Mopar 340 Engine Specs

Intro

The Mopar LA 340 is the small-block that proved Chrysler didn’t need big-block cubes to make people nervous. Introduced for 1968, the 340 took the LA small-block layout and gave it the one thing the 273 and 318 didn’t have in the same way: real factory performance intent. It was Chrysler’s serious small-block answer, built with the bore, breathing, compression, cam, and attitude to run hard from the start instead of pretending a mild workhorse had suddenly joined the fight.

Chrysler needed the 340 because the muscle era had moved into lighter cars, and not every answer could be a 383, 440, or Hemi. Darts, Barracudas, Dusters, Demons, Challengers, and other lighter Mopars needed an engine that could rev, breathe, and make power without dropping a big-block anchor over the front tires. The 340 gave Chrysler that answer: a compact, high-output wedge small-block with a big bore, short stroke, strong parts, and enough attitude to make bigger engines stop smirking.

The architecture is the point. The 340 used the same 3.31-inch stroke as the 318 but opened the bore to 4.04 inches, giving it better breathing room and helping support larger valves and stronger airflow. Early versions came with high compression, a four-barrel carburetor, performance cam timing, good heads, and hardware that made the factory rating look conservative to anyone who’d actually driven one. The 340 didn’t need a giant displacement number. It needed airflow, rpm, and a car light enough to let it work.

That’s where people get careless. A 1968–1971 high-compression 340, a 1970 Six Pack or Six Barrel T/A/AAR engine, a 1972 lower-compression 340, and a 1973 emissions-era 340 don’t all tell the same story. Same 340 badge doesn’t prove compression, heads, crank, carburetion, camshaft, year, or application. The reputation is real. The hardware still has to show up.

The plain-English identity is this: the Mopar LA 340 is Chrysler’s serious small-block performance engine, built to give lightweight Mopars a high-revving, hard-breathing alternative to big-block brute force. It’s not a 318 with good public relations, and it’s not automatically a T/A Six Pack prize. It’s the LA engine that made small-block Mopar performance mean something.

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Overview

The Mopar LA 340 sits in the LA family as the performance small-block, not the default workhorse. The 273 started the LA line. The 318 made it common. The 340 made it dangerous. Chrysler didn’t build the 340 to be the engine that powered every wagon, truck, and fleet sedan until the ashtrays filled up. It built the 340 for buyers who wanted small-block weight with enough horsepower to embarrass cars wearing bigger badges.

The 340’s architecture explains why it worked. It used the LA small-block layout with the same 3.31-inch stroke found in the 318, but the 4.04-inch bore changed the breathing story. More bore meant more room for larger valves and better cylinder-head airflow, and Chrysler backed that up with performance-minded parts: high compression in the early years, a strong four-barrel setup, aggressive cam timing, dual exhaust, and hardware aimed at real output instead of showroom decoration. The 340 wasn’t magic. It was a well-matched package, and that’s usually better.

In the right car, that package made the 340 feel sharper than its displacement suggested. A 340 Dart, Barracuda, Duster, Demon, or Challenger could run hard because the engine didn’t have to drag around big-block weight to prove a point. The short stroke helped it rev, the heads let it breathe, and the cars around it were often light enough to make the whole combination feel nasty instead of merely noisy. That’s the kind of small-block performance Chrysler needed while everyone else was busy shouting cubic inches across the parking lot.

The 1970 T/A and AAR versions gave the 340 its loudest factory headline. The Six Pack and Six Barrel setup, special block and head changes, and Trans-Am image made those engines famous for good reason. But that fame needs a fence. A regular 340 four-barrel isn’t automatically a T/A engine. A T/A or AAR claim needs the right block, heads, induction, application, date codes, and documentation. Three carburetors and a story don’t make the parts true.

The 340 also changed across its short production run. The early high-compression engines carry most of the reputation. Later versions dealt with lower compression, emissions pressure, and changing ratings, especially as the industry moved into the early 1970s hangover. That doesn’t make every later 340 junk. It means the factory package changed. A 1973 340 isn’t the same conversation as a 1969 340, even if both badges look good in an argument.

Compared with the LA 318, the 340 has more bore, better breathing, and far more factory performance intent. Compared with the 360, the 340 gives up stroke and low-speed torque but usually carries the sharper muscle-era reputation. Compared with a big block, the 340 doesn’t win the cubic-inch contest, but it can win the light-car street fight when the chassis, gears, cam, and tune all agree. Same small-block family doesn’t mean same job.

The biggest bad assumption is treating the 340 like either a miracle engine or a dressed-up 318. It’s neither. A real early 340 is a serious factory small-block. A T/A or AAR 340 is a special package that needs proof. A later low-compression 340 still has value, but it doesn’t get every 1968 bragging right by inheritance. The 340 earned its reputation because the parts were right, not because the decal had good posture.

