[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.
More information about the 200q20v