Some pointers in choosing AAH/AFC heads for building...
One of the best things
about having an extra set of heads around is the ability to take plenty of time
in re-building them and not find yourself in the position of needing head-work
with the associated down-time of your daily driver.
Also many of the facets of head work can be accomplished in stages at a head
shop and don't have to all be done in one fell swoop. You can have them clean
and disassemble your heads initially. Then pick them up take em home and start the porting/polishing/gasket-matching
tasks along with smoothing the area on top of the head (under the valve cover)
so that oil doesn't have so much rough surface to cling to and can drain back
into the oilpan quicker. There's MUCH room for
improvement in that area alone on these heads.
Later, as time & funds become available you can return to the head shop to
have them install new valve guides and/or valve seats if you plan on going with
larger valves. Then return home and back to porting/polishing around those
newly installed pieces.
Back to the head shop a final time once you have your valves, lifters and
springs in-hand to let them install the new valves, seals, springs, retainers,
keepers then lap the valves and get everything to the correct installed heights
and fly-cut the head mating surface flat and square.
Now you start aquiring all the bits and pieces
necessary to do the installation. You should do Audi exhaust studs, nuts and
washers. DO NOT use just any stud, nut and washer that will work. Audi exhaust
studs/nuts and washers are the best you can do bar none! You'll also need all
new cam seals, head gaskets, IM gaskets and valve cover gaskets. Wouldn't be a
bad idea to consider an EGR clean-up/replacement while in there as well as a
new valley pan gasket. There won't be any easier time to do these things.
As far as head selection here's some good/bad things about each head:
I would personally still choose AAH
heads today to rebuild rather than AFC even tho the AFC in stock form would
outflow the AAH heads. But in "rebuiding heads" to "stock" specifications today
I'd opt for the AFC heads on an AAH or AFC engine with AAH cams.
Basically if yoiu're going for 270-300+ HP the AAH heads would be easier/cheaper
to accomplish it. For ANY HP number lower than that the AFC heads are the way to
go and for the very same reasons.
AAH uses a dual valve spring while AFC uses a single spring. The AAH has
hardened spring seats sitting below the springs that allow using even heavier
springs which allows higher RPMs (if stand-alone is used)while preventing valve
float. The AFC heads would require machining and hardened seats made to
accomplish that. Any valve stem diameter can be installed in either head with
appropriate valve guides. And a total of 2mm larger valves can be used, ie; 1mm
larger int and exh or 2mm larger intake or 2mm larger exhaust. It is possible to
use 1mm larger each and retain stock valve seats tho larger seats will always
flow better.