VAP

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) ***:

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.

patience

Probably dumb, but... Will 30 valve manifolds fit?

VAP

and what it would take to make them fit. If not it's a dozen or so down on this page. But I wouldn't be in too big a hurry to start shuffling 30V parts on to a 12V car. I've flow tested both throttle bodies and the stock 30V TB flows less than the stock 12V TB and has even less inlet diameter than the 12V's seriously small inlet diameter.

Just because something has equal displacemment and 150% more valves is no indication it can or does flow more. It has the "potential" to do so but that doesn't "make" it do so. Their MAF after 1998.5 is another great example of that. Way, way more restrictive and smaller housing I.D. than the one used on 93.5 & up 12V.

Gotta be hard to make a lot of exhaust when the engine is dying of asypxiation on the intake side.

JWG

I know, rub it in, but, if did not have have headers, I would definitely ask Mr. D about borrowing his uninstalled headers to take measurements and make a template to replicate them.

VAP

I see it happening like this:

"Ummm Mr D I don't know how it happened and I feel just awful but (insert preferred carrier here) seems to have crushed your headers during shipment. Yeah, both of them. Looks like 747 tire marks across the box. No, I told the driver I would accept them with the tire marks and he shouldn't fret about the container damage as the contents were steel parts. Oh not to worry about that, I'll cover your costs and file a claim with (insert preferred carrier here again).


;-)

JWG

LOL, that's evil.

MikTip

AAH manifolds point downward. AFC point straight back?

So, us B4 90 crowd, cannot use the later ACF(A4) manifolds?

DigglerD

http://www.extrudehone.com/price_list2.pdf

VAP

My UrS4 had extrude honed heads, turbo housings (both sides), exhaust manifold, intake manifold and throttle body.

But it won't buy the 12V manifold much. Maybe 5-8 HP for a EH Level II process that'll run $800+... not a great return on investment. I even posted about extrude honing the very exh manifolds I'm writing about today at this forum less than two weeks ago on June 1.<ul><li><a href="https://forums.audiworld.com/12v/msgs/27859.phtml">previous post/extrude honing</a></li></ul>

VAP

Yes and unknown until attempted. Prolly can but it won't be a cake-walk