Hi has anyone got opinion on KandN on 4.2 advice please pros and cons appreciated

I have read just as many reviews that say they're a waste of money as I have that say they add bhp and torque.......you pays your money etc.

Having said that I have put one in Mrs B's Cooper S  Current: FF Westminster money pit

Gone:
RRS 4.2 SC
RRS 3.6 TDV8
FL2 TD4

I did have K & N filters on my 3.9 classic with re-jetted carbs (engine / gearbox was installed by specialist so had a choice carbs or injection). I didn't have any problems, cleaned and re-oiled them as per instructions.

However, I personally would not fit them to a engine that has a MAF (as most do now) having heard stories that, if they are over oiled the oil mist can contaminate the MAF and screw up the mixture settings,
Some people do run them dry which I think defeats the whole purpose of protecting the engine.

If you want to improve performance there are a number of things you can do :-

1/ Remap the engine.
2/ Custom made stainless exhaust with tubular manifolds with different silencers.
3/ Get the supercharger pulley wheel changed for a smaller one = higher rpm = higher inlet manifold pressure
(there are companies that can do this) approx. £500 or you can get a DIY kit.
4/ Replace the pipe between MAF and inlet with 90deg stainless pipe + silicone piping joiners improves airflow.
An old forum member did this and reported a marginal increase in performance (The all Seeing Pie).

Bottom line is, tuning is a money pit you have been warned .

They work if you have modified the engine enough to benefit the the extra air. On a standard engine its pretty much a waste. You may get marginally better throttle response. Just put a good quality paper one in.
As mentioned, over oiling them wrecks the MAF sensors. 2007 Vogue TDV8 in Zermatt Silver with Charcoal

This is some tuning posts you might find interesting:-

www.fullfatrr.com/forum/topic19069.html

www.fullfatrr.com/forum/topic15296.html

Our mappers have always said that K&N's don't work on modern RR's...

I had them on mine and could not feel difference one way or another, so I went back to paper elements. 2012 TDV8 Black Edition
2000 Disco. TD5
1975 series III