The Mopar LA 340’s real place is as Chrysler’s high-output LA small-block: short-lived, respected, and built with enough honest performance hardware to make the displacement look bigger than it was. Give it the right car and the right proof, and it earns the grin. Start handing every 340 the Six Pack crown or dismissing it as a 318 with ego, and the little LA will make you look dumber than the guy who thought the air cleaner was documentation.

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What makes it different

The Mopar LA 340 is the small block that showed up already tired of being called little. The clean comparison is simple: the 273 started the LA line, the 318 carried the everyday work, the 360 brought the torque, and the 340 was the factory performance piece with the sharpest teeth. Big bore, short stroke, serious breathing, and high-compression attitude are why “small block” stops sounding like an apology here.

Compared with the 273, the 340 is what happened when Chrysler stopped just starting the LA family and decided to make the little engine mean. The 273 had crisp early-LA character and the Commando versions had their own bite, but the 340 brought more bore, more airflow, stronger factory hardware, and a much clearer performance job. A good 273 can be sharp. The 340 was built to show up in A-bodies and make bigger engines look over their shoulder. Same family doesn’t make the 273 a baby 340. It makes the 340 the point where the LA family quit being polite.

Compared with the 318, the 340 is the one that got the performance parts from the factory instead of just a reputation for surviving abuse. The 318 is a good workhorse. It runs, lasts, and keeps doing jobs while people argue about what’s cooler. But the 340 had the bore, heads, compression, cam, induction, and attitude to play a different game. That doesn’t make the 318 junk. It means a 318 needs a plan to chase what the 340 was born closer to doing. Common doesn’t beat purpose just by hanging around longer.

Compared with the 360, the 340 gives up stroke and easy low-rpm torque, but it keeps the sharper factory performance identity. The 360 is the bigger-cube LA that makes more sense when the car is heavy, the gearing is lazy, or the builder wants street torque without spinning the thing like a shop fan. The 340 is the shorter-stroke, big-bore engine that wants airflow, rpm, compression, and a car that lets it work. The 360 is the bigger wrench. The 340 is the sharper one that doesn’t need to be huge to draw blood.

Compared with the 340 Six Pack / Six-Barrel versions, a regular 340 needs to keep its paperwork in order. The 1970 T/A and AAR pieces are their own deal, with special induction, application-specific hardware, and collector value that doesn’t come free with every 340 block. A four-barrel 340 is already a serious small block. It doesn’t become a Six Pack because somebody bought three carburetors and started talking faster. If the price smells like T/A or AAR money, the block, heads, intake, dates, application, and paperwork need to stand up straight.

Compared with Magnum and later small-block thinking, the LA 340 belongs to the real muscle-era performance world. Later engines can be useful, cheaper, and easier to live with in some builds, but they don’t carry the same factory 340 identity. The 340 was built when displacement, compression, breathing, and factory performance image all lined up before emissions, insurance, and fuel reality started taking bites out of the fun. That doesn’t make every 340 sacred. It means you don’t judge one like it’s just another smog-era small block with nostalgia sprayed on it.

The 340 makes sense when the job rewards small-block response with real factory performance behind it: A-bodies, E-bodies, lighter B-bodies, period street cars, correct restorations, and builds where the engine’s identity matters as much as the dyno sheet. It makes less sense when somebody wants cheap torque, heavy-car laziness, or 360-style grunt without giving the 340 the gear, compression, airflow, and rpm room it expects. The 340 isn’t magic. It’s just honest enough to tell on a bad combination.

That’s the 340’s place. It’s the LA small block that earned the performance crown the hard way: not by being the biggest, not by being the most common, and not by borrowing big-block noise. Verify the real high-performance claims, don’t let every 340 pretend it’s a T/A or AAR piece, and don’t insult it by building it like a wheezing grocery motor. The 340 came with teeth. The builder’s job isn’t to file them off.

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General Specs

Displacement	340 cu in / 5.6L	Bore / Stroke	4.040 in × 3.3125 in
Production / Use Era	1968–1973 Mopar LA small-block performance era	Bore Spacing	4.460 in
Engine Family	Chrysler / Mopar LA-series small-block wedge V8	Deck Height	9.600 in LA small-block baseline
Block Material	Cast iron	Rod Length	6.123 in LA small-block baseline
Cylinder Head Material	Cast iron	Main Journal	2.500 in
Factory Rating / Strength Lane	High-output small-block performance lane; early 4-barrel engines built the reputation, 1970 AAR / T/A Six Barrel and Six Pack pieces carry special proof requirements, and 1972–1973 versions need year-specific checking	Compression Ratio	Early high-compression versions, 1970 AAR / T/A packages, and later lower-compression versions need exact year, piston, head, cam, and application verification
Fuel / Induction	Gasoline; factory 4-barrel, plus 1970 Six Barrel / Six Pack AAR and T/A packages	Rod Journal	2.125 in
Cam Location	In-block camshaft	Main Bearings	5
Valve Layout	OHV / pushrod, 2 valves per cylinder; wedge chambers	Firing Order	1-8-4-3-6-5-7-2
Common Main Caps	2-bolt	Distributor Rotation	Clockwise
Rear Main Seal	2-piece	Balance	Forged-crank engines are normally internal-balance; 1973 cast-crank engines need the correct external-balance parts
Weight	~500–550 lb complete iron LA small-block with typical accessories	Dimensions	~28H / ~26W / ~29L
Primary Identity Check	Verify 4.040-inch bore, block casting, pad stamping, date context, heads, crank type, intake, carburetion, and vehicle application before paying 340 money	Common Use	Performance-era A-body, E-body, and selected B-body Mopars; correct restorations, period street builds, and documented 340 cars

