Don:
A 3-angle valve job is when the valve seat is ground
at three different angles; The tuners all have their favorite
recipe, but it is usually something like 25-45-65 degree cuts,
with the valve contacting only the 45 degree cut.
At low lifts, this reduces the 'sharpness' of
the corner that the air must go around to get past the valve. In
effect, you 'streamline' the valve seat by making three narrow
cuts rather than a single wider 45 degree cut.
Since your valve spends more time at low lifts (less than
0.100") than at moderate or high lifts, this technique is more
effective than you might guess.
Remember though that each change of this sort has a small
impact. It is the sum of several small details such as this
that really start to make a difference. As I recall a 3 angle
valve job was worth about 2-3% more flow at low lifts (on a
small block Chevy 350 - dunno on our LBC 6-pots).
However, when you add a 3 angle valve job to a 30 degree
back cut on the valve, necked down stems, polished valve faces,
and a mild pocket port, you can achieve improvements on the order
of 10% more HP, with no impact on emissions or drivability.
If you are into the engine anyway, it is very little
additional money to get a 3 angle valve job versus a standard
valve job.
Some racers really get fanatical about it; Five angle
valve jobs are not unheard of, and the "ideal" valve seat is
is completely radiused so that in effect the valve seals
against a metal 'o-ring like' surface. This type of seat
is very difficult to machine and get a good seal. Try to
find a machine shop that can do it for you =:-)
Cheers,
Vance
------------------------------
1974 Mimosa Yellow Triumph TR6
Cogito Ergo Zoom
(I think, therefore I go fast)
-----Original Message-----
From: Don Malling [mailto:dmallin@attglobal.net]
Sent: Tuesday, December 03, 2002 10:09 PM
To: 6-Pack; mgs@autox.team.net
Subject: 30 degree valve seats and 3 angle valve grid
David Vizard and others on these lists recommend 30 degree valve seats
rather than 45 degree valve seats because they improve flow at low valve
lift. Rimflow and/or back cutting the valves also help during low lift.
On page 207 of David Vizards "Tuning the A series Engine", he points out
that a single angle 30 degree valve seat reduces the port diameter as
compared to that of a single angle 45 degree seat. So while the single
angle 30 degree valve seat increases flow at low lift, the resulting
narrower port reduces flow at high lift. He then points out that a 15
degree sharp angle has the same flow characteristics as a smooth radius
bend. He then adds multiple 15 degree angles to the 30 degree valve seat
such that the port diameter approximates that of a single angle 45
degree valve seat.
Is this what a "3 angle valve grind" is, or is it something else???
Thanks,
Don Malling
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