Boosting AMC AMX Performance with AFR 225 Heads: Cost, Installation, and Gains

The AMC AMX remains one of the most distinctive muscle cars of the late 1960s and early 1970s, offering a compact two-seat body and potent V8 engines like the 390 and 401. For modern enthusiasts looking to unlock serious horsepower, upgrading the cylinder heads is one of the highest-return modifications available. AFR 225 heads have become a top choice for AMC builders seeking significant airflow gains without sacrificing reliability. This comprehensive guide covers everything you need to know about selecting, installing, and tuning these cylinder heads on your AMC AMX, including a detailed cost breakdown, step-by-step installation advice, and real-world performance results.

Understanding AFR 225 Cylinder Heads

Air Flow Research (AFR) has built a reputation for producing some of the best aftermarket cylinder heads on the market. Their 225cc intake runner head is specifically engineered for big-inch AMC engines, including the 390, 401, and stroker combinations. These heads are available in several configurations, with options for straight or angle plug locations, different combustion chamber volumes (typically 58cc or 64cc), and valve sizes up to 2.08″ intake / 1.60″ exhaust.

Design Features and Flow Numbers

The AFR 225 head features a patented CNC-profiled intake port that delivers outstanding airflow right out of the box. On a 4.180″ bore, these heads flow approximately 285–295 cfm at 0.600″ lift on the intake side and 210–220 cfm on the exhaust side. This represents a massive improvement over stock AMC iron heads, which typically struggle to reach 220 cfm intake flow even after heavy porting. The compact combustion chamber design promotes better flame propagation and reduces the risk of detonation, allowing for higher compression ratios on pump gas.

Compatibility with AMC AMX Engines

AFR 225 heads are designed to bolt directly onto AMC V8 blocks (1966–1970 290/343/390, and 1971–1978 360/401). They use standard AMC exhaust port spacing and intake manifold bolt patterns, though some intake manifolds may require minor port matching. The heads are available with 2-bolt or 6-bolt exhaust flange patterns—the 6-bolt version fits most headers and manifolds designed for later AMC engines. Deck height and valve clearance are optimized for factory camshafts, but aftermarket cams with lifts above 0.550″ often require checking piston-to-valve clearance and may need upgraded valve springs (AFR ships heads with springs rated for 0.600″ lift as standard).

Cost Analysis for the AFR 225 Upgrade

Upgrading to AFR 225 heads is not a simple $2,000 bolt-on. The total investment often includes supporting parts and labor. Below is a realistic budget for an AMC AMX owner planning the swap.

Parts List with Estimated Prices

  • AFR 225 cylinder heads (pair) – $2,200–$2,600 depending on options (bare or assembled, chamber size). Available direct from Air Flow Research or distributors like Summit Racing.
  • Head gaskets – $80–$120 (Fel-Pro or Cometic MLS recommended for your bore size)
  • Head bolts or studs – $60–$150 (ARP stud kit preferred for consistent clamping)
  • Intake manifold gasket – $30–$50
  • Exhaust gaskets – $20–$40
  • New spark plugs – $30–$60 (AFR heads use 3/4″ reach plugs; angle plug heads demand short-style plugs)
  • Pushrods (if needed) – $60–$120 (aftermarket heads often require length checking)
  • Performance camshaft (optional but recommended) – $400–$600 (e.g., a hydraulic roller from Comp or Howards)
  • Intake manifold upgrade (optional) – $300–$700 (Weiand Stealth or Edelbrock Performer RPM)
  • Exhaust headers (if needed) – $400–$800 (Hooker or Hedman for AMC)
  • Professional labor (if not DIY) – $1,200–$2,000 depending on shop rates

Total estimated cost for a full upgrade (parts only): $3,200–$4,800. With labor and supporting mods, plan on $5,000–$7,000 for a turnkey installation.