Verify the block, bore, crank, heads, intake, carburetion, camshaft, compression, balance hardware, converter or flywheel fit, exhaust, stampings, dates, and vehicle application before ordering parts or believing the seller. A real 340 can justify real money. A dressed-up small-block with orange paint and a bedtime story can justify a long walk back to the truck.

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Variants

The 340 LA needs a variant section with the brakes working because this is the small-block Mopar people love to overprice, overclaim, and dress up until the casting numbers start looking embarrassed. The engine earned its reputation, but the reputation doesn’t identify a block, prove an AAR or T/A package, or turn every orange small-block into factory performance history.

The useful split is simple: 1968–1971 high-compression 4-barrel 340s, the 1970 AAR / T/A Six Barrel and Six Pack branch, 1972–1973 lower-compression 340s, and the service / swap / clone confusion that follows any engine valuable enough to make people creative. That gives the reader the map without turning the section into a casting-number swamp.

1968–1971 4-Barrel 340

The 1968–1971 4-barrel 340 is the version that built the legend. It used the LA small-block layout, a 4.040-inch bore, and a 3.3125-inch stroke, giving it the breathing room the 273 and 318 never had. Chrysler backed that bore with performance heads, healthy compression, serious cam timing, a strong 4-barrel package, dual exhaust, and a factory personality that wasn’t built for grocery-store obedience.

This version worked because the pieces agreed with each other. The short stroke let it rev. The big bore gave the heads room to breathe. The compression, cam, carburetion, exhaust, and gearing made the engine feel sharper than its displacement suggested. In a Dart, Duster, Demon, Barracuda, Challenger, or other lighter Mopar, the 340 didn’t need big-block weight to make somebody nervous.

The early 340 deserves respect because it was one of Chrysler’s best factory street small-blocks. It doesn’t need fake Hemi talk, fake Six Pack claims, or a barstool story about beating everything ever made. Quick, compact, well-matched, and nasty in the right car is already plenty.

Claims still need proof when money gets brave. Block casting, pad stamping, date codes, bore identity, heads, intake, carburetor, exhaust, vehicle application, fender tag, broadcast sheet, and paperwork should support the story. A seller whispering “probably original” isn’t documentation. It’s just noise with a price tag.

1970 AAR / T/A Six Barrel and Six Pack 340

The 1970 AAR / T/A 340 is the special branch, and this is where the proof burden gets heavy enough to dent the bench. Plymouth called it Six Barrel. Dodge called it Six Pack. Either way, the package belongs to the AAR ’Cuda and Challenger T/A Trans-Am homologation story, not every 340 with three carburetors and a seller grinning like he found buried treasure.

This wasn’t just a normal 340 with a center carb and two decorations for cruise night. The AAR and T/A package had specific induction, special small-block details, application context, exhaust treatment, and documentation requirements. The package looked wild, sounded serious, and carries collector weight because it was tied to a specific factory purpose.

That’s why the lies are so easy. A regular 340 can grow a six-barrel intake. An E-body can grow stripes, spoilers, scoops, and a story that gets more expensive every time someone opens the hood. Parts can be bought. Factory identity has to be proven.

For a real Six Barrel or Six Pack 340 claim, the car and engine both need to testify. Engine stamping, block date, heads, intake, carburetors, linkage, air cleaner, exhaust, transmission, rear axle, fender tag, broadcast sheet, VIN context, body details, and paperwork all need to point in the same direction. “Look at all those carburetors” doesn’t get to be the witness list.

1972–1973 Lower-Compression 340s

The 1972–1973 340s are still real 340s, but they live in a different factory world. Compression dropped, emissions pressure tightened, gross horsepower bragging gave way to net ratings, and the performance party had already heard the cops outside. The badge stayed good. The package changed.

That doesn’t make the later 340 junk. A 1972 or 1973 340 still has the 4.040-inch bore, LA performance bloodline, and enough foundation to make a strong street engine when the build is honest. It can be a correct restoration engine, a good driver engine, or a solid performance foundation if the compression, cam, heads, induction, exhaust, and gearing are matched.

