The term "351 Cleveland" refers to a unique series of 351 cubic inch passenger car production engines built using advanced engine block and cylinder head castings first produced at Ford's Cleveland, Ohio plant. The term "Cleveland" is appended to the engine displacement in order to distinguish the 351C from Ford's 351 "Windsor", an entirely different engine. The 351C is part of Ford's "335" engine family, which includes 351C engines of various configurations from 1970-1974, plus smog-era 351M (1975-1982) and 400 cubic inch engines (1971-1979). 351C engines were produced in standard "2V" and several distinct "4V" versions, where the numbers 2 and 4 indicate the number of venturis of the engine's associated carburetor. A 2V engine is one that was factory-configured with a 2- barrel carburetor, while a 4V engine is one five 351C engine configurations that came with a 4-barrel carburetor. There are significant internal differences between 2V and 4V versions - more on this in a minute. It's important to keep in mind that Ford 335-family engines are distinctly different from Ford's Windsor family engines, and very few parts are interchangeable. How to tell a 351C from a 351W? Easy: The 351C has eight bolts per valve cover, where the 351W has six. 351C engines utilize a dry intake manifold, which is why the top radiator hose routes across then downward (through the water outlet) to the engine block, where with the 351W the top hose routes across, via water outlet, to the intake manifold. The 351C engine is physically larger, has a less rounded, more angular appearance and weighs considerably more than its Windsor counterpart. Even though a lot of people consider the term "351 Cleveland" a reference to a highperformance engine, the truth of the matter is that 92% of North American 351C engines were basic, 2V, low-compression, medium horsepower engines installed in sedans, station wagons, taxis, etc. Only 8% were 4V configurations - all of which are considered high performance. Here are the six basic 351C engine configurations by engine codes. The engine code appears as the 5th digit of the vehicle serial number. For example, a VIN of 1F02R217854 is a '1' = 1971, 'R' = Boss 351. 351C Engines by VIN Tag Engine Code Code Engine Type Comment H 351C-2V Generic low compression 351C, '70-74 M 351C-4V Somewhat Rare, High Compression, '70-71 only R 351C-4V Boss Very Rare, High Compression, Solid Lifters, "Boss 351", 1971 only R 351C-4V HO Very Rare, Low Compression, Solid Lifters, "High Output" 1972 only
Q 351C-4V CJ Rare, Low Compression, "Cobra-Jet" May 1971/, '72, Q 351C-4V CJ Rare, Low Compression, "Cobra-Jet", 1973-74, decreased performance, see 4V section notes. General Notes: 1) Engine code "H" was used for both 351C-2V and 351W engines, and both engine types were produced simultaneously during the 1970s. For certain car models, this means that one should lift the hood to indentify which engine type (Windsor or Cleveland) is installed in a car. Note that engine code H was also used to identify the 351M engine, which appeared in 1975. 2) The 351M engine has been left out of the above list because while related to the 351C, it was only produced after the end of the 351C production and has significant differences owing to the use of a taller engine block. The 351M, a smog-era engine, should not be confused with the 351 Cleveland. 3) All hydraulic lifter 1972-1974 351C engines equipped from the factory with 4-barrel carburetors are "Cobra Jet" types. '73-74 CJ engines have a few, significant internal differences from the May '71/ and 1972 351CJ engines (see notes). 4) The "M" in the 5th digit of the VIN signifies the '70-71 351C-4V, and is not a reference to the 1975 & later 351M engines. Beginning in 1975, Ford's Cleveland, Ohio plant began producing a 351 engine related to the 351C, called the 351M, where M simply stands for the term "Modified". Don't confuse the "M" in 351M with the engine code for the '70-71 351C-4V engine. The VIN code for the 351M engine is "H". 5) People sometimes refer to their cars by VIN tag engine code. For example, a '72 Mustang equipped from the factory with a 351 Cobra Jet engine is commonly referred to as a "Q-Code Mustang." High Performance 351C Engines Today most of the interest in 351C engines centers around the range of 4V 351C engines Ford produced between 1970 and 1974, which encompasses engines such as the Boss 351 and the 351-CJ. As you can see from the table above we actually have five 4V 351C engine variants to consider, so we'll definitely need to break all this down. But before we can talk about them, we need to talk about the principal difference between 2V and 4V 351C engines, it's a true fork in the road. Here it is: 4V Engines Versus 2V Engines: Radically Different Cylinder Heads All 351C engines equipped from the factory with 4-barrel carburetion received radically different "4V" style cylinder heads. 4V heads are not merely improved versions of 2V heads, they're an entirely different design requiring 4V-specific intake and exhaust manifolds. Unlike GM and Chrysler powerplants of the era, in which cylinder heads destined for high-performance versions were were essentially "improved" versions
