[200q20v] Re: Camshafts, was [urq] UrQ failed emmissions on HC
b.m.benz at prodigy.net
Tue May 22 14:17:22 EDT 2001
The other recieptents of this response and I have recently had a spirited
exchange on cams and cam timing, so I am including them in my critique and
questions interlaced below.
> From: "Bruce Bell" <bbell at surview.com>
> Reply-To: <bbell at surview.com>
> Date: Tue, 22 May 2001 01:56:18 -0600
> To: "Julian Stafford" <julian.stafford at virgin.net>, "audifans @ urq"
> <urq at audifans.com>
> Subject: RE: Camshafts, was [urq] UrQ failed emmissions on HC
> We measure them the same way in the US. For example: the
> Hydraulic Schrick 272 cam is an asymmetric grind with a total of
> 272 degrees of duration on the intake side and 268 deg on the
> exhaust side. This cam cam has 110 deg. lobe centers with an
> 11.2mm lift. Valve timing is 26 deg. BTDC and 66 deg. ABDC for
> the intake and 64 deg BBDC and 24
> deg. ATDC for the exhaust. Valve overlap is 50 degrees. The
> 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
> All that said, you could grind many camshafts that fit the above
> description yet behave very different. While the above profile
> fixes the lobe center (distance in degrees between the intake and
> exhaust lobe), the lift, and at what point on the curve valves
> begin to open or close, it says absolutely nothing about how fast
> or where the valve is accelerated on opening or closing.
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)
> Since a hydraulic (think self adjusting) lifter is designed to
> run at "0" clearence and a solid lifter requires clearance to
> allow the valve to close properly, it follows that the shape of
> the lobes must be quite different to achieve the same action at
> the valve. Often, and I'm not sure about Audi, there is also a
> different base circle (size of stock from which the cam is
> ground) between a hydraulic and solid lifter cam. They should
> never be interchanged.
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. 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.
> Getting back to the original theme of this thread, valve overlap
> (time and how far both valves are open simultaneously) has a very
> big affect on volumetric efficiency at low rpms and will reduce
> vacuum at idle. On a high compression motor the above profile
> will reduce engine vacuum at idle to somewhere around 15 inches.
>> Does anyone happen to know the meaning of 268/272?
>> In the UK people tend to talk about cam profiles
>> w.r.t. valve opening before
>> or after bottom or top dead centre. I reckon u have a
>> different system in
>> the US.
>> Also would anyone happen to know why a hydraulic cam
>> is not compatible with
>> a solid head and vice-versa?
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