What it doesn’t get is every early high-compression bragging right by inheritance. A later 340 shouldn’t be dismissed as fake, and it shouldn’t be priced or described like a 1968–1971 high-compression example unless the hardware supports that kind of talk. Year, compression, crank type, balance package, and documentation still get a vote.

The right way to handle a later 340 is to respect it honestly. Verify the block, year, crank type, balance package, heads, intake, carburetion, exhaust, emissions equipment, application, and paperwork. A later 340 can be very worth owning. It just needs its own facts instead of borrowed nostalgia.

Service, Swap, and Clone Confusion

The 340 has one problem the 318 usually doesn’t: value attracts fraud. A plain 318 can be misidentified by accident. A 340 gets faked on purpose. That means every expensive 340 claim needs more proof than a fresh coat of paint and a story that smells like a swap-meet hot dog.

After decades of swaps, rebuilds, blown engines, race use, replacement blocks, restamps, clone cars, tribute builds, and “the original motor is around here somewhere” nonsense, a 340 claim has to be handled like money is standing in the room because it usually is. The better the story, the more proof it needs.

A real original 340 car needs the car and engine to agree. A real performance package needs the hardware to match the tag and paperwork. A restored car needs replacement history explained. A loose 340 needs inspection before anyone starts paying original-car money for a bare block with a campfire story.

The clone problem gets especially ugly with Dusters, Demons, Darts, Barracudas, Challengers, AARs, and T/As. A clone can be fun. A tribute can be a fine build. It stops being harmless the second the price starts pretending the factory built it that way.

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Key Points

The LA 340 is the small-block Mopar people like to brag about, and for once the bragging has machinery behind it. This is the performance-minded LA small-block with the bore, airflow room, factory hardware, and collector baggage to make the number mean something. It deserves respect, not blind worship.

The basic fingerprint is 340 cubic inches from a 4.040-inch bore and 3.3125-inch stroke. The stroke matches the 273 and 318, but the bigger bore changes the attitude. More bore gives the heads room to breathe, gives the valves room to work, and gives the engine a higher performance ceiling than the smaller LA engines. That hot reputation wasn’t born from a decal and a loud air cleaner.

The 340’s value comes from the package. Early engines had the compression, cam, heads, induction, exhaust, and factory purpose to back up the badge. The 1970 AAR and T/A branch added its own special proof-heavy lane. Later 1972–1973 engines still count, but lower compression, emissions pressure, crank type, and balance details change the conversation. Same number, different evidence pile.

A real 340 can define the car it came in. A correct Dart, Duster, Demon, Barracuda, Challenger, or other factory 340 application may carry value because the engine, body, paperwork, and hardware all tell the same story. A loose 340 core can still be worth owning, but it shouldn’t be priced like a born-with drivetrain unless the evidence is standing there with clean hands.

The first buyer mistake is letting paint name the engine. Orange paint, a 4-barrel intake, finned valve covers, headers, a 340 air cleaner, or a seller’s favorite bedtime story can’t prove the block. Bore size, casting numbers, pad stampings, date codes, crank type, heads, intake, carburetion, exhaust, application, and documentation have to carry the claim. The air cleaner doesn’t get a vote just because it looks confident.

The second mistake is confusing the 340 with its LA relatives. A 318 can be a fine workhorse, but it doesn’t become a 340 because the top end got dressed for church. A 360 can make excellent street torque, but its longer stroke, larger main journals, and balance rules are different business. The 340 is the big-bore, short-stroke performance piece. Same family, different troublemaker.

The AAR / T/A / Six Barrel / Six Pack lane needs extra suspicion because the money jumps fast. A real claim needs the right car context, block, heads, induction, carburetors, linkage, air cleaner, exhaust, transmission pairing, dates, stampings, fender tag, broadcast sheet, and paperwork. Three carburetors can be cool. They can also be a costume with fuel lines.

As a build platform, the 340 is genuinely good. It likes compression, airflow, camshaft, ignition, exhaust, carburetion, and gearing that let it rev and pull like a performance small-block should. It doesn’t need big-block cubes to be mean, but it still needs a matched plan. Throw random “race” parts at it, and the engine will happily explain that famous numbers don’t fix bad math.

Money decides whether a 340 is the smart answer. If originality, documentation, period-correct value, or factory performance identity count, the 340 may be worth the premium. If the goal is cheap torque, a 360 can make more sense. If the goal is a mild cruiser, a 318 may do the job without dragging collector pricing into the garage. Paying 340 money is smart only when the 340 part of the answer actually matters.

Parts support is good compared with oddball early Mopars, but the details still need checking. Heads, intake, exhaust, crank, rods, pistons, balancer, flywheel or converter, oil pan, brackets, pulleys, timing cover, water pump, and year-specific pieces can affect fit and value. A pile of “340 parts” isn’t automatically a correct 340 engine. Mopar didn’t install your swap-meet assumptions at the factory.

