Re: OHC vs others

[George Graves <gmgraves@domain.elided>]

Very good explanation. Well written.

There is nothing inherently wrong with push-rod engines, and everything 
that John says about the differences between the two design 
philosophies is correct. The main reason why the US has always 
gravitated toward pushrods over OHC designs is because in the States, 
the emphasis has always been on displacement and low-end torque in 
engine design. It has been said many times, that all else being equal, 
there is simply no substitute for cubic inches. In Europe, big engines 
are, historically, the exception rather than the rule for a number of 
economic reasons (fuel costs and tax structures, mostly). In order to 
get horsepower and torque out of small engines is at high-revs. But at 
high engine speeds, valve train mass takes its toll, so if one is going 
to build engines that make their torque at the top third of their 
rev-range, reciprocating mass must be reduced as much as possible. 
Hence the reliance on overhead cam designs. In America, most engines 
are fairly big, Low fuel costs over most of the automobile's history in 
this country has led Americans to a taste for large cars. These big 
engines produce their horsepower at relatively low RPMs essentially due 
to their huge displacement. Torque is torque and power is power and 
when it can be produced at relatively low engine speeds, the effect of 
reciprocating mass upon the engine is simply not very important. Since 
these large cars have always been sold to the middle classes, the 
importance of keeping manufacturing costs down becomes paramount. Since 
sophisticated valve trains are not needed to maximize engine output, 
they have simply been. largely dispensed with. The pushrod straight six 
and V-8 engine designs go back almost to the dawn of the American motor 
industry. It works, its reliable, and many great engine designs have 
resulted from this simple and cheap technology. Look at the difference 
between the motors of the winning Ford GT-40 MkII at LeMans in 1966 and 
the losing Ferrari P4s. Sure the Ferrari with its 4-liter DOHC V-12 is 
a lot prettier than the 427 Ford 'lump' in the GT-40, but look which 
one won. Both technologies have their place, and while we Alfisti are 
naturally impressed with the jewel-like Italian precision and sheer 
beauty of the Alfa 4s, 6s and V-8's , remember that these engines are 
less than 180 cubic inches; tiny by US standards. The OHC designs are 
necessary to get the performance out these lovely engines that we enjoy 
so much. They are certainly more interesting to most of us than the 
"crude' American pushrod V-8, but remember that both approaches can 
work equally well, all things considered.

George Graves
'86 GTV-6



On Sunday, Feb 16, 2003, at 00:39 US/Pacific, alfa-digest wrote:


> Date: Sun, 16 Feb 2003 09:19:36 +0200
> From: "John Fielding" <johnf@domain.elided>
> Subject: OHC vs others
>
> 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
George Graves
'86 GTV-6
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