172 5k kick by BenR 2006
By no means is the 5k kick the result of any mechanical/electronic chance withing the engine system, whether that be mapping paremeters or valve timing.
I think to fully understand, a base understanding of the operating system of the vvt is needed. The vvt system employed on the clio is what we refer to as cam phasing, this is where the whole camshaft (inlet or exhaust or both) is advanced/retarded. This can be in a single step as it is in the clio (single stage 16 degree advance on the inlet), or variable. The camshaft is hollow and is used as a galley to feed the front pulley (called a phaser) with oil pressure, this pressure simply acts on vanes inside to rotate it in a desired direction, and returned under mechanical pressure. The cam does not always carry oil pressure, but a vvt switch is used, and is basically a plunder activated by 12 (grounded by the ecu (a lowside switch)) which allows the passing of oil from the lifter galley to an area of the cam bearings with holes that can feed the inside of the cam, which then passes through to the nose of the cam and into the pulley (phaser).
Variable setups (like the new 197) will use the same base components as what is used today, but instead of the vvt plunger being used as a switch, the same plunger has the ability to open and bypass oil to either side of the vanes in the phaser. By using a PWM signal, you can gain full control of the phaser to advance the cam in a near infinately variable curve vs rpm vs manifold pressure vs throttle angle etc.
Honda's Vtec system is a cam 'changing' system where the actual cam profile is changed in its entirety (hence the 3 lobes per cylinder), the actual 'timing' of the primary lobe remains the same at all times. And now with I-Vtec and VVTL-i the benefits of both phasing and changing is being used at the same time to build some monster VE (volumetric efficiency) curves.
Anyway, in the clio the cam sits in its dephased state until the required parameters are met. This is above 1450rpm and 800mb manifold pressure. When it is required though, the cam is phased and the cam timing effectively advances 16 degrees, at 6800rpm it is dephased again and power drops off like a stone. No official reason, but my thoughts are that they do it so you guys shift up once there is a loss of acceleration. The reason for that is because the stock pistons just fall apart with prolongned high rpm use.
The 5k kick is the result of a few natural phenomenons within the engine. At 5000-ish rpm VE suddenly reaches a higher %'age and the resultant torque increase gives you that wahey feeling. VE increases due to the cam comming 'on'. This term has nothing to do with VVT and is quite an old term, whereby the reference is to the rpm region that particualr engine/cam combo requires to process the valve timing events efficiently.
VE refers to volumetric efficiency, or the %'age of the cylinder that you can fill with fresh charge. For a 2ltr, thats 500cc per cylinder. If you can only fill that cylinder with 430cc's at peak efficiency (normally at peak torque point) thats 86% VE. The higher the VE on any engine, the higher the torque output at that specific RPM, and all engine tuning revolves around increasing or rather sustaining a good VE for as long as possible, and upto as high an rpm as possible.
Anyway, back on track. The clio with its advanced cam timing (phased) operates with an overlap value that is larger than when it is dephased (infact it has no overlap when depahsed). This overlap is part of cam design basics, and larger overlap periods are used to help generate higher VE's via scavenging and inertia ramming. At 5000 rpm the natural effects of the port velocity and pulse tuning al reach a level where they start to actively enhance the torque production with the valve timing it is running. Put in a wilder cam and it will push the 'kick' higher up if nothing else changes, but lower rpm efficiency will loose out, and peak power will be pushed up the rpm scale, along with a higher peak bhp figure. To an extent, there will be a point where you can go wilder on the cam and just loose out everywhere as the engine system as a whole does not work with the cam profile you are running.
Not very clear sorry, basis is that the 5k kick is the result of natural phenomenons within the engine, primarily port velocities and pulse tuning harmonics.
