Posted by daanesh on December 29, 2000 at 13:57:14:
In Reply to: Why don't the masters (BMW) change/improve this? posted by Agent99 on December 29, 2000 at 11:49:20:
simply 'opening up' an intake or exhaust is not neccessarily a good thing unless you only drive your car at WOT or between stock power peak and redline.
and don't forget the role of the HFM in all of this -- the smoother the charge in the intake tubing, the more accurately the flow through the measuring channel reflects actual intake volume and the more afinely controlled you air/fuel mixture can be.
also, if the HFM is sending coarse signals (i.e. the numbers jump around alot rather than repeat, the DME is coded to be more conservative with ignition advance, etc.
Bosch specs a certain distance of straight tubing upstream of the HFM for laminar flow through the housing. this number is based on the use of a panel filter. a 360 cone style filter is even less conducive to laminar charge (and laminar is faster than turbulent if you paid attention in first year physics, and charge velicity is more important that intake capacity.
the oil from the K&Ns has been known to form a film on the hot wire of the HFM, changing it's response to flow and screwing up the works. the itg filters have a velocity stack at the output and the multilayered foam is a hell of a lot more effective at straightening out the charge as it exits the filter than K&N's pleated media with no stack.
i've been writing a class on Engine management for VW-Audi so trust me on this one (the suppliers/technologies overlap).
95 M3, Euro airbox/HFM stock filter
00 M5 stock intake/filter