The Amazing Mopar (Dodge/Plymouth) 340 V8 Engine
Thanks to Danny Frost, Steve Silver, Joshua Skinner, Carl Payne, Dan Stern, Steven Havens, H. David Braew, Jim Forbes, Jamie Bergen, Mike Sealey, and Peter Duncan.
Introduction to the 340
One of the best engines of the 1960s and 1970s for performance enthusiasts was the 340 V-8. It had high-flow heads, big ports, a two-level intake manifold, and a six-barrel option (three two-barrel carbs). The package allowed for high speed with the light weight helping handling.
The 340 cars gave away nothing to the 383 cars in a straight line, and were ahead of the 383 cars on anything involving turns — and spark plug access.
When the 340 came out in late 1967, it was a street fighter from the start. Separating the 340 from the standard-performance 318 were not just 22 cubic inches, but also:
- a dual timing chain with a windage tray to improve top end engine RPM by keeping the crank counter weights from 'churning the oil' in the pan.
- 2.02 inch intake valves and 1.60 inch exhaust valves
- a high-rise dual plane intake
- an 850 cfm carburetor (from 1971 to 1973)
- a forged steel crank (through 1972’s engine #39118000, when a cast iron crank was used)
- high-performance heads
- a revised oil pump with a 90 degree adaptor
- a special carburetor and cam
The 340’s best power rating was 290 horsepower; even in 1973, it still managed 240-245 net horsepower. A good-running, early 340 in a lightweight A-body or Road Runner embarrassed many big block engines. Though relatively few were made, many parts interchange with 318s and 360s.
1970-71 engines were painted orange; they changed to blue in 1972-73, although some late 1971 engines ended up blue also.
In 1971, the 340 came with the J heads and 2.02/1.60 valves. Since 1971 saw the introduction of the "360 style" J head, they used the same casting for 1971 340s and 360s, with different machining for the different sized valves. The 360 head actually saw first limited use in the 1970 340 Six-Pack AAR/TA, which also had 2.20/1.60 machined heads.
||1968 318 V8
| 1968 340 V8
|Gross Horsepower||230 hp at 4400 rpm||275 hp at 5000 rpm||275 @ 5,000|
|Torque, lbs.-ft.||340 at 2400 rpm||340 at 3200 rpm||340 @ 3,200|
|Compression ratio||9.2 to 1||10.5 to 1||8.8:1|
|Displacement, cu. in.||318||340||same|
|Carburetor type||2-bbl.||4-bbl.||Carter AVS-49335|
|Air cleaner type||Silenced||Unsilenced||same|
In 1972, the 340 was seriously detuned, ostensibly for emissions reasons (but possibly also for insurance reasons). It went from a 10.4:1 to a 8.5:1 compression ratio, got smaller intake valves, and seriously fell in performance. 1973 was its last model year.
Parts from the 340 were transferred to a muscle version of the 360 in 1974. The high performance (HP) 360 went on to appear in A-body and F-body police cars, as well as the Little Red Truck and other fast pickups.
Micomlar wrote: “The windage trays supposedly resulted in an approximately 650 to 800 fouth gear top end finish improvement. These trays were also suspected to cause engine damage as they starved some critical bottom end components unless the engine was in peak mechanical shape.
I own 3 340s (In a 1972 Challenger, a 1969 Dart and a 1973 Duster) and all 3 had the trays when I rebuilt them. One engine has genuine X heads and the other two have AAWJ 360 with the 1.88 intakes removed and the 2.02 stainless valves and the intake runners worked to match the larger foot print X head gasket outline. By increasing the intake valves and opening up the intake runners, the magic reported answer to the question of how much improvement is approximately 35 ponies.
With reworked heads and a set of headers, the factory 275 horse animal (that the insurance companies at that time corrected to be 325 horsepower) becomes 330 according to Dodge and 380 horsepower according to the insurance companies. These numbers were 1970/75 numbers, so I am sure with my 750 CFM Double pumper, KB pistons, single plane Weind intake, it’s well over 400 horses! Also, all parts will interchange with all LA blocks, except the crankshafts (and pistons) on the 360s have larger mains. (Heads will need o-ringing). I have put 340 heads on 318s with a stroker kit and ended up with a killer little engine. Also, the 340 and 360s have the beefed up HD rods. The 273 and 318s have the light weight rod versions.”
340 V-8 engine parts identification (Steve Silver)
If you find the J heads at a swap meet, they could be either original 2.02 valve heads (1971) or original 1.88 heads (1972). If you find a set of 1.88 heads and want 2.02 valves, just take them and have them machined for the larger valves.
Some of the 1970 340 Six-Pack engines had heads with a U or O casting letter in addition to the J, but they were still the same part number head (the T/A head had the intake valve pushrod holes drilled in a offset location).
