[200q20v] Re: Camshafts, was [urq] UrQ failed emmissions on HC
b.m.benz at prodigy.net
Tue May 22 19:28:42 EDT 2001
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
> 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
> 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
> 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?
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