351C is a member of the Ford 335 series engine family.  It
was built in a number of configs.  The CobraJet designation
was a marketing designation applied to any one of several of
the 351C 4V engines from year to year.  The CJ designation
was never used on any of the other engines of that family
and, did not necessarily designate the highest output engine
in the family.  They had closed chamber heads until the 1972
year when the CJ got the low compression open chamber heads
along with a big drop in output.  IIRC, in the '72 and '73
model year they produced a CJ HO for the Mustang which was
rated around 266 HP while the regular CJ was rated at 248
HP.  The closed chamber engines were much better performers
but, had detonation problems as time went by and fuel specs
were altered making it relatively unhappy with pump gas.
All of the factory 4V CJ's were strongest above 4000 RPM and
relatively weak below that because of the big ports and
valves reducing the charge velocity at low RPM and the
accompanying fuel drop out problems.  The 2V heads are much
better for a street driven engines because of smaller intake
runners and valves which improves the intake velocity ram
effect and better fuel distribution characteristics with the
smaller intake sytem.   The low compression 4V CJ's had the
initial timing spec'd at 16 deg BTDC to cover up an off idle
stumble and power hole while the 2V engines were happy at
6-10 deg on regular gas of the times.  This resulted in a
much less aggressive timing curve than you might like to
avoid detonation in the open chambered 4V engines.

These engines are also discussed regularly in the Ford big
block group.

HTH a bit
Lugnut

The Q-code "351 Cobra Jet" was available in the 1971 through 1973 models.
The main differences were that the 351CJ was a low-compression version of
the 4V, had more duration and overlap in the camshaft, featured open
combustion chambers, cast-iron intake manifold, special camshaft hydraulic
lifters, dual-point distributor and 4-bolt main bearing caps. The 351CJ did
retain the large 4V valves/ports.
Within the range of 4V heads produced, there are two basic categories: the
closed-chambered "quench head" design associated with high compression 351C
M-code and 1971 Boss 351 R-code engines, and the open-chambered design
associated with the low compression, 1972  HO R-code engine and the 351CJ
Q-code engines.  The closed chambered design is one in which the combustion
chamber is of considerably lower volume than its open-chambered counterpart,
and in which the valves are tightly shrouded by the casting at the mating
surface of the head.  Open chambered heads feature a larger combustion
chamber that does not tightly shroud the valves and instead extends to a
full circle equal in diameter to that of the cylinder bore.

Both designs have been widely used, so the decision is really one of
deciding upon the compression ratio you want to end up with.  There is a
concern that the 4V cylinder head design is actually overported and exhibits
poor low-rpm flow characteristics.  Many suggest use of the factory 351C-2V
head (an open chambered design) or the Australian heads for street use.

My 351C 4V M-code does not really come alive until about 3,600-3,700 RPMs
but then it takes off and is a monster. You definitely want to use a dual
plane intake manifold for the street - a single plane is a beast around town
(been there, done that).

Yet another $.02 worth from a proud owner of a 1970 Mustang Mach 1 featuring
a 351C 4V M-code @ http://community.webshots.com/album/18644819fHAehGJAjt