Mopar 331 FirePower Hemi Engine Specs

Intro

➤ See the Family Page for specs common to all engines in this family.

The Chrysler 331 FirePower Hemi is where all the Hemi noise actually starts. Not the 426, not the 392 everybody wants to find behind Grandpa’s shed, and not whatever old Dodge engine somebody has already promoted because the valve covers are wide. The 331 arrived for 1951 as Chrysler’s first modern overhead-valve V8 and its first production Hemi. Before there was Hemi mythology, there was this enormous iron contraption sitting in a Chrysler and making the company’s old flathead thinking obsolete almost overnight.

Chrysler did not sneak into the overhead-valve era with some timid little economy engine, either. The 331 used hemispherical combustion chambers, opposed valves, centrally located spark plugs, dual rocker shafts, and cylinder heads that look as though they were designed after somebody decided ordinary valve covers were not intimidating enough. The result was 180 horsepower in its first year, along with the kind of smooth torque a big New Yorker or Imperial actually needed. In 1951, 180 horsepower was not something to apologize for. Plenty of manufacturers were still trying to explain why their old engines were perfectly adequate.

That is the 331’s real personality: expensive-car muscle before “muscle car” existed. It was built to move large Chryslers with authority, not to idle badly outside a hamburger stand. Smoothness, torque, durability, and prestige came first. The fact that racers and hot rodders later looked at the big heads, strong bottom end, and airflow potential and started getting irresponsible was a bonus Chrysler probably should have seen coming.

The 331 also has one feature guaranteed to separate the experienced buyer from the fellow shopping by valve-cover shape. The early 1951–1953 engines use Chrysler’s extended-bellhousing block, with a great lump of cast iron continuing out the back where later engines become much more cooperative. It is not a decorative attachment that unbolts after lunch. It is part of the block, and it changes transmission planning enough to make a supposedly cheap early Hemi start charging admission before the rebuild even begins.

Later 1954–1955 331s are friendlier in that department, but nobody should mistake “friendlier” for “small-block Chevy easy.” Heads, cooling pieces, crank details, intake hardware, front dress, exhaust, rocker gear, and transmission parts still belong to an engine designed when Chrysler expected the original car to keep all its original pieces. Seventy years of swapping, racing, industrial use, and people putting important hardware in coffee cans has not improved that situation.

Then Chrysler built the 1955 C-300 and showed what the 331 could do when the engineers quit worrying about chauffeured passengers. Dual four-barrels, a hotter mechanical camshaft, more compression, and the right supporting hardware produced the famous 300-horsepower version. That engine gave the 331 a legitimate performance résumé. It also guaranteed that every ordinary 331 with two carburetors added sometime later would eventually become “probably a C-300 engine” five minutes before it was offered for sale.

A good 331 today makes sense because somebody wants a 331, not because it is the economical route to horsepower. In a correct early Chrysler, a period custom, or a traditional hot rod, the engine has history and mechanical presence no modern crate motor can counterfeit. If cheap power, light weight, and easy parts are the assignment, there are dozens of better answers. The 331 already did its important job: it made Chrysler’s Hemi name worth remembering. It does not need fake 392 ancestry or 426 daydreams stapled to the valve covers.

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Overview

Chrysler built the 331 because by 1951 the flathead era was running on reputation and borrowed time. Cadillac and Oldsmobile had already moved into modern overhead-valve V8s, and Chrysler’s premium cars needed more than another round of improvements to yesterday’s architecture. The FirePower was Chrysler’s answer, and subtlety was apparently not on the drawing board.

The big engineering idea was upstairs. Hemispherical combustion chambers let Chrysler put the intake and exhaust valves on opposite sides of the chamber and put the spark plug near the middle. That arrangement gave the engine excellent breathing potential for its day and a good place to keep developing as compression and rpm increased. The price was complexity: two rocker shafts per head, lots of valvetrain hardware, enormous cast-iron heads, and an engine wide enough to make later small blocks look as though somebody forgot half the machinery.

