When the American Motors Company debuted the AMX as a mid-year 1968 model, it became the only steel-bodied two-seat vehicle being made in the States--or maybe anywhere, but we didn't research it that far.
The AMX, which stands for American Motors eXperimental, was supposed to be AMC's halo car, similar to Chevrolet's Corvette. That may sound ludicrous, but the car actually competed in the SCCA's B Production class against cars like the Shelby GT350 and actually wound up second in the 1969 final standings. Equipped with AMC's 390 engine the car was capable of a quarter mile in 14 seconds and zero-to-60 in less than seven.
But despite AMC's best efforts at making a pretty cool car that was still affordable to the masses, low sales doomed the AMX after just three years and total sales of just over 19,000 cars. Those low production numbers, however, have only served to make the AMX more beloved by AMC fans as well as Mopar nuts who have adopted the make.
So when we found out engine builder Keith Dorton at Automotive Specialists in Concord, NC, was building an AMC 390 that will be going into a 1969 AMX we dropped everything to go see it. Of course, Dorton is a dyed in the wool racer--among his many accomplishments includes building an engine that won the Daytona 500--so a boring, straight-up rebuild was certainly out of the question. But the fact that there is very little in the aftermarket for old AMC engines means a little ingenuity is required.
"Rebuilding an engine stock is no fun, so we put a few tricks to it to squeeze out some more horsepower," Dorton says matter of factly. "It's all stuff I've learned from racing, but really, there's nothing magic about it. Anyone can do it."
In 1969 the AMX 390 equipped with a four barrel carb was rated at 315 horsepower and 415 foot/pounds of torque, making it the most powerful in AMC's arsenal. When Dorton finished with the engine, the dyno revealed corrected numbers of 525 horsepower at 6,500 rpm and 492.3 foot/pounds of torque at 5,100. Not bad for an engine that runs on pump gas and will be equally at home at the local cruise-in or an autocross track.
The 390 V8 AMC put together for its new AMX sports car used a cutting-edge (at least back then) thin walled block casting, with a 4.165 inch bore and 3.574 inch stroke. Even with a forged crank and rods, it produced just 315 horsepower. Dorton's updated rebuild upped the ante to over 500 horsepower while remaining pump-gas friendly.
Dorton says he had trouble finding an AMC connecting rod that met his requirements, so he went with a 6.125 inch Chevy H-beam rod from Molnar Technologies. Of course, the problem with running a Chevy rod in an AMC engine is the offset is wrong, pushing the rod off-center on the wrist pin.
Finding a tri-metal performance bearing with a harder babbit than stock also proved problematic. Instead, Dorton found that Ford bearings have the correct housing bore and journal diameters. The tangs don't match up, so they were machined off in a lathe. Dorton says cutting the tangs off doesn't increase the likelihood of spinning a bearing--the proper amount of bearing crush will lock them in place--you just have to be careful about lateral placement before bolting down the rod and main caps. Calico coated the ACL bearings for added protection. Because of the unique width of the crankshaft main journals, Dorton had to bite the bullet and use a standard AMC unit for the thrust bearing.
The only custom piece on the build is the set of pistons Diamond made to get the right compression height to match the long connecting rod and extra stroke (more on that later). These forgings are set up with Chevy-size .927 pins and use a 1.240 compression height. Instead of cutting a shallow dish, both valve pockets are cut to intake valve size to get the compression down a bit. The ring pack is a lightweight .043/.043/3 mm setup, which used to be the exclusive territory of race engines but now is becoming more widely accepted in street engines.
To get the connecting rod centered on the piston's wrist pin, Dorton chucked each rod in a lathe and cut a new chamfer on the opposite face. The rods are installed "backwards" in the block, but now everything lines up appropriately.
The original forged crank was kept, but to make it fit up with the Chevy rods, it was sent to Shaftech to have the rod journals offset ground down to standard 2.100-inch Chevy size. This also brings the stroke up from 3.547 inches to 3.625. Besides grinding the crank, ShafTech also nitrided it to help protect the crank from the 390's less-than-ideal oiling gallery design.
The final 4.200-inch bore combined with the additional stroke brings total displacement up to 402 cubic inches. Dorton ditched the original cast iron cylinder heads for a more modern design from Edelbrock cast from aluminum, so MLS head gaskets from Cometic are laid in place.
Modern ports and combustion chambers are a must when trying to make naturally aspirated power, so the cast iron bricks were set aside for a pair of Edelbrock Performers. The heads come set up with 2.020/1.60 inch intake and exhaust valves mated to 45 degree seats which are proven to flow more than the 30-degree seats cut into the stock heads.
The valvetrain is built around a solid roller camshaft from Crane Cams with 248/256 degrees of duration at 0.050 and 0.400 lift. With a set of Comp Cams Ultra Gold aluminum rockers in 1.6:1 ratio, that brings total valve lift to 0.640 inches of lift for both the intake and exhaust valves.
Automotive Specialists' Tony Corrente installs the valvetrain, including pushrods from Trend Performance. It may be difficult to find high-performance roller lifters specifically for AMC engines, but the blocks share the same lifter bore diameter as Mopar engines, so Dorton sourced a set of solid rollers from Crane Cams originally designed for Dodges and Chryslers.
The AMC V8 uses a distinctive timing cover that also incorporates mounts for the oil pump, fuel pump and distributor. Instead of trying to refurbish the grungy old one, Dorton sourced this new cover from Summit Racing.
Corrente drops the MSD distributor into its location in the timing cover. Incidentally, the 390 uses a distributor that's interchangeable with Ford's. Note also the sweet finned valve covers that were supplied by Edelbrock to go with the cylinder heads.
Milodon offers a road-race pan for the AMC with baffles and trap doors to help maintain proper oil control even under high-G loading, so that was the obvious choice for this build. The shallow design with kickouts allow for adequate ground clearance in a lowered chassis while still holding six quarts of oil (not counting the filter).
A March Performance front accessory drive setup uses a serpentine belt to spin the alternator on the right and an air conditioning compressor on the left. Neither are hooked up for the dyno session. A power steering pump can be added later if the driver wants it.
Except for the Holley fuel pump, everything that will be installed in the AMX was also run on the dyno. That includes the Holley Ultra XP carb, a pair of Hooker long tube headers, MSD plug wires and those fantastic Edelbrock ribbed valve covers.
Automotive Specialists; 704/786-0187; (engine assembly)
ACL Performance Engine Bearings; 800/847-5521; (bearings)
ARP; 800/826-3045; (fasteners)
Calico Coatings; 704/483-2202; (bearing coatings)
Centerforce; 928/771-8422; (clutch, pressure plate)
Cloyes Gear; 479/646-1662; (timing set)
Cometic Gasket; 800/752-9850; (gaskets)
Comp Cams; 901/795-2400; (rocker arms)
Crane Cams; 866/388-5120; (camshaft, lifters)
Diamond Pistons; 877/552-2112; (pistons, rings, pins)
Edelbrock; 800/416-8628; (cylinder heads, valve covers, intake, water pump)
Holley; 270/782-2900; (carburetor, fuel pump, regulator)
Hooker; 866/464/6553; (headers)
March Performance; 888/729-9070; (accessory drive)
Milodon; 805/577-5950; (oil pan, pickup)
Molnar Technologies; 616/940-4640; (connecting rods)
MSD Performance; 915/857-5200; MSDPerformance.com; (distributor, plug wires)
ShafTech; 419/435-6372; (crank modification)
Summit Racing Equipment; 800/230-3030; (front cover, oil pump, various)
Trend Performance; 586/447-0400; (pushrods)