Anyone know where to get this SAE doc? I think I have seen it before but I can't find it now.
From http://forums.maxima.org/showthread.php?t=262261
Example of NVCS Operation, 1990 Nissan 300ZX TT
Paraphrasing from SAE documentation I have on the VG30DETT engine from the 300ZX TT, here is how the NVCS system worked.
NVCS ON
Intake Valve Open: 19 BTDC
Intake Valve Close: 49 ABDC
Exhaust Valve Open: 59 BBDC
Exhaust Valve Close: 9 ATDC
NVCS OFF
Intake Valve Open:
-1 BTDC (1 ATDC)
Intake Valve Close:
69 ABDC
Exhaust Valve Open: 59 BBDC
Exhaust Valve Close: 9 ATDC
Note, example follows VG30DETT engine, but NVCS is a little more
clearly illustrated with the VE30DE from the 92-94 Maxima SE below:
Light Load
At light loads and below 6100 rpm, NVCS is
OFF with the intake valves retarded. This helps to promote a smooth idle and engine stability.
Medium to Heavy Load, < 6100 rpm
At higher loads but lower RPM's, it is possible for air that has flowed
into the cylinder to bounce back out again. This decreases volumetric
efficiency and therefore an earlier valve closure is desired. So under
these conditions NVCS is
ON and the intake valves are advanced by 20 degrees. Doing this can increase engine torque in this range by close to 20 lb-ft.
Medium to Heavy Loads, > 6100 rpm
Now at higher RPM's, whereas before the problem was that air was
bouncing back out through the intake port, now the problem is that not
enough air has had a chance to flow in yet. In order to take advantage
of inertia effects, a later valve opening is preferred, even if it is
the same duration. So at high revs, NVCS is now switched
OFF
and the intake valves are retarded back 20 degrees to their original
position. This can help with top-end torque by as much as 10 lb-ft.
Nissan NVCS and "Cam-Phasing" vs Honda VTEC
Nissan chose to focus their NVCS system mainly at low and medium speed
torque production because the vast majority of the time, engine RPMs
will not be at extremely high speeds. The NVCS system can produce both
a smooth idle, and high amounts of low and medium speed torque.
Although it can help a little at the top-end also, the main focus of
the system is low and medium range torque production.
We all know how Honda's VTEC system works, but here is where it runs
into trouble. Since it only has one profile with fixed phasing for
low-RPM's (cannot advance or retard valve opening/closure), the low-RPM
profile must be able to produce a smooth idle and stable running at
light loads. This means retarded intake valve opening. At low and
medium revs but high load, the intake valves cannot advance on the
low-RPM profile like they could on Nissan's NVCS, so volumetric
efficency is possibly lost, and torque production cannot be optimized
in this range. Finally at the top-end, a higher lift and duration cam
profile is used and the system drastically improves top-end torque
production (peak horsepower), but it is never able to optimize torque
in the low-end or mid-range.
So the NVCS system and in general cam phasing technology helps
significantly in the low-end and mid-range with torque production, but
really not that much at the top-end. Honda's VTEC helps significantly
at the top-end (peak horsepower), but not at all in the low-end and
mid-range.
Summary
Nissan's 1986 NVCS system took advantage of the fact that the optimal
intake valve opening and closure position for a given duration is
different for every RPM and load combination. Varying the phase helps
significantly with low and medium range torque production which is most
important for most people. The engine is only going to spend a very
small amount of time at or near redline, so perhaps optimizing an
engine for operation in this range does not make as much sense? Indeed,
the vast majority of variable valve timing systems in use today
(Audi/VW, BMW VANOS, Porsche Variocam, Subaru AVCS, Toyota VVT-i,
Nissan's 2nd Gen CVTC, etc) are all "cam phasing" systems just like
Nissan's original NVCS system that favor low and medium speed torque
production over all-out top-end. Additionally, these systems are also
cheaper to adopt and less complex than cam-
changing systems as well which lower manufacturing costs and costs to the consumer.
As a final punchline, even Honda has now finally adopted a cam phasing system with their
i-VTEC engines which also uses their traditional cam
changing
technology as well. Maybe Honda has finally seen the light and realized
that some of us do actually expect some power below 5000 rpm?
Now if only they would put i-VTEC systems in their V-6 lines
(4-cylinder only so far). They still need some help in the torque
production department, especially when pulling 3300-3500+ lb
automobiles with only 3.0-3.2L engines! ;-)