How To Rebuild Your Ford V-8 351C-351M-400-429-460.pdf - Index of

PARTS 35
Compression ratio is determined by a
piston's swept volume and clearance
volume. Swept volume is displaced by
piston as it travels from TDC to BDC. It is
also an engine's displacement divided by
its number of pistons. Volume above a
piston at TDC is clearance volume.
heads results in a 351C that has almost a
2-point lower compression ratio at 9.22:l.
Its power and mileage will be lower, however
the engine's octane requirements will
also be lower. Put the 351C-4V head on a
400 and its compression increases to 9.6:l.
This requires premium fuel, but performance
increases noticeably. Unfortunately,
a problem arises:
To get the most out of this head swap,
you should install a four-barrel manifold
and carburetor. This is no problem in itself
because aftermarket four-barrel manifolds
are available. However, if you want to install
a 351C manifold, you'll need spacers
between the manifold and the heads and
block. These are available from Weiand,
part 8205. If you can't find them at your
local speed shop, write to Weiand
Automotive, P.O. Box 26977, Los
Angeles, CA 90065.
Because of the swept-volume-to-clearance-volume
relationship, you'll find that
oversize pistons are usually dished whereas
the standard piston is flat. This is because
as a bore is enlarged, its swept
volume increases. Consequently the clearance
volume must be increased to maintain
the S.V.-to-C.V. ratio to keep the
original compression ratio.
Other Considerations- When interchanging
heads, pushrod lengths must be
matched to the engine and the rocker-arm
type used. Cylinder-block deck-height is
directly reflected in pushrod length, so in
Two bask 351 C combustion chambers-open and closed. Open, or large chamber, is used
on the 2V, CJ and '72 and later 4V cylinder heads. Closed-chamber heads are on Boss
and early 4V engines.
the case of the 351C and 400, the pushrod
that goes with the engine must be used.
The catch here is 351C Boss or HO heads
on the 351Ml400 require pushrod-guided
rocker arms. Hardened pushrods are not
available from Ford for the 351Ml400 but
can be obtained from aftermarket
manufacturers.
Cylinder-Head Evolution-Now that I've
covered the ins and outs of interchanging
cylinder heads, let's look at the evolution
of the 335 and 385 Series cylinder heads
beginning with the 351C and 429,
respectively.
351C-2V-This head remained basically
unchanged throughout its production run.
2.04-inch and 1.66-inch intake and exhaust
valves are used with 0.342-inch stems.
Stamped-steel, pivot-guided rocker arms
are used. Its 76.2~~
combustion chamber
was used with a flat-top piston to produce
9.5:l compression ratio.
351C-4V/CJ-The '70 351C-4V cylinder
head has a relatively high 11.0:l compression
ratio due to its somewhat small 62.8~~
combustion chamber. Its intake and ex-
haust valves are'l'arger than the 2V version
at 2.19 inch and 1.71 inch. Ports are larger
than the 2V head. Like the 2V, stampedsteel,
pivot-guided rocker arms are used.
The '71 CJ, the only year it was
produced, essentially used 4V porting and
valves with the larger 2V combustion
chamber. Consequently, it has less compression
than the early 4V head. The CJ
name was dropped after '71, but the
engine was continued under the 4V name
through the '74 model year, then it was
discontinued.
351C Boss and HO-The '71 Boss and '72
HO heads are essentially the same with exception
of combustion-chamber size. The
Boss combustion chamber is smaller at
66.lcc which, when combined with its
pop-up piston, produces an 11.7:l compression
ratio. The HO combustion-
chamber volume is 75.4~~ resulting in a
9.2:l compression ratio with a flat-top
piston. Both heads use stamped-steel,
pushrod-guided, adjustable rocker arms
and 4V-size ports and valves-2.19
and 1.71 inch.
inch