351C and 351Cobra Jet engines?

Can somebody tell me the difference between the 351C and the 351 Cobra Jet? I think the CJ had closed chamber heads, but what does that mean?
TIA.

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On Thu, 09 Nov 2006 04:07:23 GMT, "Craig"

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
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What Ford Big Block group?
I searched but can't find anything that begins with Ford, and has to do with Big block.
Thanx.
wrote:

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Craig wrote:

alt.hi-po.big-block-ford-mercury
Rob
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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

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