of generic designs (employing for the most part, subtle design differences), Ford produced a completely different base cylinder head design for its high performance 351C engines. 4V 351C heads have enormous intake & exhaust ports that mate to drastically different, 4V style intake and exhaust manifold flanges. When viewing a complete engine installed in a vehicle, 4V heads don't appear much different from 2V heads, though with a few pointers they are easily distinguished. But remove the intake manifold and you'll be stunned by the size of the intake ports. If you've worked up until now with early Windsorfamily engines only (small valves, small ports), prepare for an extended wave of intoxication as you come to view the 351C for what it was: an exotic, next-generation engine. 4V heads manufactured between 1970 and 1972 (see special notes for 1973/74 4V heads) feature massive 2.19" intake valves - that's a 41% increase in cross-sectional are over 351W heads of the same era. Exhaust valves are 1.71" in diameter. Because of port size differences, if you replace your 2V heads with 4V heads, you will need to supply mating exhaust manifolds. Likewise, you should use an intake manifold with runners that match the larger 4V style intake ports. With all this 2V/4V stuff out of the way, we're almost ready to break down the five 4V engine variants. Let's recap: Early on we established that a 4V engine is one that came from the factory with a 4-barrel carburetor. We also listed the five distinct production 4V 351C engines produced by Ford between 1970 and 1974: 351C-4V, Boss, HO and CJ (two versions). And we've just established that 4V cylinder heads, intake manifold, and exhaust manifolds are radically different in comparison to those installed on 351C-2V engines. We now only need to talk about one fundamental change in the design of the 4V cylinder head casting, something that took place in 1972, when the EPA clamped down on emissions. Here it is: 4V Cylinder Heads: Open and Closed Chambered Types Within the range of 4V heads produced, there are two basic categories: the closedchambered "quench head" design used on the high compression M-code and 1971 Boss 351 R-code engines, and the open-chambered design used on the low compression, EPAapproved 1972 (HO) (R-code) engine and May '71/-1974 "CJ" (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. While Ford produced the two basic types of 4V head castings under a variety of casting numbers over several years, as a practical matter the entire range of 4V head castings produced amounts to just the two basic designs (open, closed), though starting in 1973, Ford began installing smaller valves (see note below). Within each design type {open, closed} Ford produced multiple castings, with slight variations in combustion chamber volume. To avoid mismatch, select a pair with matching casting numbers. Note: Beginning in early 1973, many 4V head castings were fitted with smaller valves (2.04" / 1.65") - the same as those used in the 351C-2V. There's some debate as to
whether Ford made this change across the board in 1973 or if some 1973 engines were built with full-size (2.19"/ 1.71") valves. If you're going to the trouble to upgrade to 4V heads, a safe bet is to use one of the two basic types (open, closed) manfuactured in 1970, 1971, or 1972. Always measure the valve diameters. Look for pre-1973 casting numbers beginning with D0, D1, D2, and steer clear of D3 (e.g. D3ZE casting number) unless you can actually measure the valve diameter. If you're looking through wrecking yards, it's principally a matter of knowing how to distinguish 2V heads from 4V heads. In checking complete automobiles, check the fifth digit of the dashboard or door tag VIN on every '70-73 Mustang, '70-74 Torino, Cougar and Montego you find, though again, beware the small-valved 4V heads. Walk up to a car, check the engine code on the door or dashboard VIN number, then check the engine itself. Most 4V heads have the number "4" cast into the upper right and left corners, just outside the valve cover gasket perimeter. If you're building a 351C from scratch, you'll need to choose between open or closed chambered heads. Both designs have been widely used by hot-rodders, so the decision is really one of deciding upon the compression ratio you want to end up with. The early quench head design is said to have been more vulnerable to overheating (and thus cracking), though both designs were fairly durable. Due to decreased valve shrouding the May '71/ & 1972 open-chambered heads breathe a tiny bit better, but of course yield lower compression ratios. A 4V casting of either type (open or closed chamber) that comes from an engine equipped with hydraulic lifters is readily converted to its solid lifter Boss (closed chamber) or HO (open chamber) equivalent. Many consider the 4V design over-ported, while others cite poor exhaust-side flow (see web discussions re: "high-port" 351C heads). As such, stepping up to 4V heads is not always viewed as the most practical way to improve horsepower and many suggest use of the half-decent, factory 351C-2V head (an open chambered design) in street rods. Certain Australian 351C castings provide for high compression, yet with 2V ports and valves. I tend to look at it this way: Ford engineer specifically designed 4V heads to produce higher horsepower. And if acquire a set of 4V heads and later decide not to use them, they're easy to re-sell, though after actually touching them you may feel compelled to put one on your coffee table and just look at it for a couple of years. The Five 4V 351C Configurations, Listed by VIN Tag Designator: The 1970-1971 M-code version (300 HP) was a high compression (10.7:1) 4V configuration employing the quench head design and hydraulic lifters. The engine block used 2-bolt main caps and the intake manifold was cast iron, with non-spread bore carburetor flange. The 1971 R-code version (Boss 351, 330 HP) was a high compression (11.7:1) 4V configuration employing the quench head design, solid lifter cam, special pushrod guide plates, Brinell hardness -checked crankshaft, forged pistons, improved connecting rods, aluminum intake manifold, dual-point distributor, 4-bolt main caps.
