272, 276 cams

[Robert Myers]

At 10:40 AM 7/16/2004 -0700, John Larson wrote:

>"Lift" is the height of the cam lobe relative to the base circle, how much
>valve movement (at the cam) you get, either directly or through rocker arms
>(which may have a ratio involved).  "Duration" is the number of degrees
>during which the valve is open, and "overlap" is the number of degrees
>during which the intake and exhaust valves are concurrently open.  Higher
>lift implies greater cylinder filling over the same time, but that also
>depends on the ability of the intake tract to fill the need, and the exhaust
>tract to remove the spent fuel/air mixture.  It gets complicated from here.
>The terms "advance" and "retard" have to do with the beginning of the spark
>event with regard to crankshaft rotation, measured in degrees.  HTH, John

Advance and retard relate to the timing of the plug firing in relation to 
the piston location.  Piston location is measured relative to the top dead 
center position of the lobe of the crankshaft, rod and piston.  TDC is when 
the piston is as high up in the cylinder as it will go during the rotation 
of the crank and both the intake and exhaust valves are closed.  (This 
happens once every other revolution of the crank.)  That position is 
referred to as zero degrees or TDC.  Advance is when the spark comes before 
the piston has reached this point in its travel.  Typically an engine will 
produce more power when the spark comes several degrees before TDC so that 
the compression stroke has not fully completed its travel when the plug 
fires.  Typically you will see numbers like 16 degrees BTDC (before top 
dead center) or the like.  Somewhere for a given engine there is an optimal 
point for the plug to fire.  If adjustments are made so that the plug fires 
earlier you are advancing the spark (to perhaps 20 BTDC or the like).  If 
the spark is delayed to later in the piston's travel then you are retarding 
the spark (to perhaps something like 10 BTDC).  This retarded spark may 
still be in advance of TDC but is retarded when compared to optimal for 
that engine.  Typically, to get the most available power the timing should 
be advanced about as far as possible but if it is advanced too far then you 
can get into a situation where predetonation or (pinging) can 
develop.  Pinging is undesirable because it can damage an 
engine.  So...  Advance the timing until pinging commences and then back it 
off a little until pinging stops.  Our Audis are a little more complicated 
due to electronic controls of advance and detection of pinging, etc., but 
that's the idea in a nut shell.

>----- Original Message -----
>From: "Tessie McMillan" <tessmc at drizzle.com>
>To: "CS_Rally" <cs at csrally.com>; <jeg1976 at yahoo.com>;
><j.d.larson at verizon.net>; "Hoffman Anthony J A1C 552 CMS/MXMVC"
><Anthony.Hoffman at tinker.af.mil>; <Bluemaxww1 at aol.com>; "Alan Pritchard"
><apritchard at seaeye.com>
>Cc: <quattro at audifans.com>
>Sent: Friday, July 16, 2004 9:28 AM
>Subject: Re: 272, 276 cams
>
>
> > OK, so you remind me I had another dumb question. Pardon my technical
> > inexpertise, but what is "overlap", and what is "advance"? I'm guessing
> > at least the latter might be related to timing, but I don't really know.
> >
> > If I put in a .... higher lift, right? ... cam, what am I doing to improve
> > the performance? Do I take more advantage of combustion/compression in the
> > pistons, and why? Would I need to run a specific octane gas?
> >
> > Tess
> >
> > On Fri, 16 Jul 2004, CS_Rally (Carlos) wrote:
> >
> > > I have a 272 installed in a modified NF code engine that I'm about to go
> > > through inspection and I'm also curious to see the results.
> > > As Jim's car, mine is not exactly a standard NA car, but it might be
>close
> > > to your case. BTW, I'm running 30 degrees of advance with no idle
>control.
> > > I also have collecting dust in my garage a 276 that I plan to install
>after
> > > the smog test.
> >
> >
> >
>
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>quattro mailing list
>quattro at audifans.com
>http://www.audifans.com/mailman/listinfo/quattro

_____

Bob

When Jesus said, "Love your enemies", I suspect he meant, "Do not kill them".
_____
  Robert L. Myers  304-574-2372
  Rt. 4, Box 57, Fayetteville, WV 25840 USA  WV tag Q SHIP
  '95 urS6 Cashmere Grey - der Wunderwagen  YM ID rmyers25840
  http://www.cob-net.org/church/pvcob.htm
_____