A Brief History of the AMC AMX

The AMC AMX (American Motors eXperimental) debuted in 1968 as a two-seat muscle car that stood out from the crowd. Unlike the big-block offerings from GM, Ford, and Mopar, AMC packed its own V8 power into a lighter, more nimble platform. The AMX was produced through 1970 in its two-seat form, then continued as a Javelin-based muscle coupe until 1974. Engine choices ranged from the 290 V8 up to the legendary 390 and 401 cubic-inch mills. These engines are known for their robust design: thick cylinder walls, a strong block, and a large displacement potential that makes them ideal for performance builds. With the right combination of parts, a 390 or 401 AMC engine can easily surpass 400 horsepower — and even approach 500 hp with careful component selection.

Achieving that magic 400+ hp threshold, however, requires more than just slapping on a big carburetor. The factory heads and camshaft are the primary airflow bottlenecks. By upgrading the camshaft and performing careful cylinder head work, owners can unlock the true potential of these iconic AMC V8s. This article walks through the specific modifications, supporting upgrades, and tuning steps needed to reach four-digit (in fractions) horsepower goals.

Understanding the AMC AMX Engine

The most common AMC V8s used in AMX performance builds are the 390 (introduced in 1968) and the 401 (added in 1971). Both share the same basic architecture: a cast-iron block with a 4.165-inch bore (390) or 4.165-inch bore with a 3.68-inch stroke (401), forging a strong foundation. The stock cylinder heads feature relatively small ports and valves compared to modern designs, and the factory camshaft profiles are mild for drivability and emissions. While those parts work fine in a stock cruiser, they limit airflow at higher RPMs.

To reach 400+ hp, you must improve the engine’s volumetric efficiency — how well it fills and empties the cylinders. The two biggest factors are the camshaft’s timing (lift, duration, lobe separation) and the cylinder head’s flow characteristics (port cross-section, valve size, and bowl shape). Supporting systems — intake, exhaust, fuel delivery, and ignition — must then match the increased airflow capacity.

Key Measurement: Horsepower per Cubic Inch

A healthy 400 hp from a 390 or 401 cubic-inch engine works out to about 1 hp per cubic inch. That is attainable with street-friendly parts. For example, a 401 with good heads, a moderate cam, a 750 cfm carburetor, and headers can easily make 420–450 hp at the flywheel. To go beyond 500 hp, you’ll need more aggressive cam profiles, ported heads with larger valves, and higher compression. This guide focuses on the 400–500 hp sweet spot, which is still very streetable and reliable.

Camshaft Upgrades: The Heart of the Build

The camshaft controls valve timing: how long the intake and exhaust valves stay open (duration), how far they open (lift), and the overlap period (lobe separation angle). For an AMC 390/401 targeting 400+ hp, the cam specs matter enormously.

Flat Tappet vs. Roller Camshaft

Original AMC V8s came with flat-tappet camshafts, which use a sliding contact between the cam lobe and lifter face. Flat tappet cams are affordable and work well with proper break-in (moly lube, high-zinc oil). However, aggressive flat-tappet profiles can cause excessive wear on the lobe and lifter, especially with higher spring pressures. A roller camshaft uses a roller bearing at the lifter tip, reducing friction and allowing much more aggressive lobe profiles (faster opening rates, more area under the curve). For AMC engines, popular roller cam choices come from Bullet Cams, Comp Cams, or custom grinds. Roller cams are the preferred choice for 400+ hp builds because they deliver more power with less risk of lobe failure.

For a street/strip AMC 390 or 401, target the following range:

  • Intake Duration at 0.050 inch lift: 230° to 245° (hydraulic roller) or 240° to 255° (solid roller)
  • Exhaust Duration at 0.050 inch lift: 240° to 255° (split pattern, slightly more exhaust)
  • Valve Lift: 0.550 to 0.650 inch (with 1.6 or 1.7 rocker ratio)
  • Lobe Separation Angle (LSA): 108° to 112° (tighter LSA builds more midrange torque but can hurt idle quality)

For example, a Comp Cams 268HR (hydraulic roller) offers 224°/234° at 0.050 and 0.566/0.573 lift on 112° LSA, which is a great starting point for 400 hp. A more aggressive Bullet custom solid roller with 250°/256° at 0.050 and 0.650+ lift can push power near 500 hp but requires stiffer springs and more precise tuning.

