BRP ski-doo MACH Z 1000 SDI, Handbook

Page: 132 / 493

05-8
MMC2005-001_05A.FM
SECTION 05 - ENGINE PREPARATION
To the outside of the return spring is a red plastic
adjustment knob (item 5). Turning the adjustment
in or out changes the preload on the return spring
which, in turn, will change the RPM at which the
RAVE valve opens and closes. The exhaust port
height changes a total of 4 mm to 6 mm (depend-
ing on engine type) from the RAVE valve fully
closed to fully open.
Operation
The RAVE valve does not allow an engine to make
higher peak horsepower than an engine not so
equipped, it can make moving the peak higher
practical because of its effect on the rest of the
power curve. Item 2 in following illustration is the
power curve of an engine with the RAVE valve
held fully open through its entire RPM range. Item
6 notes the peak power produced. That peak will
not change if the exhaust port time of a similar
engine without a RAVE valve was the same (with
all other features equal).
Item 1 is the power curve of the engine with the
RAVE closed through its entire RPM range. The
shaded area (item 3) is the improvement in power
at lower engine speeds that is gained because of
the lower exhaust port. If the port remains at this
height, however, the power would peak as noted
in item 5. Raising the exhaust port at the proper
RPM (item 7) will allow the engines peak power
to continue to rise to item 6.
Item P1 in the illustration is the pressure of the
return spring against the diaphragm. The exhaust
pressure must be high enough to overcome this
pressure before the valve begins opening. Item P2
is the pressure required to completely open the
RAVE valve. Between P1 and P2, the usable pow-
er curve of the engine is moving from power curve
1 to power curve 2. This transition takes place very
rapidly at full throttle and from a practical stand-
point can be considered to be instantaneous at
item 7 which for the type 583 engine is at 6300 -
6400 RPM. Gradual application of the throttle,
however, will result in the RAVE valve opening
much later, i.e. 7300 - 7500 RPM.
If the RAVE valve opens too late, the engine will
bog or hesitate momentarily as the RPM increas-
es. Full peak performance (item 6) is still available.
From a functional point of view. it is better to have
the valve open a bit early than a bit late. This fact
is due to certain dynamic conditions that exist on
the snowmobile, i.e., the clutch and torque con-
verter.
Adjustment
The red cap on the RAVE valve cover should be
turned all the way in and bottomed in normal use.
Backing the red adjuster out will reduce the spring
preload and allow the RAVE valve to open at a low-
er RPM.
At high altitudes, exhaust gas pressures will drop
and the spring preload may have to be decreased.
It is doubtful that any adjustment will be required
up to an altitude of 2400 m (8000 ft). Above that,
however, the spring preload can be reduced by
turning the red adjustment screw out up to a max-
imum of four turns.
The only other time adjustment of the spring pre-
load should be considered is if the engine has
been modified in any way.
P1
P2
7
6
4
5
3
1
1
2
POWER
RPM
A18C02A