[4v6]

Depends entirely on who's doing the job id think.

Couple of cautions tho.
Valve seat angles- although ive not done any in depth work on the 20valve's seat angles, its entirely possible they may be able to be improved upon, however its also possible that if the shop you go to is just a general reconditioners that theyll just cut a "3 angle" seat and thatll be that.
You really want to know what angles work best rather than just getting them cut with a set of generic cutters that they do all their seats with.
If youve any doubts about just how important the seat angles can be, i recently did a full set of tests on a 10 valve head and got a 14% improvement of 14.3cfm at 6.50mm lift just by optimising the seat regions, with it outflowing the stock profiles all over the range up to 1mm lift.
Thats actually more than the portwork yields on that particular head.
Best to speak to a shop in your locality that works on these motors as a daily job as theyre more likely to get it right.

Valve guides- who ever you take the head to, if theyre bashing the guides in or out with a drift and clubhammer/airhammer-walk away and take your precious cylinder head with you- im not kidding.
If the guides are really tight and they can be ( speaking from personal experience here) then warming the head and pressing the guides out carefully will work better than a drift because each blow on the drift with a hammer will spread the soft bronze alloy of the guide and screw up the fit in its bore in the head which is a very precise bit of machining, you absolutely do not want it being torn up/distorted.
Push them back out from the chamberside with some lubricant not straight through from the top downwards.
Refitting the guides using cold pressing- so no heat and no need to freeze the guides needs to have as much care taken as removal- using copper slip antiseize compound is the very best stuff for the job, on the exterior of the guide and inside the bore in the head will prevent tearing of the alloy.
If that happens due to careless removal or installation of a guide the guide can go way off centre (they always skew off a little) and the seat wont cut as easily, possibly not at all.
Cold pressing the guides in allows you to get a feel for how tight they are and you can back out of it if theyre too tight.
If the guides are frozen/cooled first and the head heated and you get one half fitted by running out of time, then youll have no idea how much interference the guide and head have, which can lead to too loose/too tight issues.
Incidentally, get the guides sized before fitment.
I nearly always have to do this with aftermarket stuff as its generally too large, audi guides on the 20 valve head are 12.04mm diameter, aftermarket ive measured as much as 12.08mm...thats a lot and can make the difference between them fitting or cracking the head possibly.
Stock audi guides will be correct size tho.

Hope that helps a bit.

 

[4v6]

you can test the sealing of the valves/seats first to see if it needs the work. Once mine was cleaned well, it sealed like new, so no valve job for me...and no chance of anyone screwing anything up.

High miles motors means the guides, especially on exhausts will need replacement, they tend to wear out at around the 100k mark, im not sure the material is particularly good, probably a cheap bronze instead of the more expensive colisbro variants is being used id think.
In that event , the seat will always need to be recut as itll not be possible to just lap them in, theyll always tend to be somewhat misaligned after the guides have gone in.

16 valve heads yup seem to be the same as near as dammit to me.

 

[4v6]

Really the angle is the same. You cut a three angle valve job at say 15,30,45. You can do a 5 angle job at say 15,23,30,37,45. Gives you better flow and less turbulence. You can back cut or flycut the valve as well to pick up some easy flow. The air rolls over the valve smoother. Then you can get in to valve margins. You can move it closer to the valve edge giving higher flow but less heat transfer(na heads). Or you can move it in to gain more contact to draw heat out of the valve.

Like most things, its not really that simple.
The best angle combination varies with the port downdraught angle and valve design which is why a generic 3 angle cut likely wont be optimal, and you also have the widths of them to consider.
5 angle cuts are unlikely to give any benefit as anything of 15 degrees and under the air simply doesnt "see" as a direction change.

The reason to move the contact area of the valve outward to the margin is simply to acheive the aim of maximising the useful flow capability of the valve diameter.
Effectively youre using a smaller valve if the seat doesnt extend to the maximum valve diameter.
The heat transfer mostly occurs between the valve and its seat so it cares not where the seat is located as long as the seat is of a suitable width to conduct the heat and matches the one on the valve.

Note of caution regarding backcutting intake valves: I found on the 10 valver that if the valve backcut angle and the seat combination dosnt match up (ie compliment each other) the port can oscillate which kills low lift flow stone dead ( it drops below stock levels) before it recovers at around 4.5mm lift.
On test it actually squeals loudly enough to hurt your ears, a very loud whistle was quite a shock when it started up as ive heard the toilet cistern sounds on other heads just never heard one go off like that, i took some video of it happening and its that loud it starts to overload the cameras mic, ill see if i can compress the video file and put it up as its a bit big at the moment.
I also found it can happen with a modified seat and stock valve combination, but as of yet ive not experimented with optimal seat angles or widths of the 20 valve head.

@Mcstiff- 20 valver has 38mm long guides.

 

[4v6]

Video of noisy port. Starts to oscillate at 2.5mm lift to about 4.5mm.

 

[4v6]

Couple of photos showing the issue i always seem to find with aftermarket guides.
At this rate i think id be better off getting a custom made item, at least itd save keep messing with them.

Stock audi guide, 12.04mm (consistent along the length).

Replacement item made by Febi in germany.

After some work to resize the diameter.

This is importnat stuff to address because a small discrepancy here can throw a valve miles off centre and gives us all sorts of hassles to put right., including pushing it out and starting again if its bad enough.

Anyways sorry about the thread hijack, if it helps someone out then im happy.