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FW: Head work - Porting, Polishing etc. & Bench Flow Tests, Measurements, Charts - ?? FLOW DATA
In a follow up to a post on the S-Car list re 20valve heads, and in further
search of relevant information or personal experiences, I though I would
check to see if anyone on this list has had experience with, or information
to share about the porting, polishing & tweaking of Audi cylinder heads for
better performance. By way of background and as a preface to the queries
- I'm (obviously, as you'll note from the question(s)) not an engineer or
- I've heard people who I believe to be knowledgeable on this offer of the
cuff comments to the effect "Well, I don't know about the I5 Audi, but you
know 4 cylinder VW heads respond well to P&P etc...." so, the analysis
goes, since the heads are sort of the same... I'd GUESS that the Audi head
- I have a car that the PO had the head "Ported, Polished and Gas Flowed"
well I at least have a copy of an invoice that charged for the service.
- The cams for the engine were also changed to "sport" cams - I've been told
the cams may have, to the best of the PO's & service peoples recollection
increased duration from the stock (??) 224 degrees to 254 or perhaps 258
degrees duration - I'm guessing that may refer to the exhaust cycle.
- I have no idea what stock "lift" is or if the sport cams would tinker with
something like that. FWIW, the cams that were put in the car are from
"PIPER." I've tried sending their offices an e-mail to ask what their
"sport" cam profiles for a 1993 Audi S4 might be, but have heard nothing
- I have a new engine coming for the car. The head on the new engine is
- One of the considerations is swapping the modified head on the old engine
over to the new engine. But I would like to try to understand what the
"modified" head might offer over the new "stock" head.
- One alternative at the time of the swap is to try to have the two heads,
the stock and the P&P modified bench flow tested to check the differences.
- It would be nice to have some kind of bench mark(s) or other flow data for
the Audi heads to check the bench test data, if I can arrange to have both
the heads off & taken to a bench flow site, for reference.
- If anyone knows of data like this in the list Archives (I've tried
looking, but haven't found any) or of any publication I can try to chase
down on this, I would appreciate a pointer in the right direction.
From: MLP [mailto:firstname.lastname@example.org]
Sent: Friday, September 24, 1999 1:43 PM
Subject: 20 valve heads - Port, Polish & Bench Flow Measure & Chart
Does anyone on the list know if there are any bench flow numbers or tests or
perhaps even somekind of a "flow" chart for the S4 / S6 heads ?
Particularly with a view towards trying to make an informed decision between
the "stock" head and what, if anything might one see from a modified head?
FWIW, in talking with a bench flow person who primarily works on "big
American iron," but has on occasion seen and done work on the 4 cylinder VW
16 valve heads, I was regaled with numbers and opinions along the following
- Ya gotta know what yur doing;
- Isn't enamored with working on foreign stuff - fingers are too big, ports
are too small, takes different tool size than the Chevy & Ford blocks he
prefers to work with;
- None the less, from time to time, as a special favor, he has worked on,
and had notes about numbers he had seen on "slightly" modified VW heads.
- He did not think that he had ever seen the VW heads he had worked on in
stock form, so he couldn't say for sure what the heads had flowed before any
porting and polishing work. His stated personal preference was to measure &
provide his customers with some before & after flow figures; but
- As a 'rule of thumb' (& please any errors and mistakes in this are going
to be mine, not the bench guys)
You can test at (apparently) any level of water (a measure of the vacuum or
pressure driving the flow) you want, but this fellow preferred using 28" of
water (as opposed to 25" which I gathered/assumed must be a standard many
flow benches or charts must use or are adjusted to) - the 28 inches is tied
to 1 Atmosphere being 13.6" of water, being 1" of mercury (??Hg)).
He found that on an already "mildly" ported 1,800 cc, 8 valve VW head I
THINK (if I got any of the numbers right) that:
Valve Exhaust Exhaust
Lift in ? As Is / Was After P&P
.01 of lift 71 cfm 88 cfm
.02 of lift 118 cfm 160 cfm
.03 of lift 134 cfm 184 cfm
.04 of lift 139 cfm 186 cfm
The lift figure should probably be TENTHS (.1) of an inch, not 1/100ths He
also had, but I've lost & don't specifically recall rules of thumb for
translating flows to approximate HP potentials.
He had some other, rough rules of thumb regarding "general" thoughts about
how to translate the cfm figures into "potential" power gains - by
potential, that did not mean the P&P job would in fact, by itself produce
that kind of change, but it would give guidance in making further
development & tweaks to the engine setup. He also had a lot to say about
the art of keeping flow speed up vs. simple "quantity" of move able volume.
Can we get some bench marks or intelligent information together to share
about what the stock Audi 5 cylinder head will do vs. a ported and polished
Does this work, or are we just "PORTING & POLISHING" our wallets? If this
has already been thrashed and killed in the archives, PLEASE disregard &
just give me a hint where I should look.