[200q20v] RE: Camshafts, was [urq] UrQ failed emmissions on HC

[Bruce Bell]

I'm not currently subscribed to the 20v lists so someone may have
to forward this if it bounces back to me.

> -----Original Message-----
> From: Bernie Benz [mailto:b.m.benz at prodigy.net]
> Sent: Tuesday, May 22, 2001 3:17 PM
> To: bbell at surview.com; JShadzi at aol.com; QSHIPQ at aol.com
> Cc: urq at audifans.com; audi-20v; 200q20V mailing list
> Subject: Re: Camshafts, was [urq] UrQ failed emmissions on HC

> > duration is measured at 1 mm valve lift.
>
> I have never run into cam duration being measured at 1
> mm lift before,
> in past dealings limited to mechanical clearance
> adjustments.  Is this
> just Audi/Bentley procedure, or is it a common
> practice with all hydraulic
> cams?

Well, I think you caught me some here as I should have said 1 mm
cam lift as opposed to valve lift. Measurement at the cam removes
valve clearance and the "hydraulic variable increasing rate
spring" (I like that Bernie) as uncontrolled variables.

Duration is the only value that makes sense to measure at 1 mm
lift. Lobe centers won't change and lift at 1 mm is just that
...lift at 1 mm.:-)

Standard practice? I'm not sure how standard. VW and I presume
Audi use the 1 mm lift for reporting their numbers as does Dr.
Schrick. I'm sure many (some) vendors don't use the 1 mm lift so
they can sell with big numbers.  Since there is very little air
flow at small lifts it doesn't make sense to start measuring
duration on the clearance ramp. If I were buying I'd be sure to
ask what the specs were at 1 mm.

I do remember some suppliers from my old days (Isky?) which
measured at .01" to account for valve clearance. Of course the
cam would still be on the clearance ramp and the valve still in
the seat for all practical purposes.

If I remember correctly "The VW Water Cooled Handbook" has a
pretty good discussion on cam geometry and if anyone wants
further reading.
> >
> > All that said, you could grind many camshafts that
> fit the above
> > description yet behave very different.
>
> Sure, grind possiblities ar infinate, but the optimum
> is that which
> gives max. valve acceleration and deceleration within
> the valve train
> momentum, return spring force, and cam to follower lubrication
> constraints.  Thus, the above acceleration constraint
> and the desired
> duration determine the max. possible lift.  One reason
> why NA cams of
> long duration have higher lifts than FI short duration cams.
> (NA = naturally asperated, FI = forced induction)

We are definitely in agreement here, though I suspect there is
more to a good cam profile in this "grey area" of the curve than
maximizing valve acceleration. Keep in mind we can have two
valves open at the same time. How they work together makes a
significant difference in how the exhaust is scavenged and the
pressure wave starts filling the cylinder. This is a grey area
for me, but I doubt max open and max close is best or we wouldn't
have cams with asymmetrical lobes.  I found it interesting that
during the first 8 years I had my 4kq, highway mileage was almost
always in the 27-28 mpg range. Once I installed the Schrick,
mileage jumped to 28-29 mpg. I know, small jump, but certainly
not what I expected when installing a performance cam. When I
added the urq exhaust a year or two later mileage dropped to
26-27 mpg.

> A lobe designed for a mechanical follower uses a low
> rise rate to
> absorb the design clearance without pounding the valve
> stem with
> high impact velocity, before the lobe profile pours on the
> acceleration.

This would be the clearance ramp. I think both types of cams use
them, just differently.

> With a hydraulic lifter the "clearance" is a
> hydraulic variable increasing rate spring which becomes solid
> when the rise is sufficient to cutoff the oil supply
> to the lifter.
> Maybe that is the 1mm measurement point?  Just a guess.

Schrick <www.drschrick.com> will build a cam in small production
runs....anyone interested in exploring a group buy for the WR/WX?

Bruce