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      09-30-2012, 09:45 PM   #392
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Drives: E92 M3
Join Date: Sep 2006
Location: San Diego, CA USA

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Originally Posted by NISFAN View Post
Yes, exactly....people expect boost from idle to red line.

Where they feel the biggest deficit of boosted power is just off idle. This is not lag, this is a Turbo below it's boost threshold. i.e. in a rev range where the turbo is not getting enough exhaust energy to produce positive boost. If the engine were kept below this threshold boost would never be produced.
I don't see much harm in labeling this all lag. Lag comes from impeller inertia, friction, efficiency, load and plumbing losses. The boost threshold is simply an rpm range where flow does not produce significant turbo rpm, flow and pressure. Both create a perceived lag in throttle response and rapid onset of vehicle power feeling like some "extra" amount of throttle has been applied.

Originally Posted by NISFAN View Post
This statement proves how common the mis-labeling of 'LAG' is. This electric tri turbo arrangement will be to reduce boost threshold, not to counter lag, but as the common misconception is that lag is boost threshold it gets labelled as an anti lag device.
It is also unknown if an implementation of and electric turbo charger (or perhaps supercharger) will exist on the car and it is also unknown if it will target lag or lowering of the boost threshold or both.

Originally Posted by NISFAN View Post
Right I need to explain this better. Lets say you take an S65 engined M3 and turbo charge it. You leave everything about the engine exactly as it is, cams, compression ratio, everything. But you now have a large sized turbo that comes on boost (the boost threshold thing once again) at 4000rpm, and runs hard til the redline.

Now if you were to drive this Turbo S65 M3 you would get used to the intoxicating power that the turbo produces. After a few hours behind the wheel you would start to notice the how poor the response is below 4000rpm. This is your same beloved S65 but without boost, that seemed to have a lovely spread of torque, but now feels flat un boosted.

This is what I mean, a Normally aspirated engine has lag all the way to the red line, as it never gets that high torque boost hit.

Make sense now?
That is what I figured you meant anyway and of course it makes sense. It is however not a particularly lucid way of thinking about engine performance. Either way you slice it, it is better to not have lag. Also more power at any rpm is always better. These are nothing more than tautologies! One other very relevant point is that most production turbo engines are much smaller displacement than a competitive NA engine or the NA engine they have replaced. Typically to have a less expensive engine and to improve fuel economy. The makes them notoriously down on torque when not producing significant turbo pressure. Hence why turbo lag and rpm ranges outside of the boost threshold are more annoying.