Interchange needs discipline because LA parts tempt people into lazy combinations. Some 318 and 360 pieces can be useful when the whole build supports them, and some can hurt compression, airflow speed, balance, value, or rpm range. The 360’s external-balance parts don’t belong on a normal forged-crank 340, and 318 pieces don’t preserve 340 value just because the bolts line up.

A used 340 core needs serious inspection because optimism gets expensive fast. Check for cracks, freeze damage, thin cylinder walls from previous overbores, deck cuts, lifter-bore wear, bad main saddles, crank damage, broken exhaust bolts, missing original parts, VIN stamping issues, restamping suspicion, and machine work done by somebody who owned a grinder and too much confidence.

The 340 belongs in the performance-and-proof lane. If it’s correct, complete, documented, and tied to the right car or build, it can justify the money and attention. If it’s incomplete, overbored into danger, missing the valuable pieces, or wearing a story instead of proof, don’t let the price tag outrun the evidence. A real 340 is worth caring about. A fake or tired one is just an expensive lesson with small-block valve covers.

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Common Problems

The 340 LA’s trouble starts because it really was good. That’s the dangerous part. It had bore, heads, compression, camshaft, attitude, and enough factory reputation to make every seller stand straighter beside a small-block Mopar. The 318 crowd wants to borrow its glory. The 360 crowd wants to call it outdated. The 340 doesn’t need either argument. It just needs proof.

• 340 Identity Claims Without Proof
  The 340 is valuable enough that plain small-block parts start wearing 340 stories fast. A real 340 claim needs more than orange paint, big valve covers, and a seller who suddenly remembers every magazine article from 1970.
  
  Block casting, displacement, bore, stampings, date context, vehicle application, heads, intake, exhaust, carburetion, and documentation all need checking. A dressed-up 318 or 360 doesn’t become a 340 because the air cleaner got ambitious and the asking price learned to flex.
• Six Pack / T/A / AAR Claims Without Proof
  The 340 Six Pack story is real, but it’s also one of the easiest ways to make the price start doing push-ups. Challenger T/A and AAR ’Cuda hardware, induction, heads, application, and documentation matter. The claim has to be proven, not admired.
  
  A real T/A or AAR-related 340 claim needs the correct year context, block identity, intake, carburetors, linkage, air cleaner, heads, valvetrain details, exhaust, vehicle context, stampings, and paperwork. A 340 with three carbs isn’t automatically a T/A engine. It may just be an expensive induction setup trying to sneak into a better neighborhood.
• X-Head, J-Head, and Big-Valve Story Inflation
  The 340’s head reputation causes its own mess. X-heads, J-heads, big valves, T/A heads, and later swaps all get tossed around like every casting number came with a trophy.
  
  The heads need actual identification. Casting numbers, valve sizes, pushrod holes, port work, chamber condition, date context, and application need checking before the story gets expensive. A head can be useful without being rare. It can also be rare without being correct for the engine someone is trying to sell.
• 318 / 360 Parts and Value Confusion
  The 340 belongs to the LA family, but it’s not just a 318 with better lungs or a 360 with less stroke. Its 4.04-inch bore and 3.31-inch stroke give it a different personality and a different parts/value argument.
  
  That matters when mixing heads, pistons, cranks, dampers, converters, flywheels, intakes, camshafts, and compression plans. Some parts interchange. Some parts create balance problems. Some parts technically fit while quietly making the engine worse. The catalog may say LA. The engine still wants the right combination.
• High-Compression Reputation Versus Later Reality
  Early 340s earned a hard-running reputation, but not every 340 carries the same compression, rating, parts, or year-specific identity. Later engines can bring emissions-era changes, different expectations, and different build math.
  
  Compression ratio, piston height, head chamber size, camshaft choice, ignition curve, carburetion, exhaust, and gearing all need checking by year and application. A later 340 can still be a good engine. It just should not be priced, built, or bragged about like every year is the same high-compression street bruiser.

For broader Mopar LA-series problems shared across the engine family, use the Mopar LA-series family page under Common Family Problems.

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Common Misconceptions

The 340 LA gets misunderstood because it earned its reputation honestly. That makes people careless. They hear “340” and start seeing X-heads, Six Packs, AAR ’Cudas, high compression, and magazine-test heroics before anyone checks the block. The 340 is special. That doesn’t mean every 340 story is true.

• “Every dressed-up small-block Mopar is a 340.”
  No. A dressed-up 318 or 360 can look convincing to somebody who wants the story to work. Orange paint, aftermarket intake, headers, and a loud air cleaner don’t get to name the block.
  
  A real 340 needs identification. Bore, casting numbers, stampings, application, heads, dates, and documentation all need to agree. The parts bolted on top don’t get to name the block.
• “Every 340 is a Six Pack, T/A, or AAR engine.”
  No. The Six Pack / T/A / AAR 340 story is real, valuable, and specific. It’s not a blanket laid over every 340 block.
  
