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Old 3rd April 2014, 18:39   #1
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Default Turbo lag v/s Boost threshold

Until a few days ago I thought I knew what turbo lag meant until I saw an online video where the host stated that often what we refer to as turbo lag is not actually lag but it is high boost threshold. So at present I have the following understanding of the terms in my head:

Generally we tend to call the time gap between the time we hit the throttle and the time when the turbo comes on as turbo lag however this is wrong. We should refer to it as boost threshold.

On the other hands once you're past the boost threshold and momentarily go off throttle causing a drop in rpm but continuing to remain in the boost threshold, the time taken for the turbo to spool up back to full boost level is referred to as turbo lag.

So it this correct?
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Old 3rd April 2014, 20:50   #2
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by Bookkeeper View Post
Until a few days ago I thought I knew what turbo lag meant until I saw an online video where the host stated that often what we refer to as turbo lag is not actually lag but it is high boost threshold. So at present I have the following understanding of the terms in my head:

Generally we tend to call the time gap between the time we hit the throttle and the time when the turbo comes on as turbo lag however this is wrong. We should refer to it as boost threshold.

On the other hands once you're past the boost threshold and momentarily go off throttle causing a drop in rpm but continuing to remain in the boost threshold, the time taken for the turbo to spool up back to full boost level is referred to as turbo lag.

So it this correct?
Wikipedia's got a pretty good explanation between lag and boost threshold

http://en.wikipedia.org/wiki/Turboch...rbocharger_lag

In practical terms, I don't think as a driver you can distinguish between threshold and lag. Both introduce a lag to throttle response. Also, modern turbo have a pretty low threshold anyway, so I'm not sure how much is threshold and how much is lag to start with.

Go for a supercharger and do away with this theoretical stuff!

Jeroen
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Old 4th April 2014, 21:50   #3
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by Jeroen View Post
In practical terms, I don't think as a driver you can distinguish between threshold and lag. Both introduce a lag to throttle response. Also, modern turbo have a pretty low threshold anyway, so I'm not sure how much is threshold and how much is lag to start with.

Go for a supercharger and do away with this theoretical stuff!
Well thank you for your reply. I had gone through that explanation earlier but given Wikipedia's falling standards I thought it better to seek first hand expert reply.

Don't know about lag but a high threshold can definitely be felt.

However, why is it that folks tend to use the word lag even on the forum? Are they actually referring to the lag or have they got their concepts mixed up as well?
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Old 5th April 2014, 19:06   #4
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Default Re: Turbo lag v/s Boost threshold

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Don't know about lag but a high threshold can definitely be felt.
How do you feel high threshold and how do you distinguish between lag?

Thanks,

Jeroen
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Old 7th April 2014, 18:03   #5
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Default Re: Turbo lag v/s Boost threshold

Lag is the phenomenon and threshold is one of the factor causing the phenomenon? Does this make sense?

This is as simple as i can put after going through wiki, though i also got confused couple of times when some techies started throwing these terms
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Old 7th April 2014, 20:50   #6
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by Bookkeeper View Post
...when the turbo comes on...
A turbocharger (TC) is always 'on'. You can't 'switch it off', as you can a supercharger. It will continue to spin as long as the engine is running and the exhaust gases are flowing. It is another thing that the TC will not be compressing the intake air adequately when it is spinning below a certain rpm - hence the engine does not tend to pull as well from lower rpm as it does after the TC crosses its boost threshold (i.e. it produces an useful amount of compression of the inlet air).

Quote:
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...once you're past the boost threshold and momentarily go off throttle causing a drop in rpm but continuing to remain in the boost threshold, the time taken for the turbo to spool up back to full boost level is referred to as turbo lag.
So it this correct?
Full boost, or maximum boost pressure for a given TC, is determined by a number of design features, including size, construction, the presence of a wastegate / blow-off valve etc. What you seem to infer is that turbo lag is the gap between boost threshold and full boost - which is not correct.

At a given engine rpm, if the TC rpm is fast enough, it crosses the boost threshold. Much research has gone into (and continues to go into) designing & manufacturing a TC that can spin fast enough at low engine rpm to produce useful intake air compression - twin turbos, VGTs, lightweight materials to reduce frictional and inertia losses (such as ceramic bearings, TC vanes of lightweight alloys etc.) - the sole purpose being to eliminate turbo lag.
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...some techies started throwing these terms
Just because there are a few microprocessors inside each car nowadays, doesn't mean that one can continue on one's way after a car crash by closing and opening the Windows a few times - or even disconnecting and reconnecting the battery! Google isn't getting into manufacturing cars anytime soon either!

