AAH/AFC exhaust manifold weights, specs, and flow rates...
If your choice of exhaust
is dictated by pocketbook alone then there is a clear winner between exhaust
manifolds.
AAH manifolds can flow 290 CFM before they run up against the wall. That limits
you to sub-200HP unless you can "force" via turbo/supercharger more
air thru them. If you're "pulling" all your air like the rest of us
the exhaust manifold is a huge limiting factor.
AAH flow rates:
Right exh manifold: 135.5CFM
Left exh manifold: 154.5CFM
Please understand these numbers correctly and what they're saying. This is the
amount of air you can flow thru these manifolds before a measureable
amount of back-pressure starts to build withinin the
exhaust manifold. Yes a well-built engine could conceivably "push"
more air thru them under brute force but it would come at the price of extreme
power loss over what a better-flowing/better designed manifold or header would
allow.
Also it's important to understand this is a "manifold only" flow and
makes no statement whatsoever about the downpipes or
exhaust sytem behind it.
Now here's a little light at the end of the tunnel and probably not at all
un-related to how Audi kept the same HP rating on the AFC car with less
compression and milder cams.
AFC flow rate: 378CFM
Left exh manifold: 194CFM
Right exh manifold: 184CFM
Literally a 30% flow-improvement over the AAH
manifolds and they weigh less:
AAH left: 6.2lbs
AAH right: 6.4lbs
AFC left: 5.8lbs
AFC right: 5.6lbs.
The AAH casting is obviously a more restrictive and narrower-ported manifold
than the AAH design and this carries thru to the collector body as well. Also
the AAH is a far rougher casting internally than the AFC. That's not to be
confused with the AFC is a smooth exhaust manifold internally however... it's
just significantly smoother than the AAH. I wouln't
say either manifold's internals are "slick/pretty."
The outlet flange ports on the AFC are larger as well:
AAH exh manifold outlet bores: L/R: 1.780"
AFC exh manifold outlet bores: L/R: 1.810"
A .030" larger tube is a good size flow enhancer.
Moving to the individual flange ports we find the one place the AAH clearly
hands the AFC exh manifolds their (collective) ass:
The AAH runner ports and passages inside are slightly larger than the AFC
ports/runners from the factory. Nothing major but a good
.010" diffrence on both height and width of
runner tubes. But apparently not enough to allow the AAH manifolds to
flow anywhere near as well as the AFC's. Also means it's likely a place some
even better flow numbers could be gained with grinding tools.
Summary:
If you want a little more exhaust manifold flow the AFC is obviously a better
choice than the AAH. For those with AAH engines however this is not a direct
swap as the AAH/AFC vary between their EGR plumbing
routes and locations. Doable? Probably yes, but not a direct
plug-n-play swap.
Undoubtedly even more gains to be had by "deep" porting and polishing
as well as gasket matching. And by all means cut any new exhaust manifold
gaskets into individual gaskets rather than the one-piece design they come from
the factory in. No matter who's gasket you buy it does
NOT bolt up to the head without interfering with the exhaust ports on the
exhaust manifold. If you gasket-match then use a one-piece gasket you've just
wasted ALL your gasket matching time.
One other snippet is that you can just look at the two designs side-by-side and
see/know the AFC is a much more highly efficient design without peering in to a
single runner or its collector outlet. It's smoother, slicker has no 90degree
angles whereas the AAH's outlets are both cast into
90degree angles whiles the AAH's are straight
parallel-to-port exits.
And I have no idea if the manifolds are intechangeable
beyond the "known" EGR caveat. After all we are talking not only
AAH/AFC but also a B4/B5 difference as well.
I'm going to do a "deep" port/polish/gasket-matching job on my AFC exh manifolds. Not because I want to but because I still
have no headers in-hand or looming eminent. I need these in the event they're
my only option on heads/IM installation day. Man's
GOTTA have a contingency.