[20v] 7A vs 3B C/R Head chambers, now head games
mlped at qwest.net
Fri May 30 22:51:29 EDT 2003
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The 35-40 cc still sounds too small for the combustion chamber volume in a
stock head Javad, unless the valves are sitting way up (i.e. the seats have
been cut high, & probably narrow, in the head, causing the valve faces to
sit further out in the chamber) in their seats or something. Depending on
what the piston might yield for additional static volume, if you use 35 to
37 ccs as the head volume, and a compressed gasket thickness of 1.6 to 1.7mm
I come up with:
Static Volume Range:
35cc in the head + (1.6 * 5.152778) for the gasket, assuming its bore to
be 82mm = 43.244 cc static
40cc in head + Gasket 1.7 * 5.12778) = 48.759 ccs static volume
exclusive of piston dish
43.25 to 48.76 cc's of static volume puts the CR range in the 11.3 to 1 to
10.1 to 1 area.
I haven't got a measurement for an AAN piston, but they have pretty flat
tops, and I doubt the valve reliefs add much to the static volume. I don't
know that you can find another 5+cc's in the piston dish that one would need
to get the CR down to the factory stated 9.3 to 1, but maybe its there.
Well here's hoping some one else may have some additional grist for the
mill. If I recall correctly, the factory AAN motors, or at least the early
ones, didn't come standard with the steel head gaskets did they?
Do we have any opinions on what differences in height, if any, there might
be between the use of the steel head gasket and the original material? Same
installed crushed heights? Every mm ='s 5.15 cc's in additional static head
volume and each 5 cc's is equal to roughly 1 full point in terms of final
I do know that the two local shops that did both the TAP head and the ported
head were asked, and claim they paid extra attention to doing the valve jobs
on the heads, i.e. supposedly a lot of attention was to be paid in
maximizing the diameter of the valve seats. That might explain some of the
additional static head volume.
From: JShadzi at aol.com [mailto:JShadzi at aol.com]
Sent: Friday, May 30, 2003 6:28 PM
To: mlped at qwest.net; ackramer at hotmail.com
Cc: brogers at terrix.com; 20V at audifans.com; 200q20v at audifans.com;
avangerb at zoo.uvm.edu; b.benz at charter.net
Subject: Re: [20v] 7A vs 3B C/R Head chambers, now head games
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
porting. My guess best guess is the head would have been subcontracted
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
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
fit a cam position sensor for use in an AAN UrS4 application.
I didn't ask the shop to try to figure out a deck height comparison
the heads, if that were even possible. At the time I thought that given
stock 7A's higher compression ratio (? 10.? to 1 vs. the AAN/3B's stated
to 1) it sort of made sense that the suspect 7a head would look like it
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
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
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 &
# 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
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
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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