351C 351M or 400, What's the Difference?

The potent 351C-4V can often be found under the hood of unsuspecting vehicles such as this 1972 Torino.

Engineers took the Cleveland motor as a starting point and modified it in several ways:

400 cid.

  • Bellhousing pattern same as 429/460 (to allow use of already existing heavy-duty transmissions.
  • Main journal diameter increased by 0.25" to 3.00" for greater bearing surface area (incidentally, same as a 351W)
  • Crank stroke increased from 3.50" to 4.00" to gain 50 cid.
  • Deck height increased from 9.206" to 10.297" accommodate larger stroke
  • New intake manifold for wider deck.
  • Heads are identical in design to 351-2V, but have larger chambers to reduce compression, may also have air injection ports.
  • Different harmonic balancer and flexplate than 351C


  • Same block as 400, but crank is back to 3.50" stroke.
  • Pistons have thicker compression height to maintain compression a 8.2:1 in the tall deck.
  • Different harmonic balancer than 400

Production of the 400 and 351M engines ceased in the early 80's, as oil prices began to sky rocket and federal EPA regulations became stricter. Ford turned to its Windsor series engines as the primary V8 options for passenger cars and trucks.

About the Heads
Note that heads are interchangeable between all Cleveland and 400, 351M engines. All 335 series engines use the same camshafts as well.

All US made 2V Clevelands have open-chamber heads with about 76cc combustion space. 351M/400 heads, while similar in design to 351C-2V heads, had larger 78-78.5cc open combustion chambers.

Closed chamber head (open chamber heads have a circular bore rather than a chamber curved tightly around the valve and spark plug.)

The "Aussie" 351C-2V heads are highly sought after due to their closed chamber design, and resulting 60-62cc chambers. On a late 70's 400 motor these heads will increase compression ratio from 8.4:1 to a thumping 10.5:1. The challenge is finding a set!

In 1970-1971 the 4V Cleveland heads were all closed chamber (aka "CJ" heads)-highly desirable due to the increase in compression ratio (63cc chamber) and more efficient (read powerful) burn characteristics. In 1972 some 4V engines had closed chamber heads, while others had open chamber 4V heads to reduce compression. By 1973 all 4V heads were open chamber. Open chamber 4V heads had 75-76cc combustion chambers.

The chart below sums up the differences in the US 335 engine series heads:

Everyone knows that 4V heads are for "high rpm" use. Designed for NASCAR, the ports on these heads were large, perhaps too large to be offered in heavy cars which never saw the high side of 5000rpm.
The port on the left is a 4V intake port, while the right pic is a 2V. A 4V port measures 2.47" x 1.47". A 2V port measures 2.05" x 1.37". Now consider that the Fel Pro 1228 intake manifold gasket for 4V heads allows porting to 2.65"x1.88"....yeah, you can just about stick your hand down in there!

One solution to maintain velocity in the 4V intake port is to either build up the floor with epoxy or install "port plates" (aka tongues) made by places such as MPG head service. Similar plates are available for the exhaust ports on the 4V heads.

For a 351M or 400 motor, the 4V heads offer a double edged sword. If they are closed chamber, they can raise compression significantly, however this comes at the expense of large ports and poor atomization and velocity.

Contrary to popular belief it is possible to use a 351C manifold on a 400 or 351M, despite the wider deck. This is made possible with Holleys 351C to 400 intake adapters (pn: 8205), shown below:

Finally, what exactly does "M" stand for? Ford says it has no official meaning. Some think it means "modified" -due to the new 351 being modified from the 400. Others think it signifies the Michigan foundry where the block was cast, however the 400 and 351M were cast at both the Michigan (code MCC) and Cleveland (CF) plants. The casting codes can be found at the rear of the block, behind the intake seal ridge (MCC shown in picture.)

Remember that the 351M and 400 were designed to bolt up to Fords existing big-block (385 series) transmissions. However in '73 Ford produced a small number of 400 engines with a small-block (Windsor) bellhousing pattern, such as the one shown right. Rumor is this was done for the Panteras in case 351C supply fell short. Casting number is D3AE-B, if you think you've got one.

Retarded timing chains (literally!)
As a Ford enthusiast you should always remember that the 70's and early 80's were not great times for factory performance. When a 400 cid engine is offered with an embarrassing 173 horsepower, there is some drastically wrong! The problem of course was emissions standards, and Fords attempt to make huge displacement V8's meet the same requirements as imports vehicles. These new standards resulting in Ford doing some strange things to the engines. One of these things was timing chains with built in cam-retard. The crank and cam sprockets were setup so that the cam would actually be as much as six degrees retarded. This was not only on the 351M/400 engines, but even 302's and other Ford V8s. If you want a immediate increase in performance from a stock 70's or 80's Ford V8, replace the timing chain with a aftermarket piece (not a stock replacement, which probably will have the same retarded gears.

Shown above and below is a 400 motor we freshened up (stock specs basically) for a 1973 Torino we're working on. Yes, that is a little two-barrel carb on a honking 400 cubes.

Is a 351M or 400 worth modifying?
Of course! At FM we are strong believers of the idea that if you own an engine, you should make an effort to optimize it before moving on. Would we yank a 302 to drop in a 400? Not likely, since it would require a lot of peripheral costs and modifications. However if you have a car with a 400 or 351M in it, you can easily get it 400 horse and an asphalt shredding 500 lb.ft. of torque with some basic mods.

These engines respond to the same enhancements as any other engine. As we mentioned, replace the stock retarded timing chain, and while you are at it put in a mild cam. Any Cleveland cam will work. Step up to a 750cfm four barrel carb and a Edelbrock Performer intake, or by using the intake adapter plates shown above, you can fit any 2V Cleveland manifold on there.

If you have a 351M that needs rebuilding, go out and get a 400 crank and new replacement pistons - talk about a cheap and effective stroker!

Low compression ratio was a major hindrance on these engines, and there are no off-the-shelf high compression pistons. However you can get into the 9:1 range by milling the heads down 0.030" and boring the cylinders to 0.030". Better yet, source some Aussie Cleveland 2V heads, their 62cc chambers will bump you over 10.5:1.

A local machine shop, Sheldust of Pacheco,CA, worked on a mild 400 buildup which produced 380 horsepower and 486 lb.ft. of torque. We've got a copy of the article for you here.

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