Valve Train Upgrades

With a performance cam, you must upgrade the valve springs to handle higher lift and prevent float at high RPM. Spring pressure should be around 140–160 lbs on the seat and 350–400 lbs open. Also consider using a hardened pushrod (3/8-inch diameter, 4130 chrome-moly) and a roller timing chain set. For AMC V8s, aftermarket aluminum rocker arms (such as Harland Sharp or T&D Machine) with roller fulcrums reduce friction and allow precise geometry adjustment.

Cylinder Head Modifications: Freeing Up the Intake and Exhaust Flow

The stock AMC cylinder heads (casting numbers like 319-5, 322, 291-C, etc.) have decent iron but small ports, especially the exhaust. The intake port volume is roughly 160–170 cc, which is fine for 400 hp with proper work. For 500+ hp, you need aftermarket heads with larger runners and improved bowl design.

Porting and Polishing

Professional porting focuses on three areas: the valve bowl (just below the valve seat), the short-turn radius (the floor of the port as it curves around the valve), and the pushrod pinch area (the narrowest part of the port on AMC heads). The goal is to increase airflow without sacrificing velocity. Removing casting flash and smoothing the port floor can gain 30–50 cfm on the intake side. On the exhaust side, enlarging the port just enough to match the header primary tube size is critical. A good set of ported AMC heads should flow 280–300 cfm intake and 200–220 cfm exhaust at 0.600 lift.

Larger Valves

Stock AMC valves are typically 2.02-inch intake and 1.60-inch exhaust. For 400+ hp, upgrade to 2.08-inch intake and 1.65-inch exhaust (or even 2.10/1.70 with aftermarket heads). The larger valves improve flow but may require new seats and a valve job (multi-angle). Use quality stainless steel valves with swirl-polish stems to reduce carbon buildup.

Aftermarket Cylinder Heads

If your budget allows, aftermarket aluminum heads provide the best path to high horsepower. Companies like Edelbrock, Indy, and Rocket Brand offer heads designed for AMC V8s. For example, the Edelbrock Performer RPM (part #60199 or #60599) have 190 cc intake ports, 2.08/1.65 valves, and flow over 290 cfm out of the box. They also save about 50 lbs over iron heads, reduce detonation risk, and accept larger springs for aggressive camshafts. With these heads, achieving 450+ hp is straightforward with a matching cam and intake.

Supporting Modifications: Don't Starve the Beast

Once the cam and heads increase airflow, the rest of the engine must keep up. Think of these as the foundation for the cam and head work to shine.

Intake Manifold and Carburetor

For a street-driven AMX, a dual-plane intake like the Edelbrock Performer RPM AMC (part #7511) or Holley Street Dominator AMC provides excellent torque and mid-range power. A 750 cfm vacuum-secondary carburetor (Holley 4150 or Edelbrock AVS) works well for 400–450 hp. For 500 hp, step up to an 850 cfm carburetor. If you prefer fuel injection, options like the Holley Sniper EFI or FiTech Go EFI are available for AMC V8s with a custom throttle body adapter.

Headers and Exhaust System

Headers are non-negotiable for a 400+ hp build. Look for 1-3/4-inch primary tubes with 3-inch collectors, such as those made by Hedman Hedders or Hooker (available for AMC Javelin/AMX). For even more power, 1-7/8-inch primaries can help at higher RPM, but may sacrifice a bit of low-end torque. The exhaust system should use 3-inch mandrel-bent tubing with low-restriction mufflers (Flowmaster 40 series, Borla, or MagnaFlow). A crossover (H-pipe or X-pipe) balances pressure pulses and adds torque.

