Now this is not Hard Fact that I can Quote, but I do remember reading some about it, and have used existing knowledge to deduce some as well. SO here goes my explanation with some background first.
Any object made is made to fall within certain tolerances. On mass produced items like our beloved 250bhp PSD
motor are going to far more than say on a turbine part for a jet engine. The higher the tolerances, the less the production costs(less precision costs less). Besides costing less to make, the more tolerance you allow for, the less waste you will have. Again- cheaper. Now you consider all the parts that go into the motor, made to fall within the "is it good enough to work" column and you have a lot of variables. Look at Camshaft stroke and balance, piston height, wrist pin height, centering of bearings, crank bore locations, cylinder ring sealing, deck height, valve lift, pushrod length, rocker geometry, the list goes on. And then we get into the injector itself and all of its parts, P&B, inensifier piston, nozzle, solenoid strength, etc. Now lets look at the HPOP
. Again a whole lot of parts that can have an ever so slight tolerance. When put together this all adds up as Tolernace stacking and can become significant. Allow me illustrate it this way. Say has a Carpenter cuts 16 2x4's to 9ft, plus or minus a 1/16th. Well depending on who did the cutting on that day, the saw blade, etc all the boards can be within spec(1/16) but when put end to end could add up to be an inch off, which is way out of tolernace. So you can see how tolernace stacking can add up even when nothing is wrong or out of spec. The the higher the manufacturing tolernaces, the more the potential effect of this.
Anyone who has an AE
probably has noticed something called "rotational change velocity". This feature measures in percentage how much rotational velocity is at each cylinder. This is something that the PCM
constantly monitors for each cylinder. 0 means that all the cylinders are contributing equally. The higher the number, the more a cylinder is not contributing. While we use this to check on injector performance, it is inteded to monitor cylinder performance. Due to the tolerance stacking mentioned earlier, if all cylinders are treated the same, there could be potentially a significant output differance from your weakest cylinder to your strongest cylinder, but nothing at all wrong. The engineers needed a way to compensate for this. So in came
"rotational change velocity" The computer will attempt to keep all these figures at 0, within a certain tolerance. So obviously we can push the weak link harder, so the computer will bring the others down to match the weak link. Again only within a minimal range. Once the weak link exceeds whatever the predetermined acceptable range is, it will show up on the "RCV" Remember, RCV does not care why a cylinder is not as strong, be it injector, CP, leaking valve, etc.
So finally we get to the moral of the story. As we all already know, thanks in large part by the early work of Hutch, the OEM fuel design is poor, and #8 is in a constant state of starvation, and exposure to air. And guess what, this cylinder is weaker than the rest. However the PCM
by doing what it's programmed to do, masks this by matching all the cylinders to the weakest link. #8. Remove the weak link and over the course of the next several miles, the computer gradually relearns that #8 is not as weak anymore and again follows it programming.
Time and time again I have heard and read about how the CCK takes a few days to break in. After digging as much as I could ontrying to explain why, this is what I came up with. What I do not know is if aftermarket tuners have any affect on this or not.