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      06-23-2013, 09:55 PM   #76
CanAutM3
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You bring interesting points

Quote:
Originally Posted by NISFAN View Post
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.
My understanding:

Fuelling lag is pretty much a non-issue on the S65 since it relies mostly on RPM and throttle position to establish the required fuel schedule. So as soon as the throttles open, the right amount of fuel is sent to the injectors for that given RPM. BTW, even on older carburetor engines, more sophisticated units had an "acceleration pump" to push an extra burst of fuel when the throttles were abruptly opened.

While I agree that NA engines do have some lag, the configuration of the engine has a big impact on this lag. The S65 with high revving (good breathing) capability and individual throttle bodies positioned very close to the intake valves greatly minimize the lag to almost nothing.

You might be confusing absolute pressure with total pressure. Manifold pressure is a static pressure measurement. While an intake tract has some inherent pressure loss, the main reason the static pressure drops is because of the velocity of the intake charge. The total pressure in the manifold still remains close to 1 bar.

To repeat our previous scenario with the S65, with fully closed throttles, the static pressure in the manifold is at about 1 bar (no flow). When the throttles open, the ECU instantly adjusts the fuel flow and the full (max) intake charge only has a few inches to travel to reach the cylinder to produce power. Clearly much faster than the turbo scenario.

So not sure myth is "busted" here .

Quote:
Originally Posted by NISFAN View Post
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.
IMO for a turbo to surge, either the turbo is wrongly sized for the engine application and/or the blow off valve is not properly calibrated. I am unsure of the point you are trying to make here.

(I work with aircraft turbine engines, so I very well know what a surge is: )

Quote:
Originally Posted by NISFAN View Post
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.
I was not suggesting exhaust fuel injection for street application, I simply quoted an example of technology used to minimize turbo lag. I am not sure anyone would tolerate the machine gun staccato on decel

I am not concerned about lag on initial acceleration from a standing start or from a roll. It is more the precise control of the power delivery that I am referring to. I understand that not everyone tracks their M3, but I do. I have always enjoyed the very precise and predictable nature of the power delivery of my M3s. When cornering at the limit, I can precisely and smoothly adjust the cornering attitude of the car by modulating the throttle. It is this behaviour I hope will be preserved in a FI F80/F82 ///M.

Quote:
Originally Posted by NISFAN View Post
Plus side is the DCT will feel manic going from full boost to full boost straight away.
Agreed. The DCT will definitely contribute to make this new power plant shine. The quick shifts will keep the engine on boost and if the gear ratios are well chosen, the acceleration curve will be very linear going from gear to gear.

Last edited by CanAutM3; 06-23-2013 at 11:02 PM..
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