[20v] 7A vs 3B C/R Head chambers, now head games

JShadzi at aol.com JShadzi at aol.com
Sat May 31 01:59:03 EDT 2003

[ Picked text/plain from multipart/alternative ]
Mike, a few thoughts..the factory stated the AAN and 9.3:1, but if you've
ever done a compression test on a fresh motor you'd swear that's a little on the
low side that maybe the factory ran higher compression than they stated.

I'll also have some access to some 20v heads in the near future, I'll do some
more cc'ing on them to see if I can get a good sample, the heads are mostly
stock and untouched.

If the head has been lapped or the seats have been ground extensively, that
will lower the valves in the seat, thus giving a slightly greater amount of
volume, that may be the case you have.

Now, here is the big question..if you're going with a bigger bore than the
81mm stock spec, what head gasket do you use, Mike, do you have any
recommendations, what do you plan on doing?  I've been beating the development trail
pretty hard on this one, and aside from using a garden variety copper HG (which I'm
not very excited about) or the NG/7A head gasket (for an 82.5mm bore) which
I've managed to blow up pretty well, what are the options?


In a message dated 5/30/2003 8:57:17 PM Pacific Standard Time,
mlped at qwest.net writes:

> 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 CR.
> 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 m
> aximizing the diameter of the valve seats.  That might explain some of the additional
> static head volume.
> Mike

More information about the 200q20v mailing list