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      06-23-2013, 03:55 PM   #72
NISFAN
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Drives: BMW M2
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Good post

Quote:
Originally Posted by CanAutM3 View Post
I am not sure anyone is debating the fact that the S63Tu makes more power and has a broader power band than the S85 as is clearly demonstrated in the vid you posted.
Firstly the video I showed wasn't really to show an f10 M5 being faster than an e60 M5. It just showed in my view a turbo charged engine giving a pretty good account in the first tenths of a second. Of course there are other factors at play here, but a good display to help reduce the big bad lag issue.


Quote:
Originally Posted by CanAutM3 View Post
My personal understanding of lag, or turbo lag, is the delay between throttle pedal movement and power output change. A high revving NA engine with individual throttle bodies will have a very sharp response to throttle pedal movements. Such a characteristic makes the car very enjoyable and easy to control and balance at the limit.
Don't forget, even an NA suffers from lag, two main reasons are:
  1. Fuelling lag - the delay for the ECU to adjust engine parameters in vastly different transient states.
  2. Pressure drop - don't forget although we humans base our pressure readings from ambient, an engine runs at absolute pressure. On WOT even an NA engine, manifold pressure will drop to around -0.7 bar. So even a 4litre engine will be producing not much torque at that low pressure. This fact alone busts the instantaneous myth.

Quote:
Originally Posted by CanAutM3 View Post
On the other hand, a turbocharged engine will have inherent delay between the throttle pedal application and the power increase. Some operating conditions will make this lag worse. For example going from a fully closed throttle position to a fully opened position, regardless of RPM. When the throttle is closed, very little gases pass through the engine and the turbo spools down. When the throttle is re-opened, there is a time delay for the lower pressure intake charge to enter the engine, go through the combustion cycle, travel trough the exhaust and spool up the turbine; and then for the compressor to push more air in the intake, for the higher intake charge to reach the cylinder and finally for the engine to make more power. With your background, I am sure you are familiar with the various methods used to minimize or counter this delay, like using smaller turbos or exhaust fuel injection to keep the turbo spooling (those lovely pops that rally cars do on rundown) just to name two. The engineers at ///M seem to have to done a pretty good job to minimize this on the S63Tu, but the lag is still there to some extent according to most reviewers.
Yes good summation of sequence of events. It is true that at very low RPM turbo lag is greater. In fact you could theoretically size a turbo so big that it wouldn't produce positive boost all the way through the revs, upto the redline. Clearly turbo sizing is very important. The lower the 'boost threshold' - the revs at which enough energy is supplied to get the turbo onto the flow map. The lower the 'turbo lag' will be.

No I am not going to change the mind of those that religiously believe in the 80's turbo lag phenomenon (carburettor application has a lot to pay for), but I have personally seen turbo's on 2.6 litre straight sixes make too much boost on WOT, so much so that it caused the turbo to go into 'surge', a condition where the engine can't flow the volume of air coming from the turbo.

I am very curious about what technology the ///M engineers will use to minimize the lag on the S55 (electric turbo ??). I am sure they will be very creative about it [/QUOTE]

On a theoretical engine as swamp mentioned, an NA M4 with 3.0l displacement vs. a turbo variant of the same. The FI version doesn't need to be as 'cammy' to produce the power. Therefore the low rev cam can be profiled to suit low engine speeds, increasing like for like un-boosted torque (outside VANOS limits).

I'm not sure the 'anti-lag' you describe would be an adopted solution for road use, but even launch control holding WOT does get turbo's spooled. (clearly demonstrated in the Nissan GTR, the LC mode obliterates the non LC launch, purely because of boost at launch. On a RWD, this would be too much.

One thing for sure, unless BMW have integrated a 'flat shift' facility, a MT is going to be slower than DCT by a bigger margin. Plus side is the DCT will feel manic going from full boost to full boost straight away.

Last edited by NISFAN; 06-23-2013 at 04:52 PM..
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