Overlooked Excellence

Chrysler's Forgotten '51-53 Hemi

By Ron Ceridono

Of all the engines that have been produced by automobile manufacturers over the years, few production designs have achieved the legendary status of the Chrysler Hemi. And even though General Motors introduced overhead valve V-8s two years before Chrysler, when the big Hemi hit the showrooms, with a conservatively advertised 180 horsepower, it caught the attention of performance enthusiasts nationwide. It wasn't long before these engines were making their presence known in virtually every competitive setting from LeMans to Bonneville, NASCAR to NHRA. Even today drag racing's Top Fuel and Funny Car engines can trace their lineage to the Chrysler hemi.

During its production run from '51 to '58, Chrysler Hemis were produced in three displacements, 331, 354 and 392 cubic inches. And while all of them have been popular, it stands to reason the later, larger displacement versions are more sought after. However, that doesn't mean the early engines should be overlooked, and they often are.

What's the Difference?
There are a number of features that distinguish the various Chrysler Hemis from one another. Over the years the bore and stroke, deck height, connecting rod lengths, and crank journal diameters changed, but the most obvious difference can be found at the back of the blocks. The '51-53 331 engines are unique; they had an unusually long extension to house the fluid coupling for Chrysler's infamous Fluid Drive--that is the stumbling block when using one of these engines, but it doesn't have to be.

Oddly enough, due to the distance from the crankshaft flange to the transmission mounting surface, adapting a variety of manual transmissions to the 331 was relatively simple. A number of companies offered adapters, which were little more than flat plates drilled with the appropriate holes to bolt Ford, Chevy, Packard, Cad, LaSalle, and other manual transmissions to these blocks. Normally these adapters included a special pilot bushing (or a pilot bushing adapter) so that by using a Chrysler flywheel, pressure plate, and the appropriately splined friction disc for the transmission involved, gearbox swaps were a snap.

While the supply of Packard and Cad LaSalle transmissions and adapters may have dried up, there are still plenty of Chevy four-speeds around (not to mention five- and six-speeds). The adapters to join them to early Chryslers are readily available too. Hot Heads has new, improved transmission and pilot bushing adapters as well as flywheels and clutch assemblies to accomplish the task. And for those who want a contemporary Torqueflite behind '51-53 Hemis, they'll be introducing an adapter for that combination soon. Stick or automatic, the extended bellhousing problem is now easily resolved.

Flywheels and Starters
Chrysler Hemis used two different diameter flywheels and three different starters, and it's important the right components are combined. From '51-55 starters were 6-volt and were used with 146 tooth flywheels; '56 starters were 12-volts, however the ring gears remained the same with 146 teeth; for '57 and '58 the starters were also 12-volts but ring gears had grown to 172 teeth.

What all this means is that to convert a '51-53 engine to 12-volts, a '56 starter is required. Of course, another option is to run the 6-volt starter on 12-volts. It does make the engine spin faster and the starter and drives seem to be able to handle it for quite some time. But at about the time you read this there will be another option. Hot Heads is developing a 12-volt, high-torque starter for '51-55 engines.

Heads
All Hemis, including those from Chrysler, DeSoto, and Dodge, have one thing in common, excellent heads with their namesake hemispherical combustion chambers. It's no coincidence that all these engines used similar heads the same man, Fred Shrimpton, designed them all.

While all the Chrysler heads offer the advantages of a hemispherical combustion chamber (thermal efficiency, centrally located spark plug, big valves, and straight ports) some were better than others. And the truth is, the '51-53 heads are the least desirable. That doesn't mean they're necessarily bad, they're just not as good as the later designs.

The early '51-53 heads are identifiable by their round exhaust ports; the later engines have larger, oval exhaust outlets. Intake ports are also bigger, as are the valves. And while a pair of '54 heads on a '51-53 block would offer some performance gains, the early heads will work fine on a mild street engine with mild porting. Ironically, these heads are fitted with exhaust valve seat inserts that make them compatible with today's unleaded gas.

Intake Manifolds
Chryslers will be found with two distinctly different intake manifolds; those from '51-54 have an integral thermostat housing/water outlet, '55 and later engines don't (the water outlets are in the ends of the heads and are connected by a separate casting that also holds the thermostat).

Oddly enough, all Chrysler heads have four water jacket openings on the intake manifold face. On the '51-54 engines the front holes connect to the passages in the manifold leading to the thermostat. On the later engines the manifold blocks off these holes. However, to use a later manifold on an early engine, all that's necessary to do is drill and tap the manifold on each side for coolant lines. Virtually all aftermarket manifolds are the '55-and-later style.

Water Pumps and Front Covers
Everything on an early Hemi is massive, including the timing chain cover. The '51-54 engines used a huge, heavy cast iron assembly that also housed the water pump. While rebuilt pumps are available, a better option is an aluminum front cover form Hot Heads. Not only does this new cover knock a considerable amount of weight off the front of the engine, it also incorporates adapters for a Chevy water pump.

Cams
Camshafts are another peculiarity of the early engines. The '51-54 shafts use a long threaded snout to retain the cam gear. The '55-and-later cams use a more conventional threaded hole. To use a Hot Heads front cover, a '55-and-later cam must be used. However, be aware that even though all Chrysler cams will interchange, they're not the same. The 331 and 354 engines have the same deck height, while the 392, due to its longer stroke, was taller. As a result, the lifter bores in the "short deck" 331/354 engines are located differently relative to the centerline of the cam than the "tall deck" 392. So, even though the cams will physically interchange between all the Hemis, mixing a low deck cam with a high deck block, or the other way around, will result in improper cam timing.

Don't Overlook the Obvious
Compared to the later Hemis, the '51-53 engines are virtually ignored, primarily because of the extended bellhousing. But if the plans for your hot rod include a manual transmission, there's no problem thanks to Hot Heads. Soon, adapting an automatic will be just as easy.

Arguably, from a performance standpoint, the '54-and-later engines have more to offer, but even a conservatively built '51-53 engine with small port heads is capable of producing respectable statistics. According to Bob Walker, of Hot Heads, one horsepower per cubic inch, and considerably higher torque figures, are easily attainable. After all, in stock form they were rated at 180 hp with 312 lb-ft torque, and that was with 7.5:1 compression.

While there are good reasons to build any Hemi, the most compelling argument for the '51-53 engines is economic. Simply put, the extended bellhousing engines usually sell for considerably less than the later engines. That often makes them a bargain as Hemis go, and what could be better than that?