The bottom end was considerably more conventional and plenty stout. The 331 used five main bearings, a cast-iron block, a 3.8125-inch bore, and a 3.625-inch stroke. Rod length was 6.625 inches, with 2.500-inch main journals and 2.250-inch rod journals. Chrysler was not trying to build a featherweight. A complete iron 331 can weigh somewhere around 725 to 775 pounds depending on equipment. Anybody planning to remove one with a tree branch should first identify which neighbor owns the tree.

That 3.8125-by-3.625-inch combination gave Chrysler 331 cubic inches and established the short-deck FirePower foundation later shared with the 354. Bore spacing was 4.5625 inches and deck height roughly 10.38 to 10.39 inches. Those dimensions are more useful than the Hemi badge because the later family evolution makes sense once the measuring tools get involved.

The first production version made about 180 horsepower with roughly 7.5:1 compression. Modern numbers make that sound sleepy only because modern engines have spoiled everybody. In 1951, a 180-horsepower V8 in a large Chrysler was serious machinery. More important, the engine delivered the smooth low-speed torque expensive heavy cars required without feeling as though the drivetrain was being punished for the owner’s upholstery choices.

Chrysler put the engine into New Yorker, Saratoga, Imperial, and related upper-line duty, exactly where an expensive new V8 belonged. The 331 was supposed to make Chrysler look technologically serious and make a large automobile move without drama. It managed both. The engine’s later racing fame should not erase the fact that the original assignment involved far more country clubs than drag strips.

The 1951–1953 extended-bellhousing block is the first major version break somebody actually working on one needs to understand. The block casting continues rearward into a large integral bellhousing structure. That changes the crank and transmission relationship enough that adapter choices, starter arrangements, flywheels or converters, and swap planning all become their own little engineering project. Looking only at the front half of an early 331 before buying it is a fine way to discover the expensive half afterward.

Those early extended-bell engines are not bad engines. In a restoration or properly planned traditional build, they can be exactly right. They are simply old enough to come from a time before Chrysler had any obligation to make life easy for somebody installing a modern transmission behind one in 2026. Historical importance and convenient interchange have never been required to travel together.

Chrysler shortened the rear-block arrangement as the 331 evolved, making the later 1954–1955 engines easier to work with in swaps and later driveline combinations. Easier is relative. Water outlets, heads, timing covers, crank flanges, accessories, intake pieces, and other early-Hemi hardware still changed enough that “later 331” should start a parts search, not end it.

The cooling layout changed during the run as well. Early engines used the original arrangement routing coolant through the intake-manifold and thermostat system, while later passenger-car heads gained front water passages. That sort of change is exactly why collecting early-Hemi parts according to what looks close on the table eventually creates a very expensive still life.

By 1955 Chrysler had developed the 331 far beyond the original 180-horsepower flagship engine. The C-300 was the final proof. Dual four-barrel carburetors, a high-lift mechanical camshaft, higher compression, and performance-oriented supporting hardware produced the famous 300-horsepower rating. Chrysler named the car after the number, which saved the advertising department from having to pretend the engine was exciting.

The C-300 package is also where modern buyers need to stop believing in spontaneous provenance. A normal 1955 331 is not a C-300 engine because somebody bolted on two carburetors. Chrysler stamped the 300 application with its own engine identification, and the intake, carburetors, heads, camshaft, compression, exhaust, dates, and vehicle history need to agree before the expensive story deserves expensive money. Dual quads are attached with bolts. Chrysler was aware of that.

Industrial and specialty versions make identification even more important. The 331 served outside normal passenger-car duty and can turn up with IND markings, governors, heavy-duty manifolds, unusual cooling hardware, different front-drive pieces, and other equipment that made perfect sense on the machine where Chrysler originally installed it. That machinery may have run for thousands of hours under load, which is impressive history and not necessarily a recommendation for skipping the machine-shop inspection.

Some industrial engines make excellent cores. Others are missing every passenger-car piece the new owner did not realize he needed. A cheap complete industrial engine can become an incomplete automotive engine the instant it enters the garage. Front dress, water hardware, manifolds, crank details, bellhousing pieces, accessories, and mounts can swallow the purchase-price savings before anybody buys a piston.

