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

QSHIPQ at aol.com QSHIPQ at aol.com
Thu May 24 13:22:59 EDT 2001

In a message dated 5/22/01 8:09:31 PM Central Daylight Time, 
b.m.benz at prodigy.net writes:

> 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.

An exact definition of volumetric efficiency.  In forced induction, most audi 
cams are a compromise between high end HP and low end torque.  As a rule, an 
engine becomes more volumetrically efficient with higher overlap cam profiles 
as rpm increases.  With lower rpm (torque) you want less overlap, since you 
want the maximum charge to stay in the cylinder.  The schrick 272 in the 10vt 
for instance (assuming stock timing gear location) will suck wind at 5psi 
(compared to the stock cam), and will absolutely fly at 4500 and up rpm.  
Variable valve timing really addresses either 2 or 3 phases of optimizing 
DOHC systems.  The most sophisticated add lift and duration at high rpm, and 
decrease lift and duration at lower rpm's.  Changing the effective valve 
area, optimizes volumetric efficiency at different rpms.  Without VVT, you 
can optimize volumetric efficiency at a given rpm point, but usually this 
results in a decrease in low end torque or high end HP, depending on what you 
are doing to the cams.

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.

Depends on what system is employed.  The simpler VVT changes overlap without 
really changing cam profile per sae.  Cam timing changes in DOHC motors 
effectively changes the cam profile, since you are affecting the overlap 
characteristics of them.  

> 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!

Usually hydraulic cams are more limited than mechanical ones in this respect. 
 A whole bunch of variables can address acceptable cam profiles within the 
constraints of the design.  Stiffer springs, lighter valve train assemblies, 
and better lubrication are all known and proven techniques.  The cam lobes in 
audis are mostly limited by the design of the hydraulic lifter (not being 
optimum for high end reving) causing valve float at some pretty low rpms 
(especially in the 2 valve motors).    That said, the 20vt motor uses a 
smaller (>lighter) valve assembly per lifter, so the float is at a higher 
point (7500rpm or so).

Bottom line:  There are some really big gains to be had in audi turbo motors 
wrt cam design.  Schrick has done a few for ABT that are incredible, and audi 
sport themselves have put forth cam profiles (still asyymetrical) on the 10vt 
motor that yielded dyno increases of ~30hp.  I truly believe that audi has 
chosen to use the standard parts bin in the past to hamper/help the N/A and 
the turbo motors interchangeable, especially in the 10vt motors.  In the 20vt 
motor, the exhaust cam changes alone account for some massive changes in low 
and high rpm torque/hp.

All that said Bernie, I'm still not convinced that changing cam timing to 
yield compression ratio numbers isn't taking a step back, not forward in 
forced induction application.  Why not just change out the pistons and dip 
into the audi 20vt parts bin, then your volumetric efficiency loss is at 
least no worse than the way audi did it.



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