  A real claim needs the right car context, hardware, dates, stampings, induction, heads, and paperwork. Three carburetors alone don’t make the engine a T/A piece. Sometimes they just make the lie more expensive.
• “X-heads make any 340 automatically special.”
  No. X-heads matter, but they aren’t magic. J-heads, T/A-specific details, valve sizes, port work, date context, and the rest of the engine still matter.
  
  A correct head can help the value and build plan. A wrong, damaged, modified, or mismatched head can turn the whole claim sideways. Casting numbers are evidence, not a finish line.
• “A 340 is basically a 318 with better parts.”
  No. The 340 has a larger bore, different performance purpose, stronger factory identity, and a different build argument. It shares LA family blood, not the same job.
  
  A 318 can be useful. A 340 can be excellent. Pretending they are the same engine with better bolt-ons is how people overpay for fake 340s or ruin good 318s trying to chase one.
• “A 360 is always a better choice than a 340.”
  Not always. The 360 has more displacement and can be a smart build, but the 340 has its own value: bore size, rev-friendly personality, factory performance reputation, and correctness in the cars that came with it.
  
  A driver may like a 360. A correct 340 car may need a 340. A period performance build may justify the smaller, meaner engine. Bigger isn’t automatically better when the VIN, parts, and purpose say otherwise.

For broader Mopar LA-series misconceptions shared across the family, use the Mopar LA-series family page under Common Family Misconceptions.

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Upgrade Limitations

The Mopar 340 LA small-block is where people start acting like every small-block Chrysler suddenly turned into a race engine. The 340 earned its reputation. It had the bore, breathing, compression, factory performance hardware, and attitude that made it more than a grocery-getter V8. That still doesn’t mean every 340 core is gold, every 340 story is true, or every later part belongs on it just because the badge makes people grin.

The 340 uses a 4.04-inch bore and 3.31-inch stroke. That big bore is the whole point. Compared with the 273 and 318, the 340 has more valve room, better breathing potential, and a factory performance identity that actually means something. It’s not a 318 with better manners, and it’s not a 360 with a shorter stroke. It’s its own LA performance engine, and that means the build needs to respect what made it good instead of burying it under random catalog ambition.

The first upgrade limitation is value and identity. A real 340 block, correct heads, correct intake, correct carburetion, correct exhaust, correct dates, and correct vehicle context can matter a lot. A 340 from a proper factory application isn’t the same thing as a mystery small-block with orange paint and a story. Before the money gets excited, verify casting numbers, stampings, dates, bore, crank, heads, intake, carburetor, exhaust manifolds, transmission pairing, and vehicle documentation.

Compression is another hard line. Early 340s are remembered for strong performance, but later emissions-era versions had lower compression and softer factory reality. A 1970-style expectation doesn’t automatically belong to every 340. Pistons, deck height, chamber volume, gasket thickness, cam timing, quench, fuel, and actual measured compression decide what the engine can use. A 340 with low compression and too much cam can still be a lazy disappointment. Famous displacement doesn’t fix weak cylinder pressure.

Camshaft choice needs respect, not worship. The 340 can handle more cam than a 273 or mild 318, and it likes RPM better than a lazy truck motor. But it still needs compression, converter or clutch, rear gear, exhaust, carburetion, ignition, and vehicle weight to agree. Too much cam in a low-compression 340 with highway gears is just a famous engine pretending to be confused.

Cylinder heads are where the 340 earns much of its name, but they still need matching. Original high-performance heads, later LA heads, aftermarket heads, big valves, chamber size, port volume, valve springs, rocker gear, and piston clearance all matter. Big heads can help when the engine has compression, cam, exhaust, and gear to use them. They can also make a street car soggy if the rest of the combination is wrong. The 340 likes airflow. It doesn’t like being used as an excuse for bad math.

Induction and exhaust need proof and purpose. A correct four-barrel setup belongs in the 340 conversation. T/A or Six Barrel claims, factory intake claims, carburetor claims, performance-package claims, and exhaust manifold claims need documentation and physical confirmation. A tall intake, too much carburetor, bad linkage, weak exhaust, or fake package hardware can hurt drivability and value. The 340 doesn’t need costume jewelry. It needs parts that match the engine and the car.

Crank and balance details can bite. Early 340s are generally associated with forged-crank/internal-balance expectations, while later 340s can bring cast-crank and external-balance issues into the fight. Don’t mix dampers, converters, flexplates, flywheels, or rotating parts by folklore. Exact year, crank type, balance package, transmission, and converter or clutch setup need proof before startup. A 340 that shakes isn’t “just rowdy.” It’s telling you something expensive.

