OHC vs others

["John Fielding" <johnf@domain.elided>]

Hi All,

In the current debate of whether OHC has any advantages over other valve mechanisms, I so
far haven't seen the obvious answer.  Or perhaps some one has given it and I missed it
whilst speed reading the daily AD's?

The OHC format reduces the inertial mass of the valve components and allows higher
rotational speeds to be achieved.  In push rod engine designs the weight (mass) of the
push rods and the rockers arms have several disadvantages.  The inertial mass is one,
the second is the poor valve operating angles.  The need to have large lifts in the valves
is often accommodated by building into the rocker arm a  negative mechanical advantage,
that is the rocker arm amplifies the push rod movement by having the pivot point offset
towards the push rod side of the arm.  This imposes additional compressive forces on the
push rod & lifter and hence the cam lobes which means the size of the push rod and hence
mass has to increase.  This is bad news for high rpm.  As it is difficult to make the
camshaft with very high lifts, because you would then be unable to slide the cam through
the bearing housings in the block, the lift is less than the required figure and the
rocker arms provide the amplification required to arrive at the actual valve lift.  The
typical lift ratio is about 1.6:1 or more.  In an OHC design there is normally more
freedom to allow large lobe lifts as the cam bearings can be split in two to fit the
camshaft, or short rockers can be used with limited mass to get the required valve lift.

The second bad feature of the UHC (under head cam) design is that the tip of the rocker
arm swings through an arc centred around the rocker arm pivot.  This causes the rocker
arm tip where it contacts the valve stem to slide across a wide area causing additional
friction, and hence a wear pattern develops on the rocker arm tip and the valve stem.
Another poor aspect of this design is that the valve stem is being forced in a sideways
direction by the action of the rocker arm and the extra friction of the valve stem to the
valve guide causes the guide to wear more rapidly.  Fitting stronger valve springs to move
the rpm capabilities upwards aggravates the problem.

Taking the OHC design.

The cylinder head has less hardware than a push rod engine and the push rods and rockers
are normally eliminated.  (There are some "in-between" designs which retain short push
rods and/or rockers to allow multiple valve operation off one cam lobe).  The cam lobes
in a true OHC design normally bear directly onto tappets/lifters and these in turn contact
the valve stem. Running clearance is either by selective shimming (as the Alfa 4 cyl in
line
motors) or by hydraulic tappets/lifters which automatically take up any slack, as commonly
used in American push rod engines. The down side of the OHC design is the limitation
where  you can place the valves in the head and the angles of the valve stems.

There have in the past been good and bad engineering example of the OHC design.  A typical
bad design I have personal experience of is the Ford Pinto 2L OHC engine.  This is a
timing belt design and hence I need not tell you what can happen if the belt jumps off or
breaks.  However, although the OHC head cross-flow design is reasonably sound it was
flawed from day one due to lack of attention to small details.  The lubrication of the
camshaft is by a tubular steel spray-bar with tiny holes drilled in it to spray oil onto
the cam lobes.  These with time and dirty oil become blocked and the result is a cam which
runs dry and wears rapidly.  The average owner does not change the oil religously and this
aggrevates the problem.  When I owned one of these cars I bought a couple of spare spray
bars and changed them every time I changed the oil.  The one just taken off would be
immersed in a strong caustic soda solution to clean it out ready for reuse.  I also opened
up the holes a little, which provided an excess of lubricant and made the likelyhood of
blockage far less.

When it comes time to replace the camshaft service exchange kits are available at a
reasonable cost from several sources containg a reground cam, new spray bar and new
rockers etc.  However, having bought a kit and paid the extra deposit on the "core-cam",
which is refundable when the old cam is returned, the average home mechanic has a shock.
The camshaft has to be withdrawn towards the back of the engine.  With the cylinder head
in situ you cannot remove the camshaft as it is fouling the firewall when you try to pull
it out.  What a bum design!

Incidentally in the press release of the new Toyota Corolla here, the blurb went to great
lengths to tell the reader that Toyota now uses CHAIN DRIVE for the new engine.  This is
the first Toyota OHC engine in more than 20 years to use chains!  The reason given - more
compact engine, less clutter on the engine, lower cost, longer service intervals, and
quieter operation!  They claim the chain should last the life time of the car, something
Alfa owners already know. A friend who works for the local Toyota assembly plant told me
in confidence that the incidences of broken timing belts within the extended warranty
period was a serious problem and cost huge $'s every year because the belt breaking caused
valve to piston "tap dancing" with consequent serious damage, hence the decision to go
back to reliable "old fashioned technology". And it has an automatic tensioner.   The new
Mercedes diesel engines also employ triplex chains for OHC drive.  The extra initial cost
of a chain is small compared to, say, 4 timing belts and the labour to fit them over the
life time of an average vehicle.

John
Durban
South Africa
Alfetta 1.8L turbo
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