Charles D. Sorkin had this to say:
>But how does it work? What is the engineering principle?
[Disclaimer: not an automotive engineer; only OHC engine I ever had apart
was a Honda 2 cyl, although I plan to change the belt on the 3 cyl Metro
next week]
Well, I no longer have my MGA book to refer to for specifics, but in
general, a dual overhead camshaft design (DOHC or Twin Cam, as used in
the MGA Twin Cam or the Jaguar 6) is considered a *good thing* compared
to a pushrod (OHV) engine design (as used in basic MGAs, MGBs, Midgets,
A-Hs, etc). This design uses two camshafts mounted on top of the cylinder
head, driven (usually) by a belt or chain, with the cam lobes operating
directly on the valve stems (through a short lifter or shim pack); as
opposed to the single camshaft in the side of the block operating the
valves through lifters, long pushrods and rockers. The advantage is less
mass in the valve train, allowing more aggressive valve timing without
the penalty of heavier valve springs (more friction), and enabling higher
engine speeds (more RPM). By having a separate camshaft for the intake
and exhaust valves, further optimization of cam profiles is possible. The
disadvantages are the additional complexity and expense of the additional
components, and the maintenance hassle of having all this stuff atop the
cylinder head, as well as the packaging problem (taller engine).
But the problems historically associated with the MGA Twin Cam engine,
which eventually caused it to fail as a commercial product (deservedly or
not), are not (as I understand it) in themselves attributable to the
overhead cam engine layout, but more to a general lack of development of
the model before it was released to the public.
After all, before WWII, MG engines were as likely to be overhead cam as
not, at least the smaller, sporting models.
--
Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.
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