Turbo Lag vs Boost Threshold — What's The Difference?

What Is Turbo Lag? What Is Boost Threshold?

Honeywell (the makers of Garrett Turbochargers) defines boost threshold as: "Boost threshold is the engine speed at which there is sufficient exhaust gas flow to generate positive manifold pressure, or boost."

They also define turbo lag as: "Turbo lag is the time delay of boost response after the throttle is opened when operating above the boost threshold engine speed. Turbo lag is determined by many factors, including turbo size relative to engine size, the state of tuning of the engine, the inertia of the turbo's rotating group, turbine efficiency, intake plumbing losses, exhaust backpressure, etc."

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[TheCarPassionChannel]

Would love a video on how turbo technology has changed in the last 30 years, like how a basic single scroll/twin turbo 6-cylinder can now hit max boost at 1300rpm where in the 90's it took 3500rpm to spool them, yet both turbos max out around the same 400hp range. Maybe a look at an ancient GT2860RS against a more modern EFR6258 or Garrett G25. Great video as always!

[sachinchauhan]

Guys like you are the Real MVP of YouTube. Hats of To You Sir

[driffbro]

You should make a video on Kawasaki's h2/h2r supercharging system even though you're not a motorcycle guy..

[blkss]

Turbo lag is like foreplay. You know it's coming. And it's gonna be good...

[themotofixery]

Id like you to explore the relationship between engine size, turbo size, and boost threshold and where the point of diminishing returns might be if using an over/under sized turbo for a certain application.

[nickolastiguan]

When I don't understand most of the engineering explanations/terms, but still liked the video

[Inkwellish]

I get it now. I have a Chevy Sonic LT Turbo 6 speed. This is the first turbocharged vehicle I have owned, and it took me a while to get used to the sudden surge of torque when the the RPMs hit around 3000. Under hard acceleration I always thought it was just spooling up and 3000 was where it finally made boost, but now it makes sense that it can only start making boost at 3000. It all makes sense now

[sorwis]

People confusing these two rather simple things has long been a pet peeve of mine. Nice of you to make it clear.

[derfernerf]

So turbo lag is actually the length in time between when you start making boost, to when you hit peak boost? And threshold is the delay up until you start making boost.

So the term “turbo lag” seems to be widely misused as people (myself included) refer to it as the time it takes for the car to start making boost.

Really love these informative short videos Jason! I learn so much!

[Astral]

Thanks for this useful video which was much needed, I've seen many people including professional car reviewers often use the word "Turbo Lag" to describe boost threshold, hopefully enough people would watch this and learn.

[BLUEVITZrs]

How do modern turbocharged engines have such a low boost threshold yet are able to make so much power? Do they have huge wastegate openings to allow enough exhaust to exit so as not to become a bottleneck?

[hassenfepher]

good stuff. what i have been calling lag was threshold plus lag. makes perfect sense. your delivery is still just a touch on the dry side tough.

[austinallmond]

EMD did something really clever when they went to turbocharge their 2 stroke diesel engines, which up until that point only had superchargers. Since a 2 stroke diesel cannot run without forced induction, EMD got around the problems associated with boost threshold by mechanically driving the turbocharger via a planetary gear drive and an overrunning clutch at lower RPM's. The turbocharger acts as a centrifugal supercharger up until around notch 6, which is 75% throttle, after which, the overrunning clutch disengages and the exhaust gases begin to drive the turbocharger.

[jmardinly]

Fenske completely overlooks camshaft design: low overlap camshafts allow the engine to develop high torque levels at low rpm/low gas flow conditions, and rely on boost rather than overlap to fill the cylinders at higher rpm. In boosted engines like this, there is negligible perceived response lag. They top out relatively early, though.

[joshcareau]

Simple and Abstract. Great explanation! Keep up the good work!

[zefautino]

I do have a comment.
Today I finally viewed one video from you on my full hd tv. Definitely one of the best quality videos, image wise, I've seen using YouTube.
Sound, no so great. Probably on my side, believe you not recording surround, and it does not sound natural with the way I have my system set.

[madduck]

This guy's Jeopardy smart! Based on the discussion in the video with a little trial and error; I figured out what RPM my WRX hits the Boost threshold. So if I start off at those RPMs I'm instantly able to boost.

[Sold_a_dummy]

Excellent. I knew there were two different things happening when I hit the gas, depending on revs etc but I didnt know about boost threshold. Now that youve clarified things. So many reviewers get turbo lag wrong. So many!!

[XxSavageBearxX]

Jason, I play your videos at .75x speed so I can keep up with the math you put down. Great videos dude

[jeffbeasley]

Totally agree that people often misunderstand when flooring it at low engine speed and having to wait for more massflow before the turbo spools up. But I think it's a little misleading to call it a boost threshold when it's not a single point. In reality, boost pressure with a closed wastegate is basically a function of massflow so you will get some boost at a certain rpm and then more the higher you go. This is really evident in engines that run relatively high boost and have relatively large turbos. For example, I am working on a project where even at 1000 rpm there is some boost but full boost is not reached until after 2000 rpm. At 1500 you will never get close to full boost but you are still making way more torque than at 1000.