351 Cleveland Heads: 2V, 4V, Open Chamber, Closed Chamber

Every 351 Cleveland conversation starts in the same place. Somebody has a set of heads on a pallet, and they want to know if they got the good ones.

Here is the short version: there are two port sizes and two chamber shapes, which gives you four combinations, and the one everybody chases — the big-port 4V — is the wrong head for most of the engines it ends up on. The one nobody chases is the one that makes a street Cleveland feel alive.

The four heads

2V, open chamber (1970–1974). Small ports, 2.04/1.65-inch valves, an open combustion chamber in the mid-70cc range. This is the head that came on the overwhelming majority of Clevelands built. It has no quench area at all. It is also, port-for-port, a decent street head — the intake runner is sized for the rpm a street engine actually lives at.

4V, closed chamber (1970–1971). The famous one. Drive-through intake ports measuring roughly 2.50 x 1.75 inches at the face, 2.19/1.71-inch valves, and a 62–63cc closed wedge chamber. Big valves, big ports, real quench, and 11:1-ish compression as delivered.

4V, open chamber (1972–1974). Same enormous ports and valves, but the chamber was opened up and compression fell to the mid-8s. This is the worst of the factory four: you keep the port that kills low-rpm velocity and you throw away the quench that was helping you get away with it.

2V, closed chamber — the Australian head. Ford Australia kept casting Cleveland heads long after North America quit, and the Australian castings (identified by an "AR" prefix ahead of the familiar North American numbers) pair 2V-size ports with closed wedge chambers in the 57–59cc range. Small port, real quench, high compression without a dished piston. That is the combination the North American parts bin never offered.

Why the big 4V port is a problem

The 4V intake port is not a small-block port. Its cross-section is closer to what you would find on a big block, and the Cleveland's canted valves let Ford aim it almost straight at the valve. On a 500-plus horsepower engine turning 7,000 rpm, that is exactly what you want and it is why a well-built 4V Cleveland will embarrass a lot of engines with more cubic inches.

Below about 4,000 rpm, though, that same port is a liability. Port velocity is what carries fuel and what fills the cylinder on a long-duration cam at low lift. A port that big, on a 351 with a stock-ish cam and a 3,000-rpm cruise, simply runs out of speed. The engine feels lazy off idle, it does not want to pull a heavy car out of a corner, and it makes its power in a band most street cars never visit.

This is not a theory. It shows up on the dyno. A 1970 351C 4V with untouched factory heads, the factory iron 4V intake, a mild hydraulic roller and sub-10:1 compression made 433.5 hp at 5,600 and 427.7 lb-ft at 4,000 on Dale Meers' dyno at Lykins Motorsports. That is a healthy number — but look at where the torque peak sits and how close it is to the horsepower peak. That engine is doing its work in a narrow window. Put 2V-size ports under the same cam and the torque peak moves down and broadens out, and the car gets faster on the street while the dyno sheet gets smaller.

Quench is worth more than port volume

If you take one thing away, take this one. The open-chamber heads — 2V and 4V both — have zero quench. No squish pad, nothing to push the last of the mixture toward the plug, nothing to keep the end gas cool. They detonate on pump gas at compression ratios where a closed-chamber head is perfectly happy, and the usual fix is to pull timing out of it, which costs you the power you were trying to build.

A closed-chamber head at 10.0:1 with a tight quench (.035–.045 inch, piston to head, with the piston deck height and gasket thickness worked out on purpose rather than by accident) will tolerate more advance and make more power than an open-chamber head at the same nominal compression. That is why the Australian 2V closed-chamber castings and the 1970–71 4V closed-chamber castings are the ones worth paying for, and why the 1972–74 4V open-chamber head is a boat anchor that happens to have big valves in it.

What to actually do with what you have

You have 1970–71 4V closed-chamber heads and you are building a 400-plus horsepower engine that will see 6,000 rpm. Use them. Do not port them. They are already too big for a 351; a valve job, good springs, and screw-in studs is the whole list.

You have 4V heads and you are building a street car. Consider selling them and buying 2V heads or aftermarket. If you are keeping them, build the rest of the engine to suit: more cam, more compression, more gear, and accept that it is a 3,000-rpm-and-up engine.

You have 2V open-chamber heads. These are fine. Mill them to get the chamber down and the compression up, do a bowl blend and a three-angle valve job, and put a dual-plane on top. This is the cheapest real power on a Cleveland.

You have Australian 2V closed-chamber heads. You have the best factory street head Ford ever put on a Cleveland. Leave the ports alone, do the valve job, and spend the money on the cam.

You are buying aftermarket. Trick Flow's PowerPort Cleveland 195 is the sensible street head — 195cc intake runners, 60–62cc chambers, aluminum, and it fits 351C, 351M/400 and Clevor. The 225 version is the same casting family with more port for a stroker. From Australia, Cylinder Head Innovations' 3V 208 is the head that has been winning Cleveland classes for two decades; note that CHI's heads require a CHI intake, which is part of the cost.

Run your combination

Every one of these heads is in the dyno simulator, along with the intakes and cams that go with them. Pick the head you actually have, not the one you wish you had, and see what the combination gives you before you spend money.

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