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

Tue May 22 22:07:54 EDT 2001

Be Careful Here, allowing the "Bernie" to come at urq will require much more
eloquent posts, frequent use of $12.00 words, and long winded posts, I love
"Bernie" and will hug him if I meet him, and um, well I don't flame any
more, um, yeah, overly technical posts, with lots of big words, to the point
where you say "hey! We are talking cars here!" Beware, Bill ps sorry Bernie
----- Original Message -----
From: "Bernie Benz" <b.m.benz at prodigy.net>
To: <bbell at surview.com>
Cc: <urq at audifans.com>; "audi-20v" <audi20v at rennlist.org>; "200q20V mailing
list" <200q20v at audifans.com>
Sent: Tuesday, May 22, 2001 7:28 PM
Subject: Re: Camshafts, was [urq] UrQ failed emmissions on HC


> I didn't catch you, Bruce.  I misread your post as meaning 1mm cam rise,
> not valve lift.  Anyway, good to meet an old timer at cam theory.  Further
> comment below.
>
> Bernie
>
> > From: "Bruce Bell" <bbell at surview.com>
> > Reply-To: <bbell at surview.com>
> > Date: Tue, 22 May 2001 16:55:30 -0600
> > To: "Bernie Benz" <b.m.benz at prodigy.net>, <JShadzi at aol.com>,
<QSHIPQ at aol.com>
> > Cc: <urq at audifans.com>, "audi-20v" <audi20v at rennlist.org>, "200q20V
mailing
> > list" <200q20v at audifans.com>
> > Subject: RE: Camshafts, was [urq] UrQ failed emmissions on HC
> >
> >
> > 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.
> IMO, this is all a matter of cam timing (I think cams assuming the
> flexibility of twin cams, so I should say, int. lobe to ex. lobe
> center to center distance re: TDC, to include all you single cam
> guys) and has nothing to do with cam profile.
>
> > 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.
> The optimum cam profiles, both rising and falling, (excluding
> clearance ramps) are designed for valve train constant acceleration.
> The maximum constant acceleration, meaning the maximum constant force
> applied to the valve train that the three constraints that I mentioned
> earlier will allow.  Cams are only designed having asymmetrical lobes,
> meaning different acceleration and deceleration rates, to compensate for
> non-optimum acceleration constraints.  i.e. the 3 constraints
> do not all limit acceleration to the same max. value.  For instance,
> maybe the valve spring is not stiff enough to allow deceleration at
> the same rate as momentium and follower lube limits will allow
> acceleration.  Thus, the cam must have compensating asymmetrical
> acceleration profiles on both the rise and fall sides of the lobe.
>
> Enough of this, I'm in too deep!
>
> >> 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
> >
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