DeTomaso Mailing List: October 98, Message #103

Continuing...

> X-angle valvejobs.  You gotta be a little careful here since it is easy
> to get trapped in jargon and advertisement.  I could write all day on
> this topic alone, but will try to keep it as short as possible.  1-angle
> valvejobs are almost always performed by factories and shops that do
> 'stock rebuilds'.  This involves not touching the bowls or chambers, but
> just making a single angle cut (usually right around 45 degrees) for
> both the valve and the seat.  Since most production bowls come into the
> chamber at nearly 90 degrees, there are basically 2 45 degree sharp
> corners the air must turn just on the head side.  The same goes for the
> valve, though the back of the valve is rarely 90 degrees to the edge. 
> This kills airflow at ALL valve lifts.  The solution is to cut these two
> edges on the cylinder head with two more angles that approximately halve
> the angle between the valve seat and the surrounding material.  Hence
> the 3 angle valve job.
> 
> For better shops, the 3 angle valve job doesn't end there, since there
> are also sharp turns on the valve itself.  Most of the high end shops
> put little cuts to 'break' the edges on the valve that as a casual
> observer you would probably not even notice if you were to look at it. 
> They still refer to this as part of their 3 angle valvejob, but some
> shops like to capitalize on jargon and will call this a 5-angle job
> since they made 5 cuts (they usually only make one cut on the back of
> the valve and don't mess much on the side since it can narrow the margin
> too much if they don't have custom valves made for the application).
> 
> The problem comes when someone that actually DOES a 5 angle valve job
> and these other guys are really doing what is a 3 angle job and calling
> it a 5-angle.  A true 5-angle would be to make 5 distinct cuts on the
> cylinder head which in the scope of the size of the cuts, makes the turn
> very close to a true radius rather than 4 very small turns.  I
> personally don't port heads or do valvejobs, but get the opportunity to
> know some very good people that like to talk shop.  It's nice to see
> what really good work is and have it explained by the professional that
> did it rather than some advertising hype.  If you are wowed by the
> opportunity to have another couple angles on your heads because "more
> must be better" remember that there is such a thing as diminishing
> returns.  All but the very fastest competition engines make
> a lot of power with *good* 3 angle valvejobs and nobody that will talk
> to me knows what the secret engines have.
> 
> On a good head job (yeah, I know it's a very bad Freudian slip), the
> machined surfaces farthest away from the valve seat will typically be
> blended with a radius into the surrounding surfaces (bowl and chamber). 
> Some people use the 5-angle job in place of this and don't finish the
> valvejob with any hand blending.  When all is said and done, it seems to
> be a wash and subject to personal opinion of the porter what the head
> gets.
> 
> The valve job is one of the most important parts of good port flow and
> is often overlooked as such.  Porters that are serious and good at it do
> a lot of development in this area alone.  There are a lot of
> variables with respect to the actual angles and widths of the cuts. 
> Some people know what they are doing and understand the implications of
> the decisions and others just have a 'standard' valvejob.  Pick their
> brain a little before investing your hard earned dollars.  I am a big
> believer in good low lift flow and the shape of a flow curve more than a
> peak flow number that everyone likes to quote.  The valvejob will either
> make or break the low lift flow.
> 
> To be continued...
> - -- 
> Scott Knight  mailto:sknight@mich.com
> http://www.mich.com/~sknight IRC:SS396man ICQ:8353618
> '95 Black Impala SS
> '94 Ducati 900SS CR
> 
>