Compared with the 354, the 331 gives up bore and displacement while sharing the same 3.625-inch stroke and basic short-deck proportions. Chrysler created the 354 mainly by increasing bore from 3.8125 to 3.9375 inches. That makes the 354 a very logical development of the 331 rather than some unrelated second attempt. Ford engineers were not the only people who knew that a successful block can often be improved with a larger hole.

Compared with the 392, the difference goes much farther. The 392 uses a taller block, a 4.000-inch bore, a much longer roughly 3.906-inch stroke, longer rods, and larger crank journals. That is why a 331 should never be judged as a disappointing 392. The later engine carries more displacement because Chrysler substantially redesigned the rotating package. Expecting the 331 to match it with a carburetor and optimism is not a performance plan.

That does not make the 331 a poor hot-rod engine. The Hemi heads still give it real breathing potential, and the architecture earned racing respect long before the 392 became everybody’s favorite. A well-built 331 in a light traditional hot rod can be plenty entertaining. It simply needs parts selected for 331 cubic inches, the intended rpm range, compression, gearing, vehicle weight, and actual use instead of whatever combination looked nastiest in a photograph.

The real expense is usually not the block itself. Rocker shafts, rocker arms, stands, pushrods, water pieces, intake hardware, timing covers, front accessories, exhaust, starter and bellhousing parts, and the dozens of little pieces around an early Hemi can cost real money when missing. Wide valve covers hide more expensive machinery than most first-time buyers expect.

Machine work deserves the same respect. Piston-to-chamber relationships, rocker geometry, valvetrain condition, oiling, crankshaft condition, bore wear, head condition, and decades of previous repairs all need proper measurement. These engines have been raced, rebuilt, stored, swapped, industrialized, and occasionally repaired by men who thought a larger hammer was a precision instrument. “Rebuilt years ago” should therefore be treated as historical information rather than a specification.

The 331’s best modern role is one where its identity contributes to the project. A correct early Chrysler deserves it. A C-300 absolutely deserves the right one. A traditional hot rod or custom can use the enormous heads and unmistakable FirePower appearance as part of the whole reason the car exists. In those jobs, replacing the engine with something lighter and easier can improve the numbers while making the car considerably less interesting.

If the goal is simply horsepower per dollar, however, the old Chrysler Hemi becomes a fairly efficient way to turn enthusiasm into invoices. A 354 or 392 offers more displacement within the same broad family, and countless later engines offer cheaper power with easier parts and less weight. The 331’s defense is not economics. Its defense is that it was Chrysler’s first Hemi, a major piece of American V8 history, and still one of the most unmistakable engines anybody can put between two frame rails.

So judge the 331 by the job Chrysler gave it and the history it created afterward. It began as a smooth premium-car V8, developed into the 300-horsepower heart of the C-300, and gave hot rodders an architecture worth abusing for decades. That is enough résumé for any engine. Anyone still disappointed that it is not a 392 has simply walked into the wrong part of the story.

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General Specs

Displacement	331 cu in / 5.4L	Bore / Stroke	3.8125 in × 3.625 in
Production / Use Era	1951–1955 Chrysler FirePower early Hemi passenger-car era	Bore Spacing	4.5625 in
Engine Family	Chrysler FirePower Gen I Hemi V8	Deck Height	10.38–10.39 in short-deck Chrysler Hemi
Block Material	Cast iron	Rod Length	6.625 in
Cylinder Head Material	Cast iron	Main Journal	2.500 in
Fuel / Induction	Gasoline; carbureted, mostly 2-barrel; dual 4-barrel on C-300	Rod Journal	2.250 in
Cam Location	In-block camshaft	Main Bearings	5
Valve Layout	OHV / pushrod, 2 valves per cylinder; hemispherical chambers with dual rocker shafts	Firing Order	1-8-4-3-6-5-7-2
Common Main Caps	2-bolt	Distributor Rotation	Counterclockwise
Rear Main Seal	2-piece rope-style originally; rubber replacement available	Weight	~725–775 lb complete iron early Hemi, depending on accessories/configuration
Balance	Internal balance	Dimensions	~32H / ~31W / ~32L

