As popularity and marketing value of the VTEC system grew, Honda applied the system to SOHC
(Single Over Head Cam) engines, which share a common camshaft for both
intake and exhaust valves. The trade-off was that Honda's SOHC engines
benefitted from the VTEC mechanism only on the intake valves. This is
because VTEC requires a third center rocker arm and cam lobe
(for each intake and exhaust side), and, in the SOHC engine, the spark
plugs are situated between the two exhaust rocker arms, leaving no room
for the VTEC rocker arm. Additionally, the center lobe on the camshaft
cannot be utilized by both the intake and the exhaust, limiting the VTEC
feature to one side.
However, beginning with the J37A4 3.7L SOHC V6 engine introduced on
all 2009 Acura TL SH-AWD models, SOHC VTEC was incorporated for use with
intake and exhaust valves. The intake and exhaust rocker shafts contain
primary and secondary intake and exhaust rocker arms, respectively. The
primary rocker arm contains the VTEC switching piston, while the
secondary rocker arm contains the return spring. The term "primary" does
not refer to which rocker arm forces the valve down during low-RPM
engine operation. Rather, it refers to the rocker arm which contains the
VTEC switching piston and receives oil from the rocker shaft.
The primary exhaust rocker arm contacts a low-profile camshaft lobe
during low-RPM engine operation. Once VTEC engagement occurs, the oil
pressure flowing from the exhaust rocker shaft into the primary exhaust
rocker arm forces the VTEC switching piston into the secondary exhaust
rocker arm, thereby locking both exhaust rocker arms together. The
high-profile camshaft lobe which normally contacts the secondary exhaust
rocker arm alone during low-RPM engine operation is able to move both
exhaust rocker arms together which are locked as a unit. The same occurs
for the intake rocker shaft, except that the high-profile camshaft lobe
operates the primary rocker arm.
The difficulty of incorporating VTEC for both the intake and exhaust
valves in a SOHC engine has been removed on the J37A4 by a novel design
of the intake rocker arm. Each exhaust valve on the J37A4 corresponds to
one primary and one secondary exhaust rocker arm. Therefore, there are a
total of twelve primary exhaust rocker arms and twelve secondary
exhaust rocker arms. However, each secondary intake rocker arm is shaped
similar to a "Y" which allows it to contact two intake valves at once.
One primary intake rocker arm corresponds to each secondary intake
rocker arm. As a result of this design, there are only six primary
intake rocker arms and six secondary intake rocker arms.
References : http://en.wikipedia.org/wiki/VTEC
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