Heron Pistons and Larger Valves
bbell04 at attglobal.net
Tue Dec 19 11:25:51 EST 2000
Same Idea, create a little more space around the valve head where
it is in close proximity to the combustion chamber wall. also,
sometimes the valve seats are counter sunk compared to the floor
(roof?) of the head. All this removes material, makes the
combustion chamber larger and reduces static compression ratio.
Typically this would be done with a good port & polish job. 2.5
cc or about half a teaspoon should drop the CR from 10 to 9.5.
Usually everything here gets so scrutinized I'm surprised no one
caught my mistake in the original post. The older heads (solid
lifter) had smaller combustion chambers ~27 (25?) cc compared to
the ~30 cc chambers found in the JT. My CR computation was based
on the 30 cc chamber so with the early head I'd expect a stating
static CR of ~10.7:1. You would definitely want to open up the
head. RE: larger valves, the urq has 33/38 mm valves and VW
tuners used to go 35/40+ but they found longevity in the earlier
head suffered since the valves are approx. 1 mm closer together
on the earlier heads. You could use a later head and have the
water and oil galleys matched to your block. Simpler yet, get an
NF/NG block and use one of the hydraulic heads with 33/38 valves
from a JT or KX. Do not use the NG head.
From: DeWitt Harrison [mailto:de at aztek-eng.com]
Sent: Tuesday, December 19, 2000 10:33 AM
To: bbell04 at attglobal.net
Cc: quattro at audifans.com
Subject: RE: Heron Pistons and Larger Valves
On Mon, 18 Dec 2000 16:17:35 -0700, "Bruce Bell"
<bbell04 at attglobal.net> wrote:
> [ ... ]
> If you are going through this much trouble use the pistons
> an 85 or 86 GTI/GLI and bore the block to 81.5 mm. This
> yield you a 10:1 CR. which you can lower by unshrouding the
> valves while cleaning up the combustion chamber.
I'm curious about what is meant by "unshrouding" the valves in
the context of head work. I've come across this term before
where there can be a shrouding effect due to the close proximity
the valve to the cylinder wall which then acts as an impediment
to flow. But this seems different. Please fill me in. Thanks,
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