By C. J. Baker • It seems that in the world of high-performance engines, the 351 Cleveland has been one of the bestkept secrets in recent years. The 351 Cleveland is one of those series of outstanding performance engines that has apparently been overlooked by most hot rodders. Of course, not all versions of the 351C are suitable for competition usage — Ford has to build some for little old ladies too. But Ford does produce two versions that are real stormers: the 351C-CJ and the super-strong 351CHO (sometimes called the Boss 351). Before we go into a detailed examination of the CJ and HO engines, well review briefly the design basics of the 351C series. The 351C block is unique from other Ford small-blocks in that it includes an integral cast housing for the water pump, thermostat and fuel pump. This housing also incorporates water passages which facilitate the circulation 108 HOT ROD AUGUST 1972 of water directly to the cylinder heads without going through the intake manifold, providing a faster warm-up without hot spots. Eliminating water passages from the intake manifold also permits the design of more efficient manifold runners. All of the 1972 351C blocks which were to receive a four-barrel carburetor were equipped with four-bolt main bearing caps, as were the 71 CJ and HO engines. All other 351s have two-bolt caps. However, all 351C blocks are cast with extra-wide cap-to-block mating surfaces to accept four-bolt main bearing caps. Therefore, blocks with two-bolt caps can be updated to the four-bolt caps. Since the four-bolt caps are not sold separately, they must be obtained from a discarded block or machined from 1010 steel. The last major design feature of the Cleveland engines is the use of canted- LEFT — 351 Cleveland engines in the four basic configurations: A. 2- barrel; B. 4-barrel; C. Cobra Jet; D. HO or Boss 351. OPPOSITE PAGE, ABOVE LEFT — The 351 Cleveland block has an integral front housing for the water pump, thermostat, fuel pump and direct water passage to the cylinder heads. ABOVE RIGHT — Valves set at compound angles give the 351 heads their designation canted valve. BELOW LEFT —• Cylinder head on the left used the early quench combustion chamber. Later head on the right uses larger open chamber. BELOW RIGHT — From left to right, early flat-top piston had 10.7:1 compression ratio, while domed version featured 11.1:1 for high-performance engines. 72 flat-top piston has only 9.2:1 squeeze. valve cylinder heads. Canting the valves toward the ports allows another port deign and provides optimum flow characteristics. This design produces a more efficient flow pattern by reducing the sharpness of the port turns that are normally required with vertical valve replacement. The ports follow a nearly straight line to the valve centerlines; thus, a more uniform cross-sectional area can be used than is possible with rectangular ports. (See illustration.) Now that weve acquainted you with the basics of the 351Cs design, well enlighten you about the component variations between the 351C Cobra Jet engine, called the CJ, and the Boss 351CHO, called the HO (high output). We will exclude the standard four-barrel 351Cs built up to 1972, and all the twobarrel versions, as they are not generally considered high-performance engines. In 1972 the four-barrel 351C and • the 351C-CJ are identical, so well just refer to them both as the CJ. Perhaps the most significant difference between the CJ and the HO engines is that the CJ uses a hydrauliclifter camshaft and associated valve train while the HO uses mechanical lifters. The type of valve train used dictates several other component changes between the two engine versions. The CJ, with its hydraulic lifters, utilizes a positive-stop rocker arm arrangement with cylindrical-type fulcrums seated on individually slotted pads. A fulcrum guide is milled in the cylinder head pedestal for each of the independently mounted stamped rocker arms to position and lock the mating fulcrum guide. The cylindrical T-shaped fulcrum and rocker arms are bolted to the pedestal to provide a positive-stop feature which requires no adjustment. A more positive control of the rocker-arm-to-valve-tip relationship is attained by locking the fulcrum in the milled slot on the pedestal with a %;-inch bolt, thus improving durability of the valve tip and assuring high-speed valve train stability. The HO, with its mechanical lifters, requires a provision for valve lash adjustment. To facilitate this, the rocker arm stud pedestals on the HO heads are machined flat instead of with the milled slot that the CJ and all other 351C heads received. In place of the T-shaped fulcrum, the HO gets a fully adjustable cylindrical fulcrum mounted on a % flinch stud. Rocker-arm-to-valve-tip relationship is maintained by using pushrod guide plates in this system. For additional durability, the HO gets specially hardened and ground pushrods, stamped steel valve spring seats, higher pressure valve springs, single-groove tight-gripping valve keepers, and short-skirt valve seals for improved lubrication. The valve sizes on the CJ and the HO are identical, with 2.19-inch intakes and 1.71-inch exhausts. There is, however, some variance in the combustion chamber design between the two engines in different years. The 71 CJ, 72 CJ and the 72 HO heads are identical, featuring open combustion chamber configurations of 7.39 to 76.9cc. However, the 71 HO employed a quench combustion chamber configuration of 64.6 to 67.6cc. This change on the 72 HO, along with changes in piston dome configuration, was made to make the engine compliant with 72 emission requirements. Four different types of pistons have been used in 71 and 72 CJ and HO engines. In 1971 the CJ used a castaluminum, flat-top piston with a compression ration of 10.7:1. In 1972 the same style of piston was used, but the compression ratio was dropped to 9.0:1. (Continued) HOT ROD
Posted on: Tue, 26 Nov 2013 05:50:54 +0000
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