Ignition System

A high-energy ignition system ensures complete combustion under higher cylinder pressures. An MSD Pro-Billet distributor (PN 8592 for AMC) with an MSD 6AL box and a high-voltage coil (Blaster SS or HVC) is a proven combination. Set the initial timing to 12–16 degrees and total advance to 32–36 degrees, all in by 3000 rpm. Use quality spiral-core spark plug wires and iridium plugs (gapped at 0.045–0.050 inch).

Fuel System

A mechanical fuel pump from Carter or Edelbrock with a regulator (set to 6–7 psi for carburetors) is adequate for 450 hp. For EFI or higher power, use an in-tank electric pump (like a Walbro 255 lph) and -8 AN supply lines. Always install a fuel pressure gauge at the carburetor to monitor delivery.

Bottom-End: Building for Reliability

400+ hp doesn’t require a fully forged bottom end, but a few upgrades ensure longevity. The AMC 390/401 factory forged steel crankshaft is extremely strong — it can handle 600+ hp. The connecting rods are forged (in 390/401) but can benefit from resizing and ARP rod bolts. For sustained high RPM use, consider forged H-beam rods (Eagle, Scat) and forged pistons (Keith Black, Mahle) that raise compression to 10.0–10.5:1. The factory oiling system is also good, but a high-volume oil pump (Melling M864HV) and a windage tray are recommended.

Tuning for Maximum Performance

Parts alone don’t make power; careful tuning converts airflow and fuel into horsepower.

Carburetor and Air-Fuel Tuning

Start with jets about two sizes richer than a “baseline” setting for your cam and heads. Use an air-fuel ratio gauge (wideband O2 sensor) during dyno tuning. Target 12.8–13.2:1 at wide-open throttle. Adjust the idle mixture screws for highest vacuum (14–16 inches of vacuum with a moderate cam). For a cam with 240°+ duration, expect idle vacuum to drop to 10–12 inches; you may need an adjustable vacuum secondary canister.

Ignition Timing Tuning

The ideal total advance for an AMC V8 is typically 32–36 degrees, with full advance reached by 2800–3000 rpm. Too little timing leaves power on the table; too much causes detonation. Use a dial-back timing light to verify centrifugal and vacuum advance curves. With a hot cam, you may need to limit vacuum advance to 10–12 degrees to avoid pinging.

Dyno Sessions: The Final Validation

A properly tuned 390 with ported heads, a roller cam (240° at 0.050), 10:1 compression, Edelbrock RPM intake, 750 vac sec, and headers will typically produce 440–460 hp at 5800 rpm and 470 lb-ft torque at 4200 rpm. The 401, with its longer stroke, can hit 480 hp and 500 lb-ft with the same components.

Real-World Examples and Dyno Results

Forums like The AMC Forum abound with builds that prove the formula. One enthusiast paired a Bullet custom hydraulic roller (240/246 @0.050, 0.600 lift) with Edelbrock aluminum heads, a Performer RPM intake, and a 750 Holley. After porting, the engine made 465 hp at 6200 rpm and 490 lb-ft at 4500 rpm in a ’69 AMX. Another builder used stock iron heads (port-matched, 2.08 valves) with a Comp Cams 268HR and a Stinger intake, achieving 420 hp — all on a budget.

Conclusion: Enjoy the Journey

Rebuilding an AMC AMX engine to 400+ horsepower is a rewarding project that respects the car’s heritage while delivering modern performance. The key is a systematic approach: select a camshaft that matches your RPM range, invest in cylinder head flow (porting or aftermarket), and don’t skimp on supporting mods like the intake, carb, headers, and ignition. With proper tuning, your AMX will not only hit the four-digit horsepower mark but also provide a thrilling driving experience that turns heads at every stoplight. Whether you choose a mild hydraulic roller for everyday street driving or a more aggressive solid roller for weekend drag strips, the AMC V8 is more than capable of delivering the power you’re after.