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Variants

The 331 looks simple until somebody tries bolting a transmission behind one. Chrysler kept the same 3.8125-inch bore and 3.625-inch stroke throughout the displacement, but the rear block architecture changed during production, the cooling and accessory hardware evolved, and the 1955 C-300 received a genuinely different performance package. Industrial engines add another layer of hardware that can turn a bargain Hemi into a very expensive collection of almost-fitting parts.

1951–1953 331 FirePower — Extended-Bellhousing

What it was: This was Chrysler’s original production Hemi and the first version of the 331 FirePower. It powered upper-line Chrysler passenger cars beginning in 1951 and introduced the hemispherical chambers, opposed valves, centrally located spark plugs, dual rocker shafts, and stout five-main-bearing bottom end that made the engine famous.

What changed: The defining hardware is the extended rear block casting. Chrysler cast a large bellhousing structure as part of the block itself rather than using the later, more conventional rear-block arrangement. These engines also belong to the earliest cooling, crank, front-drive, and accessory configuration of the FirePower family.

What that did: Performance was aimed at smooth premium-car torque rather than hot-rodding. Early engines were around 7.5:1 compression and began at roughly 180 gross horsepower. The important difference today is not another twenty horsepower. It is whether the transmission the owner wants to use can actually be connected without turning the project into a fabrication seminar.

Where used: 1951–1953 Chrysler New Yorker, Saratoga, Imperial, and related premium passenger-car applications.

Identification caution: The extended bellhousing is part of the block. It does not unbolt. Crank details, starter arrangement, flywheel or converter, adapter requirements, and transmission choice all need to be settled before buying one because the cheap early Hemi can start collecting expensive adapter parts before the heads ever come off.

1954–1955 331 FirePower — Later Passenger-Car Version

What it was: Chrysler refined the 331 for its final two passenger-car years and moved away from the early extended-bellhousing arrangement. This is the friendlier 331 for swaps and later driveline combinations, although “friendlier” remains a relative term whenever seventy-year-old Hemi parts are involved.

What changed: The later engines use the revised rear-block configuration and evolved cooling and accessory hardware. Heads, front water passages, crank flange details, timing cover, intake pieces, and front dress can differ from the earliest 331 equipment.

What that did: The engine became easier to integrate with conventional transmission hardware while retaining the same basic 331 short-deck FirePower architecture. Chrysler continued increasing compression and output through the run, but ordinary later passenger-car engines remained smooth luxury-car powerplants rather than C-300 engines waiting for somebody to discover them.

Where used: 1954–1955 Chrysler passenger-car applications including New Yorker and Imperial duty.

Identification caution: Later rear-block architecture does not make every 1955 331 a C-300 engine. Chrysler built plenty of ordinary 331s that never received dual quads, the hotter mechanical camshaft, or the rest of the 300 package.

1955 Chrysler C-300 331 — Dual Four-Barrel Performance

What it was: This is the factory performance 331 that gave the displacement its real hot-rod résumé. Chrysler took the same basic FirePower architecture and finally stopped asking it to behave like the engine belonged exclusively in front of leather seats and power windows.

What changed: The C-300 package used dual four-barrel carburetors, a performance intake, higher compression, a high-lift mechanical camshaft, and matched heads, exhaust, ignition, and supporting calibration.

What that did: Output reached the famous 300 gross horsepower that gave the C-300 its name. The important part is how Chrysler got there. Two carburetors were only one piece. Compression, cam timing, airflow, exhaust, and calibration all had to participate. Apparently horsepower works better when the parts have met before assembly.

Where used: 1955 Chrysler C-300.

