03-20
SECTION 03 - ENGINE PREPARATION
Cylinder Boring Precaution
In its stock configuration the RAVE valve guillotine
has a minimum of 0.5 mm (.020 in) clearance to
the cylinder bore measured at the center line of
the cylinder. This is the minimum production
clearance.
There is only a first oversize piston available. It is
0.25 mm (.010 in) larger in diameter than the
stock piston. When the oversize is installed, the
guillotine will have a minimum clearance of 0.375
mm (.015 in) with the cylinder bore. This is the
minimum operating clearance the guillotine
should be used with. Clearance less than 0.375
mm (.015 in) will require reworking of the guillo-
tine to achieve the proper clearance and radius.
NOTE:
When boring the cylinders on the 787 en-
gine, the displacement will be 785.76 cm
3
after
boring and honing. The engine will no longer be
race legal for the 785 cc class racing. In order to
maintain legality, new cylinder sleeves and new
stock diameter pistons should be installed.
FUNCTION OF THE ROTARY
VALVE INTAKE SYSTEM
Controlling the opening and closing of the intake
port is also a critical factor in the volumetric effi-
ciency of an engine. Best V.E.’s are obtained by
asymmetrical intake timing (opening and closing
of intake port operates independently with regard
to piston position. Example: 130° BTDC opening,
65° ATDC closing). While also allowing for an un-
obstructed intake tract to provide maximum air-
flow into the engine. This is best accomplished by
using a rotary valve type inlet.
The rotary valve engine is one of the most innova-
tive concepts to be applied to 2-stroke watercraft
engines.
Simply stated, the design produces more horse-
power out of the same size engine displacement
at the same RPM. Because the aperture size and
degree of opening exceed that of a piston port en-
gine, a greater air/fuel mixture supply can enter
the engine and remain in the engine without spit-
back.
Engine Operation
Basically, the rotary valve engine performs the
same operation as the piston port 2-stroke en-
gine. The only difference being the location and
operation of intake.
1. The intake port is positioned directly in the
crankcase.
2. The opening and closing of the intake port is
controlled by a rotary valve instead of the pis-
ton.
3. The rotary valve is driven by the crankshaft in a
counterclockwise direction.
Intake and Secondary Compression
1. Fresh charge from carburetor
As the piston starts its upward stroke, the air/fuel
mixture is drawn into the crankcase from the car-
buretor via the intake port (the rotary valve uncov-
ers the intake port).
At the same time, the piston blocks the exhaust
and transfer ports, and compresses the air/fuel
mixture in the combustion chamber (secondary
compression).
F01D6TA
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Summary of Contents for Bombardier
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