340 Engine History & Details


Since it’s introduction in late 1967, the free breathing, high winding Mopar small block340 has forged itself a rock solid reputation as one of the great giant killers of all time.  While the Hemi guys made all the noise and the 440 crowd waved there big block performance flags, streetwise Mopar guys who could care less about bigger than life images and overkill cubes saw the 340 as the motor of choice for taking on those pesky Bow Tie rodents. According to CAR LIFE Magazine, “Rated at 275 bhp, Mopar’s 340 may have had 325 or more” and that it was “as cleverly engineered as the 426 Hemi, just not so fussy.” CAR and DRIVER called its 340 Dart GTS (4 speed, 3.91:1 gears, 14.4 ET @ 99 mph) “a giant killer.”

In a 1969 test, HOT ROD Magazine got its 340, 4 speed Formula S ‘Cuda with 3.91:1gears to turn high-13’s. It is stated – the 340 should be considered the ‘Hemi of the small block family’. Why? Simply because the 340 was designed from the outset as a performance engine. Look at the stats – high compression, big valves (2.02 intake, 1.60 exhaust), forged and shot-peened crank and rods, free flowing dual exhaust, dual plane intake manifolds and large 4 barrel carburetors. These engines were built to perform well and stay together doing it. A testament to the 340's power potential is the fact that Chrysler installed the beefy A-727 Torque flite automatic behind it instead of the usual 904, the only small block to receive such an honour.

 The 340's best power rating was 290 hp for the AAR (2724 built) and T/A’s (2539 built); even in 1973, it still managed 245 net horsepower. The real race T/A’s and AAR’s used a 305 cid engine (destroked 340) that produced a whopping 440 bhp to power them through a number of Trans Am race seasons. The street versions had a 340 topped with an Edelbrock manifold and three Holley two-barrels. It was rated at 290 bhp, but likely made 350 and was potent enough for high 13-second ETs.




• 340 released with forged crank, 4bbl, hydraulic cam (in 1968, only the 4 speed cars got a more aggressive cam, automatic versions got a cam that would become the standard in high performance engines)

• 10.5:1 compression ratio

• a dual timing chain and a windage tray

• First year for the dual plain intake

• big X heads - 2.02 inch intake valves and 1.60 inch exhaust valves (894’s)

• Carter AVS Carburetor


• virtually unchanged except for standard cam in 4 speed and automatic cars


340 gets Six-Pak (AAR Cuda & T/A Challengers only), T/A blocks had thicker webs for the ability to install 4 bolt mains

• The 360 J-head saw first limited use in the 1970 340 Six-Pack AAR/TA, which had 2.02/1.60 machined heads and revised pushrod holes for more meat in the intake runners (915’s)

• Engines were painted orange

• A Carter AVS Carburetor (3 Holly Caburetors on the Six-Pak set up)


• Engines were painted orange but changed to blue in late 1971 (why? Jury’s still out)

• Compression dropped to 10.3:1 from the former 10.5:1 due to new emission laws

and upcoming changes in gasoline lead content

Mid-year partial intro of Electronic Ignition

• Introduction of the ‘360 style’ J head, using the same casting as 1970 340’s - with machining for the different sized valves – 1.88” intake and 1.60” exhaust valves and hardened seats for unleaded gas (915’s)

• Carter ThermoQuad Carburetor – offering more throttle response and better low to mid range transition

• ThermoQuad’s bigger cfm increases NHRA index over 1970’s index numbers


• Seriously detuned, ostensibly for emissions reasons (but possibly also for insurance reasons)

• Compression ratio drops from 10.3:1 to a 8.5:1 producing 240hp (now net rated)

• Exhaust manifolds changed

• Cast crank* replaces steel forged crank

• 1:88” valved J Heads (915’s) continued

• Carter ThermoQuad Carburetor continued

*Note: In ’72, the cast crankshaft replaced the forged crank in production on April 11,1972 with 40 engine #HM340R-3911-8000. Damper stamped ‘Use with 340 cast crank only’ 


• Last year for 340, unchanged from ‘72

• 1:88” J Heads (new casting 587’s)

• Carter ThermoQuad Carburetor


• 340 gets replaced by 360 and takes along the 340’s cam, heads, intake, and 4bbl carb, along with a few other goodies - making the smog built motor a great little performer in it’s own right with 245hp

Carburetor & Intake

Ever since the 340 was introduced in 67 for 68 cars, they always carried the Carter line of carburetors. Prior to ’71, Chrysler’s highwinding 340-inch small-block came standard with the Carter AVS, rated at 650 cfm. Carter brought out its resin-bodied ThermoQuad in the late ‘60’s and soon found a home in 340’s starting in ’71. The VS did not pass the new emission tests for ’71, while the 780cfm ThermoQuad - with its 1 3/8” primaries - passed. Even with a slight drop in compression in ’71 and with the help of the new ThermoQuad, NHRA quickly realized the carb was a big plus, and ’71 340’s got a 30hp bump over previous index’s. It was fun while it lasted – in ’72, the tightening noose on performance found compression down to just 8.5:1 and more restrictive 360-type J heads - the 340 in ’73 came to an end - though the ThermoQuad remained in place, perking up the smog laden HP 360 4bbl in ’74 and ‘75. The stock dual plane 4 bbl intake manifold on the 340 engine was very good - incorporating large passages and raised over an inch to get a hi-riser effect.

Cylinder Heads

Chrysler engineers always seemed to have the knack of using just the right valve and port sizes for a given displacement size.  The ’68-’71 340 ‘894’ casting X heads have slightly larger intake valves – 2.02 as apposed to the ’72-’74 J heads, with 1.88/1.60 inch valves. In ’71 - after the factory used up the X heads, 340’s received the ‘915’ casing J heads, retaining the 2.02 valves - for only that year. In 72, both 340’s and 360’s received the 1.88 versions but a few large valve heads made it into a few early production ’72 340’s. The J head carried on after ’74 but were modified in ’75 to reduce emissions even further.  The J head (casting # 3418915) actually saw first limited use in the 1970 340 Six-Pack AAR and TA’s, which had 2.02/1.60 machined heads.  They also had relocated pushrod holes for the offset intake rockers. These were only used on 1970 T/A Challengers and AAR Cuda’s