The 1972 R-code version (351 High Output, 277 HP) was essentially a low-compression version of the Boss 351, configured with open-chamber heads, still with solid lifter cam. Rated 277 HP under a new for '72 SAE "net" horsepower rating system. This engine could be ordered from the factory as an option, though Ford never produced a Mustang specially marked "HO" or "Boss" in 1972. The up side: There are likely still a dozen or so of these out there, in fields, wrecking yards, storage lots, etc., waiting to be discovered. The May 1971 / - 1972 Q-code Version (351-Cobra Jet, 266 HP) was a low-compression 4V variant employing a cast-iron intake manifold (with telltale spread-bore carburetor flange), special camshaft, hydraulic lifters, dual-point distributor, 4-bolt main caps. A small number of late 1971 351C-4V engines were actually low-compression 351-CJ engines, and were equipped with an aluminum intake manifold. The 1973 & 1974 Q-code Version (351-Cobra-Jet, 246 HP) was essentially the same as the 1972 351CJ though with two important differences: Camshaft timing was retarded by four degrees; cylinder heads- though still with large 4V ports - were fitted with smaller vavles (2.04" Intake / 1.65" Exhaust), same as used in 351C-2V. Special Note: The Boss 302 The Boss 302 is essentially a high-performance Windsor-block engine with "Cleveland" style heads. Boss 302 heads are essentially the same as 351C-4V Boss heads, except cast to accept intake-manifold sourced cooling water and with significantly smaller combustion chambers. 1969 Boss 302 heads used a 2.23" intake valve, with 1970 heads reverting to the 2.19" valve. The 351 "Modified" - 351M 351M: This engine was produced 1975-1982 as a 2V version only, and looks almost identical to the 351C. In reality, the 351M utilized a taller block shared with the 400 engine, so the engine is both taller and wider, requiring a wider intake manifold in comparison to the 351C. To distinguish a 351M or '75 and later 400 (sometimes termed 400M), look for the telltale vertical rib cast into the top of the block, which extends from the lifter galley over to the base of the distributor. This rib accounts for the taller block deck and exists in the M versions only. Again, "M" is not the VIN tag designator for this engine; a 351M is not the same as the '70-71 "M code" 351C-4V engine. Notes on 351C Block Casting Numbers Be careful what you read on the internet. Boss 351s were produced with at least two different casting numbers, one of which is shared with 2-bolt main, non-boss 351C engines of the era - casting number is DOAE-L. For the Boss 351, Ford drilled this 2-bolt block to accept 4-bolt main caps. Most DOAE-L blocks have 2-bolt main caps. For the 351-4V CJ engine, many websites will tell you that the block casting number is D2AE-CA. Indeed, every 351C-CJ I've seen used the D2AE-CA block, but I've also seen this same block in 2V 351C Torino station wagons, with 2 bolts on all five caps. Again, it's a case of
Ford selectively drilling blocks to accept 4-bolt main caps. So if you find a D2AE-CA engine, remove the oil pan and inspect the main caps before purchasing it. Ford's Small Block C6 - A Gem Many 1972-1974 351C-4V CJ engines were mated to Ford's C6 automatic transmission. Ford used a special high-stall (3000 RPM) torque converter (D20Z-7902-A) and flywheel (D1ZZ-6375-C). I found one of these recently - in a mundane looking Torino - orange with white vinyl top - owned by a school district. It's worth the effort to check VIN tags - I paid $175 and drove the car home.