Identification caution: A regular 331 with a dual-quad intake installed later is still a regular 331 with a dual-quad intake installed later. Engine identification, intake, carburetors, camshaft, heads, compression, exhaust, dates, application, and documentation all need to agree before ordinary FirePower iron starts charging C-300 money.

Industrial and Specialty 331s

What it was: Chrysler also used the 331 outside normal passenger-car duty, where the engine could spend its life running machinery instead of carrying an Imperial down the highway.

What changed: Industrial and specialty engines can carry IND identification, governors, heavy-duty manifolds, different cooling systems, unusual front covers, accessory drives, crank or flywheel arrangements, and other application-specific hardware.

What that did: The basic Hemi architecture remained stout, which is why these engines can still make useful cores. The problem is that “complete industrial engine” and “complete passenger-car Hemi” are two entirely different sentences. Missing car-specific water pieces, front dress, manifolds, mounts, crank hardware, and transmission pieces can erase the bargain quickly.

Identification caution: Industrial service can also mean thousands of hours under load, long idle periods, heat cycles, outdoor storage, and repairs performed with whatever kept the machine running until quitting time. Inspect bore wear, crank condition, heads, cooling passages, rocker gear, and completeness before the Hemi valve covers start making financial decisions.

So the useful 331 ladder is real: early extended-bellhousing engines, later conventional-rear passenger engines, the genuinely different C-300 performance package, and industrial or specialty configurations. Same 331 cubic inches. Very different amounts of transmission aggravation, performance hardware, and proof required before the price gets ambitious.

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Key Notes

The Chrysler 331 FirePower Hemi is where Chrysler’s Hemi story starts, and that gives the engine more weight than its cubic inches can carry by themselves. It isn’t the 426 race-era elephant, isn’t a small-block, and isn’t just “a Hemi” in the modern T-shirt sense. It’s an early Chrysler FirePower V8 with real engineering importance, real restoration value, and enough mythology nearby to make sellers get brave.

The basic fingerprint is 3.8125-inch bore and 3.625-inch stroke in the Chrysler FirePower family. That matters because early Hemi engines weren’t one big happy interchangeable pile. Chrysler, DeSoto, and Dodge all had their own hemi-headed families with different blocks, heads, parts, dimensions, and headaches. Saying “early Hemi” gets you into the building. It doesn’t tell you which room has the right parts.

The 331’s chamber design is the reason people care. The hemispherical combustion chambers, opposed valves, central spark-plug location, and wide cylinder heads gave Chrysler a serious breathing and performance image in the early 1950s. That doesn’t make a stock 331 a 426 with manners. It means Chrysler built an advanced, strong, expensive-to-manufacture V8 before most buyers knew how much noise the Hemi name would eventually drag behind it.

Early and later 331s need separation before parts get ordered. The early extended-bellhousing engines aren’t the same swap-and-transmission problem as the later short-bell versions. Transmission fit, adapter choices, starter location, crank details, rear block configuration, and swap planning can turn into expensive lessons if somebody only checks displacement. A 331 can be a great engine, but the back of the block gets a vote before the transmission gets invited.

The 1955 Chrysler C-300 connection is real, but narrow. A correct C-300 331 is the factory performance prize with dual four-barrels, stronger hardware, and the famous 300-horsepower identity. That doesn’t mean every 1955 331 is a C-300 engine, and it sure doesn’t mean every dual-four setup on an early Hemi is factory gold. Carburetion, intake, heads, cam, compression, exhaust, date codes, vehicle application, and documentation all need to agree before the price starts acting famous.

FirePower identity matters. Chrysler 331 parts don’t automatically interchange with Dodge Red Ram or DeSoto Firedome / Fireflite Hemi parts just because all three have fancy valve covers and spark plugs pointed at odd angles. Intakes, heads, blocks, water pumps, timing covers, exhaust, distributors, and accessories can be family-specific. The parts may look related from across the swap meet. That’s usually where the wallet gets ambushed.

The 331 gets overvalued by the word Hemi and undervalued by people who only care about cubic inches. Both crowds are wrong in their own special way. A complete, correct 331 has real historical and restoration appeal. A frozen, missing, mismatched core with no intake, wrong accessories, missing rocker gear, and mystery machine work isn’t retirement money just because the valve covers look impressive.

