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ROLLER
ROCKERS
The
primary function of the valve gear is to open and close the valves as
close as possible to the intended contour dictated by camshaft design.
To achieve this through a broad engine speed range rigidity,
lightweight components and minimised friction are the target.
In
recent years the majority of manufacturers have swung to overhead
camshaft (O.H.C.) designs to operate the valves.
Due to pushrods, rockers etc. being eliminated, more efficient cam
profiles with higher rates of valve acceleration can be utilised, as
this design minimises flex and friction together with less weight of
reciprocal components.
The valve opening and closing cycle more closely follows the intended
cam contour, resulting in quieter, more efficient and smoother
operation.
The
BMC Range of ‘A’, ‘B’ and ‘C’ Series engines can benefit
from adopting the same principles in the valve train.
Note
A
worn rocker shaft and/or rocker bushes allow more oil to be deposited at
the top of the engine, thereby placing more emphasis on the valve stem
oil seals to control this unnecessary oil flow.
If
this oil is allowed down the valve guides, it contaminates the incoming
fuel/air mix. This contamination lowers the octane level of the
charge and reduces the amount of compression that could otherwise be
used.
Needle
roller bearings in the roller rockers cut friction and require a minimal
amount of lubricant, hence the restricted oil flow to the top of the
engine is possible.
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Why
use roller rockers?
During
valve opening and closing cycles the conventional rocker arm
’pushes’ and ‘pulls’ across the valve stem.
These forces are transferred into the valve guide, wearing the stem,
guide and ultimately the valve seat with consequent power loss. Evidence
can be seen by inspecting used components.
For example check side play on a worn valve/guide - there’s always
more wear on these components in line with the rocker arm.
Why
change the rocker ratio?
By
increasing the rocker ratio, the cam follower and pushrod do not have to
move as far or as fast (for a given valve lift) compared to a lower
ratio rocker.
This is beneficial because the main reciprocating mass in the valve
train is usually the camfollower and pushrod.
The
advertised ratio on the ’B’ Series engines is 1.4:1 (or more
precisely 1.426:1).
However, after numerous tests carried out on engines ranging from
MKI’s to the latest 18V - prefix engines, none appear to come up to
this ratio. In fact, the majority have measured between 1.327:1 to
1.367:1.
Many
hours can be spent in optimising rocker assemblies to gain a ratio
closer to the specified goal. Try and try again with different
assemblies seems the only way with standard components.
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Supercharger Kits