Valve covers for 340 engines were different than 318 valve covers. Since the 340 carb was a 4 barrel, it was wider and the carb linkages caused interference with the spark plug wires, so they raised the drivers side wire holder up about 2.5" higher than the 318, to clear the linkages. Also since the 4 barrel carb was wider, it pushed the choke well boss over close to the passenger side valve cover close to the wiring and vacuum advance hose that runs along the valve cover, so they added a rubber coated curved heat shield to the side of the valve cover to protect the wiring. This is how you can tell 340 valve covers from 318 valve covers at a swap meet.
T/A 340 valve covers were almost identical to standard 340 valve covers except the spark plug wire holders were located about an inch farther back toward the rear.
Mopar 340 V-8 Engine - Carburetors and Cranks
- 1968-70 - AVS
- 1971-73 - Thermoquad
- 1968-1971: forged crank, part number 2532457, 2128869, or 2843868; service part number, 2843868
- 1972-73: cast crank, 3462387, 3658393, or 3751841 (some 1974-86 cast cranks use the same 3462387 casting number as the 1973 340 crank); service part number 3751162
Chronology of the Mopar 340 V-8 engine (Stephen Havens and Michael Volkmann)
- 1968 - 340 released, rated at 275 horsepower, with:
- forged crank
- four-barrel Carter AVS carburetor
- hydraulic cam (a more aggressive cam was used in the four-speed cars in 1968)
- unique “X” casting cylinder heads, with 2.02” intake, 1.88” exhaust valves
- 10.5:1 compression ratio
- dual plane intake (the 318 had a single-plane manifold with a two barrel carburetor).
- 1969 - Unchanged, except manual transmission 340 engines also had the (formerly) automatic-transmission camshaft.
- Two distinct versions of the 340 high performance engine were made. The four-barrel carried over without changes, while a new Trans Am (T/A) version, dubbed Six-Pack on Dodge and 6-Barrel on Plymouth, debuted. This 1970-only 340 had:
- three two-barrel Holley carburetors
- “J” casting cylinder heads were unique to the T/A, with unique pushrod holes to allow for oversized porting on the intake runners
- a unique rocker arm/shaft/pushrod assembly
- thicker webs in the pan rail and other areas
- the ability to install 4-bolt mains on 2 3 and 4
- 1971 - The 340 four-barrel remained; the triple-carburetor version did not.
- Advertised compression dropped to 10.25:1 but horsepower remained at 275 (gross)
- Carter Thermoquad carburetor was used
- 340/360 “J” casting cylinder heads with 2.02 intake and 1.88 exhaust valves (not the same as the T/A J heads)
- 1972 - The 340 four-barrel dropped to 240 horsepower:
- Compression dropped to 8.5:1; the compression height of the piston via wrist pin location changed by 0.10”
- The crankshaft was switched from forged to cast somewhere in the production cycle, believed to be in early April 1972, with engine 39118000 (thanks, Karl Thomas); a milder camshaft was used
- 340/360 were moved to “J” casting heads with 1.92” intake valves; exhaust valves remained the same
- Paint went from orange to corporate blue
- 1973 - The cast crank had a different number than the 1972 counterpart, because it was shot-peened for greater strength.
- 1974 - The 340 did not re-appear for 1974, ending an era. Instead, the 360 four-barrel (E58) was provided, with the 1973-340 cam, carb, and intake.
The 340 reborn: “X” racing blocks
Ed Poplawski wrote:
The 355 engine was a bored and stroked 340 production 6-barrel engine block (LA) that we modified for racing use [until 2001].
[The 355 block was common to all racing applications before 2001.] Back then the engine block of choice was the 340 6-barrel block. However, by the late 1970s, 6-barrel blocks were hard to come by and we were scrounging the local junk yards trying to find blocks. It was the same thing for the Pettys. To be honest, we were using standard 340 blocks for dyno development and saving the 6bbl blocks for the race cars.
The reason we wanted to use the 6bbl block was because we had added material to the bulkheads for strength so we could use 4 bolt main caps. Otherwise the only difference between the engines, like you mentioned was carburetion, camshaft, cylinder heads and headers. The NASCAR engine could only use a flat tappet cam (we used a mushroom tappet) and a single 4 bbl carb. The NHRA drag engines could use roller cams and multiple carburetion intake manifolds depending on the class.
Headers were chosen depending on the type of racing we were doing and the engine speed at which we were running. For example, for 1978 our goal for Daytona that year was 600 HP and the engine peaked at 7200 rpm. The 8bbl drag engines peaked at 9600 rpm so therefore the header size and length was a lot different. The IROC engines were basically NASCAR 355 engines. Also, we used dry sump oil systems in NASCAR so there were some internal modifications done to the block to make this work. Those were the major differences between all the series. [Ed Poplawski interview • Related Marc Rozman interview]