Tell the techies to get some grease under their fingernails once in a while...

Last edited by SS-Traveller : 7th April 2014 at 21:02.
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Old 7th April 2014, 21:29   #7
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Default Re: Turbo lag v/s Boost threshold

Quote:
Originally Posted by Bookkeeper View Post
Until a few days ago I thought I knew what turbo lag meant until I saw an online video where the host stated that often what we refer to as turbo lag is not actually lag but it is high boost threshold. So at present I have the following understanding of the terms in my head:
Ignore the video as it has only served to confuse you.

Quote:
Originally Posted by Bookkeeper View Post
Generally we tend to call the time gap between the time we hit the throttle and the time when the turbo comes on as turbo lag however this is wrong.
No, this is not wrong.
Quote:
Originally Posted by Bookkeeper View Post
. We should refer to it as boost threshold.
No, boost threshold is the exact rpm point at which there is sufficient boost.

Quote:
Originally Posted by Bookkeeper View Post
On the other hands once you're past the boost threshold and momentarily go off throttle causing a drop in rpm but continuing to remain in the boost threshold, the time taken for the turbo to spool up back to full boost level is referred to as turbo lag.

So is this correct?
No.
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Old 8th April 2014, 13:28   #8
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by Jeroen View Post
How do you feel high threshold and how do you distinguish between lag?
Well after the above posts from fellow members most of my wrong notions stand addressed and hence I retract my earlier statement.

Quote:
Originally Posted by Jaggu View Post
Lag is the phenomenon and threshold is one of the factor causing the phenomenon? Does this make sense?
Yes it does ease out a lot of confusion. I would be grateful if you could give in some more explanation on lag now that I'm clear about threshold.

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Originally Posted by SS-Traveller View Post
A turbocharger (TC) is always 'on'. You can't 'switch it off', as you can a supercharger. It will continue to spin as long as the engine is running and the exhaust gases are flowing. It is another thing that the TC will not be compressing the intake air adequately when it is spinning below a certain rpm - hence the engine does not tend to pull as well from lower rpm as it does after the TC crosses its boost threshold (i.e. it produces an useful amount of compression of the inlet air).
I knew that a TC is always spinning but it seems in all of confusion in my head I ended up forgetting that. I never knew that a supercharger could be turned off, learnt something new, thanks.

Quote:
At a given engine rpm, if the TC rpm is fast enough, it crosses the boost threshold. Much research has gone into (and continues to go into) designing & manufacturing a TC that can spin fast enough at low engine rpm to produce useful intake air compression - twin turbos, VGTs, lightweight materials to reduce frictional and inertia losses (such as ceramic bearings, TC vanes of lightweight alloys etc.) - the sole purpose being to eliminate turbo lag.
Can you please offer a more elaborate explanation on lag?

Quote:
Originally Posted by VeyronSuperSprt View Post
Ignore the video as it has only served to confuse you.
Ignored. The video, not your advice.


Thank you everyone for your replies. Now I have a clear picture of what threshold is, would really be grateful if a little more explanation is provided for lag.
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Old 8th April 2014, 13:58   #9
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Default Re: Turbo lag v/s Boost threshold

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Can you please offer a more elaborate explanation on lag?
Too much work!

Read this thread (Turbo Lag 101).

Also ask Google. You'll get resources like...
https://in.answers.yahoo.com/questio...8103816AAWiYfb

Also read about 'flat spots' (with relation to engines, not tyres): http://en.wikipedia.org/wiki/Flat_spot#Engine_response
http://www.fiatforum.com/sedici/2931...g-egr-fix.html
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Old 9th April 2014, 07:22   #10
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by SS-Traveller View Post
A turbocharger (TC) is always 'on'. You can't 'switch it off', as you can a supercharger.
Well, you can't just switch a supercharger on/off without it having being designed so. I certainly can't switch it of in my Jaguar. It would require at the minimum a clutch so you can disconnect the supercharger from whatever drivers it, e.g. pulleys, belts etc.

