When it comes to the vintage homologation specials for SCCA’s Trans Am racing series, the Mustang Boss 302 and Camaro Z/28 typically get all the attention—but Chrysler had its own dogs in the fight.
When it comes to small-block V-8s, never underestimate the Mopar 340 Six-Pack; it delivers the kind of power that dreams are made of.
The Dodge Challenger T/A and Plymouth ’Cuda AAR were strong competitors on and off the track, with the street-going versions featuring a unique, Six-Barrel version of the 340 small-block—although the racing cars used destroked engines displacing 305 ci.
The 340 “T/A” engine used in the Challenger T/A and AAR ’Cuda had a unique foundation: a specific block casting (with a strength-enhancing, higher nickel content) with PN 3577130TA.
Significantly, the “T/A” engine was more than a 340 with a trio of two-barrel carbs. It was comprehensively engineered for high-rpm performance and featured a unique, strengthened block casting and unique cylinder heads with offset intake rocker arms that made room for larger intake runners.
It’s a common misperception that the T/A engine had four-bolt mains. It did not, but the main webbing was reportedly thicker than the other 340 blocks to accommodate them. The cylinder walls and pan rails were also thicker than other blocks, adding approximately 30 pounds to the block’s weight.
The induction system consisted of three Holley two-barrels mounted atop an Edelbrock aluminum intake manifold. The throttle controlled the central 350-cfm carb, and the vacuum-operated front and rear carbs joined the party when the center carb was wide open.
The original forged steel crankshaft was retained for this rebuild, but it wasn’t unique to the T/A engine. It was used in other 340 assemblies from 1968-71. Valley Performance simply inspected, cleaned up, polished the journals, and spin-balanced this one. It was in very good, original condition.
It was a strong combination. In fact, it was stronger than the official 290hp rating that was likely advertised for a number of reasons, from keeping the insurance companies at bay to maintaining a more plausible difference between that LA small-block engine and its RB big-block brothers. Over the years, the general opinion has been that the true output was more like 340 or 350 hp.
The original connecting rods were also forged steel and, like other small-block Mopar rods, measured 6.123 inches in length. Like the crankshaft, the rods for this engine were cleaned up, inspected, and reconditioned in this rebuild, but the fasteners were upgraded to strong ARP bolts as ordered by Summit Racing.
That was only 25 hp shy of the larger, heavier 440 engine. The proof, of course, is on the dyno, and that’s exactly what we were interested in with Michigan-based Valley Performance’s rebuild and testing of a 340 Six-Barrel engine plucked from an AAR ’Cuda.
The most significant update in the engine was the use of new ICON forged aluminum pistons, also from Summit Racing. Each is approximately 150 grams lighter than the originals. It also features a modern, thinner ring pack, with a 1/16-inch top and second rings rather than the conventional 5/64-inch size. That reduces friction and ring tension, while the more modern material in the rings enhances sealing and overall durability.
The engine was original to the car, had never been rebuilt, and was running well enough, but with the rest of the car undergoing a restoration it was time for a refresh. According to Valley Performance’s Jack Barna, the goal was to keep the engine original-looking and as stock as possible but add a few practical updates that would make the car easier to drive.
For the record, Valley Performance performed its standard machining steps on the block. That included taking the cylinders 0.020-inch over, which technically stretched the displacement to 343 ci. To accommodate modern lower-octane pump gas, the pistons were spec’d to reduce the compression ratio from 10.5:1 to 9.5:1.
“The biggest issue was accommodating today’s fuel, which has lower octane, no lead, and some ethanol compared to the fuel of 1970,” he says. “It only made sense to rebuild the engine around those changes, especially because the owner planned to drive the car regularly after the restoration.”
Also, from Summit Racing came a Comp Cams PN 20-309-4 “Magnum Muscle” hydraulic flat-tappet camshaft with a 268/276 degrees duration (advertised), 0.464/0.464-inch lift, and a 110-lobe separation angle. Those specs match the original cam’s duration but elevate the lift a bit from the original cam’s 0.429-inch/0.444-inch specs.
In practical terms, that meant dropping the compression a smidge with new flat-top pistons that would shave about a point off the compression ratio. The loss in compression was compensated somewhat by the new pistons’ lighter weight and a more contemporary ring pack that reduced ring tension and friction.
The oil pump was replaced as a matter of course. This one is a Melling M-72HV. The pickup wraps neatly over the factory windage tray, which was also cleaned up and reinstalled on the short-block. Chrysler employed the trays on all 340 engines and a number of higher-performance big-blocks.
