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


Hello everyone and welcome in this video We’re going to be talking about the difference between boost threshold and turbo lag two terms Which I think are often confused with one another and we’ll get started talking about What is boost threshold so this is the engine speed at which you have sufficient Exhaust gas flow in order to create boost boost of course being when your intake manifold pressure is above atmospheric pressure so here we have a simple depiction of a turbocharged engine and Basically that piston is going to be moving up and down as it moves faster You’re flowing a more gas through the exhaust and eventually that’s enough gas to spool up this Turbocharger and then pull an air so that rpm at which you’re producing enough Exhaust gas to start this turbocharger spooling up and producing positive intake manifold pressure that is your boost threshold Now if we’re to look at you know the intake flow going into an engine. That’s equal to its displacement multiplied by the RPM You’re spinning at multiplied by the volumetric efficiency divided by two That’s the number of crankshaft rotations you have per intake stroke so as an example if we were to have a 2.0L engine spinning at 4,000 rpm with 100% volumetric efficiency at 4,000 rpm it would be producing 4,000 litres a minute that it would be pulling in in order to use for combustion So if you were to look at your exhaust gas flow Versus rpm it’s going to be basically a nice linear line assuming you have a 100% volumetric efficiency And then once you start to build boost this volumetric efficiency increases so if you’re about 15 psi volumetric efficiency Is now 200 percent rather than 100 percent so very quickly the amount of gas that you’re pushing through the exhaust is going to increase And so you’re going to see a difference in the curve Where you’re now producing so much more exhaust flow that that’s going to spool up that turbocharger and very quickly you’re gonna have Significantly more airflow going through it so that is boost threshold now moving on to turbo lag This is the time delay above boost threshold when you are operating at an RPM high enough that you can create boost The time delay between when you open up the throttle from closed to when you actually start to receive boost So the order looking at an engine right here first of all you have your throttle closed But let’s say you’re operating at 4,000 rpm and your boost threshold is 3,000 rpm You’re well above what you need to be in order to create that boost so you press the throttle pedal you open it up it starts to then you know intake more air more air flows more exhaust gases through the Turbocharger that starts to spool up your turbocharger you start pulling in more air and then eventually that air passes through this intake through the intercooler through the throttle body into your intake manifold into your engine and Eventually you do start to create boost that time delay is what turbo lag is Now if you are operating an engine you know in this lower Rpm where you aren’t above the boost threshold and you floor it you may think okay? I’ve got this enormous amount of turbo lag because it takes until 4000 rpm when in reality you just don’t have enough gas flow in order to spool that turbo up So there’s a bit of a difference. I think a good example is the previous generation Subaru WRX Where you know below about 3500 rpm 3000 rpm, you won’t create any boost so you can floor it and nothing happens you have to be above that in order to actually feel the boost and feel that torque coming in And then once you are above that the car still does have turbo lag so you floor and you still wait for all of this to occur for the turbo to spool up and then for that intake air to reach the Engine so hopefully that kind of gives you a bit of you know a better understanding of what these two terms mean and you know the understanding of how they actually affect how the car performs if you guys have Any questions or comments of course feel free to leave them below, thanks for watching! You

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29 thoughts on “Turbo Lag vs Boost Threshold — What’s The Difference?

  1. Please test the banks power intake manifold air density gauge . Shows how a boost pressure is less than important compared to manifold air density

  2. Can you do a video on tuning Turbo engines? Specifically what these flash tunes do from 5Star, LMS and Roush? I have a 2018 F150 ecoboost and installed a 87 Performance Tune…. Totally transforms this engine…. Tell us about flash tunes!!

  3. 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

  4. When I watch car reviewers they say 'turbo lag in this supercar is unnoticable'. Do they mean boost threshold or turbo lag? Because boost threshold seems like a bigger problem.

  5. I find it absolutely amazing that turbos these days can have the boost threshold at or below 1500 RPM. So you're only operating when there's turbo lag. And these small turbos make the turbo lag almost nonexistent. It's astounding to me.

  6. IMHO you should have involved the interaction of the wastegate and diverter valve into explanation of turbo lag, as they play an important role here. Hope you will address this in a future video!

  7. Hello, can you please tell me what petrol should i put into Genesis 2017 2.0 turbo 245 HP. In the manual it states not lower than 92. In Russia we have 92,95,98 and 100. At the moment i am putting 100 and my friends telling me that its can be dangerous for the engine and i should switch to 95. Can i get some recommendations from experts please, thank you very much.

  8. Great explanation and very helpful. It seems some adjustments to our terminology is needed when talking about the behavior of turbocharged engines.

  9. Does diesels have turbo lag too? Because I'm not feeling it. I can definitely feel the boost threshold but does my engine braking keep my turbo spinning?

  10. What about when you're cruising on the highway and step on the gas pedal and your Boost goes up but then drops within a few seconds? What condition is that what would cause that?

  11. wow! i had been doing this wrong my whole life :O…
    some observations – to make more power from a smaller engine you have to use larger turbos and that increases boost threshold(eg ford ecoboost [email protected])..larger capacity engines with small or medium sized turbos give you amazing drivability because boost threshold is almost nil(toyota d4d [email protected])..

  12. How can we find out what RPM our boost threshold is at? Can we say the boost threshold is when the boost gauge hits 0 psi?

  13. So can you increase your exhaust flow/pressure and lower your boost threshold with a smaller diameter exhaust pipe? Im askin for when you see a high hp car with a rear mounted turbo that has no lag. They usually have like a 2" or 2.5" single exhaust running to the turbo making 600+hp.

  14. I hear turbo lag used often in my car scene when referring to boost threshold. Hopefully videos like this help educate.

  15. If you drive a lot in the lower RPM range under boost threshold, will it damage your engine or cause problems? Specifically the 3.5/2.7 ecoboost engine where the TCM seems to force the engine into a taller gear for fuel efficiency in the F-150 series trucks.

  16. Do turbo engines loose power in higher elevations, like in Denver, etc.. ? I read article where motortrend tested the new santa fe in both it's engines somewhere high up, and the naturally aspirated 2.4L beat it's turbo brother in 0-60 and 1/4 mile by approx. half a sec. on both runs, but when had same vehicles on their home turf, the turbo did approx. 1.5 seconds faster then it did before (a huge difference) and it beat the 2.4 this time.. I though some say the naturally aspirated engines are effected by higher elevation more than turbos ?
    Short video on this, would be great.

  17. the bigger the gain from boost lag the better. nothing better than driving around a 190 hp car with no boost then waiting even 10 seconds and getting 800 out of it…. it is so fun, I LOVE IT
    i dont care if it still runs [email protected] the spool and sound is THE BEST!!!!! i miss my over sized turbo civic

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