to spin the turbine. This intrinsic relationship means that the engine needs to be revving sufficiently before significant boost is achieved. This event is called the ‘boost threshold’.

Porsche Panamera Turbo power curve shows the boost threshold from 0-2000 RPM (Black line)

Porsche Panamera Turbo power curve shows the boost threshold from 0-2000 RPM (Black line)

This gives the F40 that characteristic power delivery where you mash the loud pedal, and wait for the surge of turbo boost to pin you to your seat.

Dr Michael Whiteley

Older turbocharged cars such as the Ferrari F40 have a boost threshold quite high in the rev range – around 4000 RPM. This gives the F40 that characteristic power delivery where you mash the loud pedal, and wait for the surge of turbo boost to pin you to your seat. Modern turbochargers use fancy technology such as variable geometry, or twin scroll channels to reduce the boost threshold or even smooth it out. Therefore, this type of turbo lag is actually the threshold for boost at a specific engine speed.

TRANSIENT RESPONSE DELAY_

The other thing that turbo lag applies to is what us engineers call ‘transient response delay’. This is just a fancy way of saying how long something takes to react to a change.

Turbochargers are complex systems and have bypass valves and wastegates. These can be used to release built up pressure from the turbo when you lift off the throttle. If we didn’t have these, the turbo would force air into the engine when you lift off the throttle, and we don’t want all that extra air going in when not needed. But this poses an issue for performance. When you reapply the throttle, the turbo has to re-spool and this takes a short while. Depending on the size of the engine/turbo ratio and what advanced tech you have to combat this, it can take up to a second for this to happen. Not good if you want full boost all the time. This event is called transient response delay, and - in my eyes - is the true form of turbo lag.

Play video
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Nissan S14. 2.0 litre engine paired to a gigantic turbo meant that I only felt full boost at around 4500 RPM. When it did get on boost, I was treated to around 450 bhp.

In this video, you can hear how the turbo expels excess charge when I lift off, and has to re-spool when I need more throttle.

Ferrari have managed to reduce this transient response delay to 0.08 of a second with their 488 GTB. That means that when you lift off the throttle and quickly reapply, the turbos only need 0.08 of a second to reach full boost again.
There are systems in some race cars that keep exhaust gasses flowing and the turbo spinning, so that when you do lift off the throttle and quickly reapply, you have a pre-spooled turbo and therefore, no lag. These systems are called ‘anti-lag’ systems (see here for more info:Https://drivetribe.com/p/Rhobbng3QXadaohsrE5AdQ/byESgNEES9Sh4-9tUSLkYg)

So there you have it. In an automotive world charging headfirst into a turbo’d future, you now know how to use the term ‘turbo lag’ properly. Now go and have fun arguing the toss with your friends.

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Comments (29)

  • Explain it to Clarkson while you're at it...

      4 years ago
  • The combination of supercharger AND turbocharger usually kills the turbo-lag (example: VW's TSI). Other turbo-lag killers are in the form of the bi-turbo systems: a small turbocharger is responsible for building boost at low RPMs and another larger turbocharger kicks in at middle/high RPMs.

      4 years ago
  • Far too many people think turbo lag is about dyno graphs instead of throttle response. We must spread the word!

      4 years ago
  • Makes sense, I like how you explained this in a nutshell. Would be a good little segment for the show

      4 years ago
  • I agree with you on the definition of Turbo Lag, great read! Thanks!!

      4 years ago
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