Parts availability is better than it used to be, but this still isn’t small-block Chevy country. Pistons, camshafts, valvetrain pieces, gaskets, adapters, intakes, water pumps, exhaust pieces, front-drive parts, and dress hardware may require specialty suppliers or careful planning. Some hot-rod parts exist. Some restoration parts are costly. Some original pieces have already disappeared into somebody’s “future project” stash, which is old-car language for “bring money and patience.”

A used 331 needs a full identity and condition check before the Hemi halo blinds anybody. Check the engine number, block casting, year, extended-bell or short-bell configuration, bore size, crank, heads, intake, rocker assemblies, water crossover, timing cover, accessories, and previous machine work. Missing rocker gear, damaged heads, wrong intake, or a bad block can turn a romantic Hemi purchase into a financial sinkhole with spark plug tubes.

Machine work deserves respect because early Hemis aren’t cheap practice engines. The wide heads, valvetrain layout, rocker assemblies, piston dome and chamber relationship, oiling details, and parts cost all punish sloppy planning. A shop that treats a 331 like a generic old V8 shouldn’t be allowed near the torque wrench.

Build judgment depends on the car and the goal. A 331 makes sense in a correct Chrysler restoration, early hot rod, period custom, vintage drag-style build, or project where the look and history are part of the point. It makes less sense when somebody wants cheap horsepower per dollar. A later 354 or 392 may be more desirable in the early-Hemi world, and a later, better-supported engine will usually be cheaper to make fast. That doesn’t make the 331 bad. It makes it honest about its lane.

The 331 Hemi deserves respect because it launched Chrysler’s famous early Hemi line and helped prove the design had real teeth. It doesn’t need 426 mythology, fake C-300 claims, or swap-meet fairy dust to be interesting. Verify the FirePower identity, year, rear block style, parts completeness, application, and rebuild condition before spending money. The 331 can be the right engine in the right build, but the word Hemi isn’t a warranty.

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Bottom Line

The Chrysler 331 FirePower is where the Chrysler Hemi legend actually begins, and that gives it a place no later engine can steal. It was Chrysler’s first modern overhead-valve V8 and the first production Chrysler Hemi, with real engineering behind the wide valve covers long before the name became a marketing department’s favorite hammer. That historical weight matters. It just does not turn every greasy 331 core into a C-300 engine or a bargain race motor.

Its best role is honest early-Hemi duty. In a correct Chrysler or Imperial, a documented C-300 application, a traditional custom, or a period hot rod where the look and history matter as much as the horsepower, a good 331 can be exactly the right engine. It has enough visual presence and mechanical character to carry a build without borrowing 392 mythology or pretending it is an antique 426.

The trouble is that “331 Hemi” still does not describe one simple pile of interchangeable parts. Early engines can bring extended-bellhousing complications, later versions are friendlier, industrial engines can carry odd hardware, and Chrysler, Dodge, and DeSoto Hemis belong to separate parts families. Missing rocker gear, water parts, brackets, pulleys, intake pieces, exhaust hardware, or bellhousing components can turn a cheap Hemi into a long-term relationship with people who know exactly what obsolete parts are worth.

That makes buying one a completeness-and-identity exercise before it becomes a horsepower discussion. A real C-300 claim needs real supporting hardware and documentation. A complete passenger-car 331 is worth far more consideration than a stripped mystery core whose best remaining component is the seller’s vocabulary. If cheap horsepower is the goal, later engines usually make more sense. If early-Hemi history, appearance, and period character are part of the assignment, the 331 earns the extra trouble.

So save the correct ones, build the complete ones, and inspect the questionable ones with both eyes open. The 331 does not need fake C-300 ancestry, 392 envy, or 426 daydreams to be interesting. It already did the important job: it gave the Hemi name something worth remembering. Just remember that history does not find missing rocker shafts, and nostalgia has never paid a machine-shop bill.

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