Core condition matters more because 340 blocks are valuable. Previous overbores, cylinder-wall condition, cracks, damaged decks, worn lifter bores, bad threads, main saddle condition, wiped cam history, and mismatched parts can turn a desirable engine into a machine-shop argument. Don’t bore a 340 casually just because pistons are available. Sonic check, measure, and preserve the block if it’s a real one. Rare iron doesn’t need a hero with a boring bar and no patience.

The honest upgrade plan depends on the engine’s identity. A correct 340 should be verified and preserved. A street 340 should use real compression, a matched cam, good heads, sensible induction, strong exhaust, proper ignition, and gearing that lets it work. A T/A, Six Barrel, or other performance-package claim needs documentation and physical proof before anybody pays legend money. If the goal is cheap torque, a 360 may be smarter. If the goal is cheap driver reliability, a 318 may be cleaner. The 340 is special because it’s a real performance LA engine, not because it excuses sloppy planning.

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When It Actually Makes Sense

The 340 makes sense when the engine’s performance identity matters. In a correct 340-equipped Dodge, Plymouth, or proper factory application, the engine can define the car. Swapping it out or casually modifying rare original hardware can damage the story the car is supposed to tell.

It makes excellent sense when the goal is a period-correct high-performance small-block. A healthy 340 has the bore, breathing potential, and factory personality to make an old Mopar feel sharp without needing big-block weight. It can rev, pull, and sound right when the combination is matched.

It also makes sense when the engine is complete and documented. Block, heads, crank, intake, carburetor, air cleaner, exhaust manifolds, distributor, brackets, pulleys, transmission pairing, dates, stampings, and vehicle context all matter. A complete 340 package is different from a bare block with a price tag wearing a cape.

The 340 makes sense when the car needs balance more than brute torque. In lighter cars, a well-built 340 can feel quick, responsive, and alive. It doesn’t need to act like a 440. It needs compression, cam, airflow, exhaust, ignition, gear, and traction working together.

The 340 makes less sense when the goal is cheap horsepower with no concern for originality. Real 340 parts cost money because people know what they are. A 360 can often make more street torque for less money, and a 408-style stroker can make the power goal easier if originality doesn’t matter. That doesn’t make the 340 weak. It means the 340’s value is performance identity, not bargain-bin displacement.

It also makes less sense when the engine is incomplete, undocumented, or already damaged. A 340 block missing its correct parts can still be valuable, but replacing rare heads, manifolds, carburetion, air cleaner, brackets, and date-correct pieces can turn the project into a parts hunt with sharp teeth. If the block is overbored badly or cracked, the famous number doesn’t save the invoice.

Plain answer: build the 340 when correctness, factory performance identity, small-block response, and documented hardware matter. Don’t build it because you think every 340 is automatically a perfect muscle engine. It isn’t. The good ones are good because the whole package was right.

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Compatibility Notes

Family identity.
The 340 is a Mopar LA-series small-block performance V8. It belongs to the LA family with the 273,318, and 360, but it isn’t interchangeable by wishful thinking. Exact year, application, heads, intake, crank, balance, front dress, and transmission details still decide what fits. The family name opens the door. It doesn’t sort the parts pile.

Bore and stroke proof.
Verify the actual engine before trusting the story. The 340 uses a 4.040-inch bore and 3.3125-inch stroke. Check casting numbers, pad stampings, bore, crank, pistons, heads, intake, date context, vehicle application, and physical measurements before calling it a 340, AAR / T/A piece, Six Barrel / Six Pack package, 318, 360, Magnum, or swap mystery.

340 versus 318 LA.
The 340 and 318 share LA-family logic, but they don’t share the same job. The 340 has the larger bore, stronger performance identity, different piston and compression planning, and different head and induction value. A 318 recipe can be too mild or just wrong for a 340, and a 340 recipe can be too much for a 318. Same family doesn’t mean same plan.

340 versus 360 LA.
The 360 has a longer stroke, larger main journals, different balance requirements, and different torque behavior. The 340 is a big-bore, shorter-stroke performance small-block. Don’t assume 360 crank, damper, converter, flexplate, flywheel, pistons, or balance parts belong on a 340. A 360 can make excellent torque, but it doesn’t get to donate parts by displacement envy.

Early versus later 340.
Verify exact year, crank type, balance package, pistons, compression, heads, intake, carburetion, emissions hardware, and transmission pairing. Early and later 340s aren’t always the same engine for build planning. Compression, rating method, emissions equipment, crank material, and balance details can change the plan fast.

AAR / T/A, Six Barrel, and Six Pack claims.
A real AAR / T/A or Six Barrel / Six Pack 340 needs documentation and physical proof. Verify block details, heads, intake, carburetors, linkage, air cleaner, exhaust manifolds, camshaft, distributor, dates, stampings, transmission pairing, and vehicle context. Three carburetors, special valve covers, or a good story don’t create factory identity. They just make the lie more decorative.

