[20v] 7A vs 3B C/R Head chambers, now head games
JShadzi at aol.com
JShadzi at aol.com
Fri May 30 21:28:25 EDT 2003
[ Picked text/plain from multipart/alternative ]
Mike, the few heads I've ever tested ran just under 40cc, in the 35-40cc
range, this being only the comb. chamber itself with valves in place. Usually its
difficult to test the cc of a head with HG in place as its so easy to
calculate mathematically. If your heads have been ported as much as you say, and
have had comb. chamber polishing, etc, then your 45+cc figures make sense. The
only stumper is that TAP head, the fact that it doesn't look like the comb.
chambers have been touched, but the fact that it measures up to known modified
comb. chambers makes me believe that it has. Also yes, typcial compressed HG
thickness is usually about 1.6-.7 mm or so.
Recently I had occasion to have three Audi 20v head's cc'd. Two of the
heads, the ones which I own, are of "known" AAN origin. Both these have
been "ported" by different parties. One I inherited on an engine was done
under the auspices of TAP for the cars prior owner. I mention this as I
regard ANYTHING done by, or under TAP's auspices as potentially suspect.
When I say "ported" by TAP, my guess is that no one at TAP actually did the
porting. My guess best guess is the head would have been subcontracted out
by TAP to someone, probably in Florida, but I really don't know.
FWIW, the other head was worked on by a local shop here in Denver.
Last, the third head, the origins of which are less certain, had a fresh
valve job, but otherwise no porting. Visually this head looked like it had
been shaved, but again, I don't know for certain. One possibility is the
head was in fact originally a 7A or a 3B since it had been machined to retro
fit a cam position sensor for use in an AAN UrS4 application. Unfortunately
I didn't ask the shop to try to figure out a deck height comparison between
the heads, if that were even possible. At the time I thought that given the
stock 7A's higher compression ratio (? 10.? to 1 vs. the AAN/3B's stated 9.3
to 1) it sort of made sense that the suspect 7a head would look like it had
been shaved but....
In every case, the static chamber volume measured for every head was,
according to the shop, at least 45cc's. I'm now a bit puzzled by how you
come up with 37ccs (http://www.geocities.com/audikramer/) for the 4 valve
per cylinder Audi heads and wondering if the shop that cc'd my heads is all
Now, FWIW the TAP head was "critically" examined by the shop that did the
porting on the other hand, and the comment was "Well, looks like there was a
lot of grinding etc. in the intake & exhaust ports, but not much was done
in, or too the combustion chamber (i.e. everything south of the valves.)
Therefore one might be justified in speculating that the TAP head in fact
represents a fairly "standard" stock combustion chamber for purposes of
estimating static compression volumes. 2nd, FWIW here are the numbers we
can up with for static head chamber only compression volume (in CC's & cubic
# Finley ?? 7A TAP's AAN P&P RR Colo P&P Head
1 45.0 9.89 46.2 9.64 49.2 9.05
2 44.2 10.07 46.0 9.68 49.2 9.05
3 44.0 10.12 46.0 9.68 49.6 8.98
4 44.6 9.98 46.7 9.53 49.4 9.01
5 45.6 9.76 46.4 9.59 49.6 8.98
Variation" 1.60 0.36 0.70 0.15 0.40 0.07
Average 44.7 9.97 46.3 9.62 49.4 9.01
Stock AAN Engine 2226.0
445.2/45 = 9.893 each 1mm = @ 4.452 cc's
445.2/46 = 9.678
In the other FWIW, guesses, if you use the 7A's 2.3 liter numbers (82.5mm
bore x 86.4mm stroke right?) => 2.309.31cc / 5 = 461.46 cc per cylinder,
461.46/45 = @ 10.26 CR subject to gasket thickness, deck height & piston
but pretty close to what you would expect from the 7A yes/no/maybe?
One other poster on the issue, Keith Lloyd, tossed out two figures for a
10valve WR rebuild, i.e:
Gasket thickness: 1.7 ?mm
Piston/deck 0.508 ?mm
I've assumed Keith's numbers are in mm's. If his figures for the usual 20v
head gasket are right, total 1.7+0.508 = 2.208 x @4.4452 cc per mm = + at 9.8
cc's to the engines static volume, then using my, apparently now erroneously
measured 45 to 46 measured cc's less that additional static volume for the
head gasket, suggests your figures 37cc or less would be a better number
apply to the AAN etc heads as a stock static combustion chamber volume.
But, .... is 1.7mm for a compressed AAN head gasket a reasonable number?
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