Hi Vance,
Thanks for the reply. You understood my question, and answered it.
So, a 3 angle valve grind is simply a generic way to smooth and increase
the flow past the valves at low lift, and there is no hard correlation
between the 30 degree valve angle and the 3 angle valve seat grind. If I
do a 30 degree valve angle rather than a 45, does the 3 angle valve seat
grind become xxx-030-yyy or am I into a 4 or 5 angle valve seat grind?
Yes, I understand that the 3 three angle valve grind is just one of many
small mods that hopefully will add up to something significant. But
thanks for mentioning it. It is always good to be clear about those
kinds of things, and I did not mention it in my post.
Kelvin Dodd over at the MG list pointed out that the 3 angle grid will
not be as durable as a single 45 degree grind. I assume he is correct,
but this will be a sunny day street car: 2500 mile/year, 30 weeks/year,
80mi/week -- 25,000 mi every 10 years. Do you think the 3 angle will be
a problem in 25-50K miles (10-20 years)?
Thanks,
Don Malling
"Navarrette, Vance" wrote:
>
> 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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