Heads, intake, and valvetrain.
Verify head casting, chamber size, valve size, intake manifold fit, rocker gear, pushrods, lifters, camshaft, timing parts, and oiling details by exact year and build plan. The 340 can use airflow, but it still needs the right compression, cam, exhaust, and gear. Big parts without a matched plan are just expensive clutter.

Crank, balance, and transmission fit.
Verify forged versus cast crank, crank register, crank flange, internal or external balance, damper, converter, flexplate, flywheel, starter location, bellhousing or case pattern, clutch parts, linkage, mounts, driveshaft, and rear gear by exact year and application. Don’t mix early forged-crank assumptions with later cast-crank parts unless the rotating assembly has been deliberately built and balanced that way.

Front dress, cooling, and accessories.
Check timing cover, water pump outlet side, pulleys, alternator brackets, power steering brackets, A/C brackets, radiator hose routing, fan spacing, oil pan, mounts, and chassis clearance. A-body, B-body, E-body, service-engine, and later swap parts can differ. The front of a small-block Mopar is where confidence often starts leaking.

Restoration-correct versus driver-correct versus performance-correct.
For restoration-correct work, the right 340 and supporting hardware can define the car. For a driver, a matched 340 can be quick, responsive, and satisfying. For performance work, be honest about the exact engine and goal. The 340 is one of Mopar’s great small-blocks, but even great engines don’t make mismatched parts smart.

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Reality Check

The 340 LA earned its reputation, but that doesn’t mean every 340 story deserves your money. A real early high-compression 340 is serious small-block Mopar hardware. A real T/A or AAR Six Pack / Six Barrel claim is even more proof-heavy. A later 1972–1973 340 is still real, but it lives under different compression, rating, and emissions-era expectations.

The danger isn’t that the 340 is overrated. The danger is that the name is valuable enough to make people lie. A dressed-up 318 isn’t a 340. A 360 swap doesn’t preserve 340 identity. Three carburetors don’t prove AAR or T/A history. Orange paint, finned valve covers, and a loud seller don’t get to replace casting numbers, stampings, dates, hardware, and paperwork.

The 340 makes the most sense when the car, engine, and proof all line up. Correct Darts, Dusters, Demons, Barracudas, Challengers, AARs, T/As, and other proper 340 applications can justify the cost and care. In that lane, the engine is part of the car’s identity, not just a way to make horsepower.

For ordinary performance use, the 340 has to compete with money. A 360 or stroker may make easier torque for less grief. A 318 may be a better mild-driver choice. That doesn’t make the 340 weak. It means the 340 is worth paying for when the performance identity, correct application, or verified hardware actually counts.

The hard truth is simple: respect the 340, but make it prove itself. Verify the block, bore, stampings, heads, intake, crank and balance package, dates, application, and paperwork before paying legend money. A real 340 can be a nasty little engine. A fake one is just a sales pitch with small-block valve covers.

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Bottom Line

The Mopar LA 340 is Chrysler’s serious factory small-block performance engine. It had the bore, breathing room, compression, camshaft, induction, and factory intent that the everyday 318 never got. It wasn’t the biggest LA engine, but it was the one that made small-block Mopar performance feel sharp, mean, and worth bragging about.

The 340 makes the most sense in cars where its identity matters. Correct 340 Darts, Dusters, Demons, Barracudas, Challengers, and proper factory applications deserve the right engine and the right proof. A documented AAR ’Cuda or Challenger T/A claim deserves even more care, because the Six Pack / Six Barrel hardware and Trans-Am connection carry real value only when the evidence lines up.

It makes less sense when the only goal is cheap power. A 360 can often make more street torque for less money, and a stroker can outrun the nostalgia if originality isn’t part of the mission. That doesn’t make the 340 a bad engine. It means the 340’s value lives in performance heritage, correct applications, and real factory identity, not bargain-bin horsepower math.

The mistake to avoid is buying the legend instead of the engine. A 340 claim needs the right bore, block casting, pad stamping, date codes, heads, intake, crank and balance package, carburetion, exhaust, car context, and paperwork. A 318 dressed like a 340 is still a 318. A 360 in a 340 car may be useful, but it changes the story. A six-barrel setup without proof is hardware, not history.

Completeness and condition matter hard. A complete, correct, documented 340 package can be worth serious money. A cracked, overbored, restamped, stripped, or mismatched core can become expensive before it becomes useful. The famous number doesn’t heal cylinder walls, replace missing heads, or make bad paperwork honest.

Bottom line: the 340 is one of Mopar’s best small-blocks when it’s real, healthy, and matched to the right car. Preserve the correct ones. Build the street ones with compression, airflow, cam, exhaust, ignition, and gearing that make sense. Don’t hand every 340 the AAR crown, don’t let a fake claim ride on orange paint, and don’t insult the engine by treating proof like an optional accessory.

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