In general there are three types of superchargers, Roots, twin-screw and centrifugal. I'm not sure if I have ever seen a centrifugal one in real life, at least not for cars. But certainly in the case of a roots and a twin-screw, stopping the turbocharger will introduce a tremendous amount of resistance in the inlet circuit, so you would need a bit of plumbing, most likely, to bypass the supercharger in case you switch it off.

I seem to recall that American hot rods and such sometimes have the ability to switch on/off the supercharger. Not sure if there are any regular production cars that have this option. If anything, why would you want to switch off the supercharger? I love my supercharger and I would not switch it off even if I could.

On very large two stroke diesels as used on merchant ships, you might find additional TC and or superchargers to help out when the engine runs on low rpm.

The diesels, as large as a three story house, can run at 30-40 RPM. Problem is at that speed the exhaust gasses have low kinetic energy and hardly drive the TC. This is a big problem for two strokes as you need to clear the cylinder of exhaust gasses when the piston is near its lowest point and get new fresh air in as well. Hence there is/are additional TC or more often superchargers, electrically driven that kick in at low RPMs and just blow a steady volume of air into the inlet manifold.

As far as I know its very specific to large two stroke diesel, I've never seen or heard of such an arrangement on a car.

Jeroen

Last edited by Jeroen : 9th April 2014 at 07:24.
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Old 9th April 2014, 11:11   #11
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Default Re: Turbo lag v/s Boost threshold

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Well, you can't just switch a supercharger on/off without it having being designed so. I certainly can't switch it of in my Jaguar.
True - should have elaborated that it's not usually switched off, but can be if the engine designer wants to exercise the option. Add-on SCs on some customized American hotrods do have an option to switch the SC off, AFAIK, using a clutch.

However, the option to turn off a TC does not exist - again, AFAIK.
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Old 10th April 2014, 07:59   #12
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Default Re: Turbo lag v/s Boost threshold

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However, the option to turn off a TC does not exist
Agree, you can fiddle around with the waste gate though. You will need to make some modifications obviously and the turbo will still spin, but will be providing less boost. Again, sometimes used on hotrods and maybe the odd racing car as well?

Now, one for the turbo-nerds: does opening the waste gate earlier introduce more/less turbo lag or lowers/increase the boost threshold or both?

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Old 11th April 2014, 21:42   #13
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Default Re: Turbo lag v/s Boost threshold

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Now, one for the turbo-nerds: does opening the waste gate earlier introduce more/less turbo lag or lowers/increase the boost threshold or both?
I'll pretend NOT to be a turbo-nerd and wait for someone else to answer that question!

BTW, each time a wastegate pops open, how long does it remain open before shutting? Also, what controls the opening and shutting of the wastegate in an ECU-equipped car and a non-ECU equipped one?
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Old 11th April 2014, 22:10   #14
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Default Re: Turbo lag v/s Boost threshold

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I'll pretend NOT to be a turbo-nerd and wait for someone else to answer that question!

BTW, each time a wastegate pops open, how long does it remain open before shutting? Also, what controls the opening and shutting of the wastegate in an ECU-equipped car and a non-ECU equipped one?
What happened to all the experts on this forum? As soon as somebody reports a problem with his/her car a dozen members jump in with possible answers. Often without understanding the topic at hand or knowing the facts. I would have thought this question would have been a great one to ponder on.

The more mechanically inclined are likely to love the non-ECU operated waste gate as in it's simplest form it is operated by a simple mechanical linkage, a lever that controls the valve position!

Slightly more advanced is to have an actuator controlled waste gate, whereby the actuator is directly measuring boost pressure and controlling the waste gate accordingly. So balancing a pneumatic force against a spring essentially.

From here on it gets more complicated, but I will leave that to the turbo-specialist members. Going by the number of post on turbo related topics, or by the number of post from members claiming the turbo is at fault for any problem under the sun, this forum is awash with turbo specialists and I will gladly listen to what they have to say.

Jeroen
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Old 12th April 2014, 08:50   #15
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Default Re: Turbo lag v/s Boost threshold

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Originally Posted by Jeroen View Post
does opening the waste gate earlier introduce more/less turbo lag or lowers/increase the boost threshold or both?
Jeroen
How will opening a waste gate earlier, say at 9 psi instead of 10psi lower/ increase boost threshold and/or turbo lag?
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