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Turbo Lag vs Boost Threshold – What’s The Difference?

That slow buildup of boost that you’ve always blamed on turbo lag?
It has to do with the boost threshold,
Which is completely different from lag.

As automakers increasingly turn to small-displacement turbocharged engines,
A lot of folks are worried. We’ve all grown up hearing horror stories of “turbo lag,” the slow,
The unpredictable appearance of boost under acceleration.

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But you may be blaming “turbo lag” for a completely
Different problem—one that has nothing at all to do with lag.

Jason Fenske of Engineering Explained is here to break it all down for you. The thing you’ve been describing as turbo lag? It’s probably got to do with boost threshold,
The RPM at which a turbocharged engine can begin to
Push enough exhaust to start spooling up the turbo.

You can calculate an engine’s boost threshold using its displacement and
Volumetric efficiency;
It’s a factor of engine design, one that you can’t change without completely
Altering the way the engine moves air.

What is turbo lag? It’s the delay between when you open the throttle;
And when the turbo begins delivering a boost
When the engine is above its boost threshold RPM. Some engines have nearly zero turbo lag, some have a ton.

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But there isn’t an engine out there that can deliver;
An immediate hit of boost below its boost threshold RPM. It’s simply not possible.

So, the next time you blame a turbocharged vehicle’s slow throttle response on turbo lag,
Think to yourself: Am I experiencing lag,
Or is this a case of sitting below the boost threshold?

Here’s Fenske with the nitty-gritty:

Ekster EU

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