Also, a Comp Cams replacement hydraulic flat-tappet camshaft from Summit Racing helped, adding a bit more lift, but essentially, everything else in the assembly, from the crank and rods to the cylinder heads and rebuilt original carbs, was kept to original specs. That made the dyno results more than a little eye-opening: 375 hp at 5,600 pm and 389 lb-ft of torque at 4,200 rpm.
The harmonic balancer was replaced because of its age, as the elastomer in balancers deteriorates and/or hardens over time. That can sometimes cause a separation of the rubber insulator and the body of the balancer. Better safe than sorry.
Not only did that match the output of some of the higher-rated Mopar big-blocks of the muscle car era, but it also delivered the same output as a contemporary Challenger R/T’s base 5.7L Hemi engine, which benefits from the precision of electronic port fuel injection and those contemporary Hemi combustion chambers. That’s no small achievement for this vintage small-block.
The hydraulic lifters, like the camshaft, were also new in the engine. They were Comp parts from Summit Racing and matched the specs of the originals.
“I was confident we’d get to around 350 horses, but I was surprised how well it performed,” Barna says. “I think it proves how well the engine was engineered in the first place and how a well-conceived plan for the rebuild and good tuning can make the most of what the factory delivered.”
The T/A engine used Mopar’s “J” head, which would also be used on later Mopar 360 engines. Valley Performance left the originals on this engine in factory condition, performing only the necessary machine work for the rebuild.
The 340 Six-Barrel “T/A” engine was a one-year wonder from Mopar that made a significant statement for small-block performance capability. This rebuilt example demonstrates it was much more capable than the factory let on, and it suggests the original 340 Six-Barrel engineers knew exactly what they were doing. -MR
For the T/A engine, the J heads were fitted with 2.02-inch intake and 1.60-inch exhaust valves. Valley Performance replaced the original intake valves with stainless steel SI valves, which arguably improved airflow slightly. They also performed a three-angle valve job but were left intact as the original valve seats showed no signs of recession.The T/A engine was unique with offset intake rocker arms (specific to the left- and righthand sides) that were necessary to accommodate offset pushrods flanking thicker intake runners.The original shaft-mounted rockers were in good shape with this engine. For the rebuild, the shafts were polished, while the rockers were touch-honed and their foot pads were radius-ground to maintain consistent adjustment.The rocker arms on the T/A engine were adjustable, and here the preload for the hydraulic lifters was adjusted to 0.030 inch. The preload compensates for growth when the engine warms, eliminating lash. Also, the original valvesprings were replaced with new Comp Cams springs.Also unique to the T/A engine were the pushrods, which were 7.325 inches long, compared to the 7.500-inch length for small-blocks without adjustable rockers.The T/A’s induction system was composed of three Holley two-barrel carburetors mounted on an aluminum intake manifold painted orange like the rest of the engine. The central 350-cfm carburetor was flanked fore and after by vacuum-actuated companions.Edelbrock supplied the intake manifolds to Chrysler, just as the company did for the initial run of 440+6 engines on the 1969-1/2 Road Runner and Super Bee “A12” cars.The “LIST-4789” identifies one of the carburetors as the front vacuum-actuated part. The center carb carries list number 4791, and the rear carb is number 4790. Notably, they are not the same Holley list numbers on the 440+6 engine’s carburetors.Valley Performance rebuilt the carburetors and reinstalled them on the engine. The engine starts and runs at low engine speed/engine load on only the center carburetor. The other two carburetors kick in via a vacuum signal, like the vacuum secondaries on a Holley four-barrel.Here’s a look at the other side of the induction system, showing the mechanical linkage that controls the center carburetor.The rest of the engine assembly is composed of mostly stock and reused original equipment, but it wears a new water pump and a new double-row timing set behind the timing cover.Another of the engine’s subtle but practical upgrades was the change from points to an electronic ignition, complete with Mopar’s ubiquitous “orange box” ignition controller (not shown). Chrysler was the industry leader in the changeover to electronic ignition, which started in 1972.Here’s the completed engine assembly, an essentially stock rebuild with minor enhancements and upgrades that maintain the original appearance.Valley Performance’s John Lohone made a number of carb adjustments on the dyno, as the rebuilt carbs were instantly too lean. That was attributed to the modern pump gas flowing into them. Four jet changes were made before the air/fuel ratio stabilized.Starting at 34 degrees of total timing, the engine ultimately made its best pulls on the dyno at 36 degrees. That resulted in 375 hp at 5,600 rpm and a strong 389 lb-ft at 4,200 rpm. As for the dyno headers, they featured 1-5/8-inch primaries, 3-inch collectors, and 18-inch extensions. They likely had an advantageous effect on the engine’s performance, compared to the stock exhaust manifolds, but the engine was also down a full point of compression compared to the original specs. We’d call that a wash and find the overall results very healthy.