Cost Comparison to Alternative Heads

Stock AMC iron heads can be rebuilt with hardened seats for about $800–$1,200 per pair, but they will never match the airflow of AFR 225s. Edelbrock makes aluminum heads for AMC (Performer RPM), but they typically flow 10–15% less than AFR 225s and cost about the same. Used AFR heads occasionally appear on forums for $1,500–$2,000, but inspecting for cracks or wear is critical. The AFR 225 upgrade offers the best power-per-dollar for naturally aspirated applications, especially when combined with a modest cam and 10:1 compression.

Step-by-Step Installation Guide

Installing AFR 225 heads requires mechanical skill, proper tools, and attention to detail. If you’ve never removed cylinder heads before, consider enlisting an experienced friend or a professional engine builder. Below is a general outline of the process for an AMC AMX 390 or 401.

Preparations and Required Tools

  • Torque wrench (ft-lb and in-lb capabilities)
  • Socket set, screwdrivers, pliers, gasket scraper
  • Engine stand (optional but helpful)
  • Feeler gauges for valve clearance checks
  • New coolant, oil, and filter
  • Shop manual for factory torque specs

Before starting, disconnect the battery and drain the cooling system. Remove the intake manifold, exhaust manifolds, valve covers, rocker arms, pushrods, and any accessories blocking access to the heads. Take photos to assist reassembly.

Removing Old Cylinder Heads

Unbolt the heads using the factory reverse-order torque sequence. Lift the heads straight off the block to avoid damaging the gasket surface or pistons. Place them on a clean workbench. Use a plastic scraper to remove old gasket material from the block deck—be careful not to gouge the aluminum surface if your block is aftermarket. Clean all bolt holes with a tap to remove debris and lubricant.

Block Preparation and Head Gasket Installation

Check the block deck for flatness with a straightedge (maximum 0.003″ warp acceptable). If your block has been previously decked, measure the piston deck height to ensure proper quench with the new heads. AFR heads typically have a combustion chamber of 58cc or 64cc; choose a gasket thickness that achieves your desired compression ratio. Install the head gaskets dry (or with a thin layer of copper spray if specified). Populate the head bolt holes carefully—some AMC blocks use different length bolts in different locations.

Installing AFR 225 Heads

Lower the AFR heads onto the gaskets, aligning the dowel pins. Install head bolts or studs hand-tight, then torque in three stages per the manufacturer’s recommendations. For example, using ARP studs on a 401: first step 50 ft-lb, second step 80 ft-lb, final step 100–105 ft-lb (check AFR instructions). Angle plug heads require careful routing of plug wires—use high-quality, heat-resistant boots. Install the spark plugs before attaching the intake manifold for easier access.

Reassembling Intake and Exhaust Systems

Use a new intake gasket (ideally the printoseal type) and apply a small bead of RTV at the front and rear china walls. Install the intake manifold with supplied bolts, torquing in a crisscross pattern to 25–30 ft-lb (or as specified). Reattach the exhaust manifolds or headers with new gaskets using a crisscross pattern at 20–25 ft-lb. Install pushrods and rocker arms after checking geometry (see next step). Set valve lash according to your cam specs (hydraulic lifters need 1/2 to 3/4 turn preload, solid lifters need 0.012–0.020″ clearance).

Final Checks and First Start

Fill the cooling system with 50/50 coolant mix, install the distributor, set initial timing around 10–15° BTDC (adjust later on dyno). Prime the oil system by cranking with the ignition disabled until oil pressure registers. Start the engine, bring it to operating temperature, and verify no leaks at the head gaskets, intake, or coolant passages. Check for vacuum leaks. After a 20-minute heat cycle, retorque the head bolts (if using a re-torque gasket) and adjust valve lash again. A professional dyno tune is highly recommended to optimize fuel and timing curves.

Expected Performance Gains

The real excitement comes after the first wide-open-throttle run. AFR 225 heads transform the AMC AMX from a respectable cruiser into a genuine street predator. Gains depend on the rest of your engine combination, but common results are well documented.

Dyno Results with Stock vs. AFR Heads

On a typical AMC 401 with 9.5:1 compression, a mild hydraulic roller cam (230–240° duration @ .050″), and a Performer RPM intake, AFR 225 heads add 50–80 horsepower over factory iron heads. Peak power often jumps from 350 hp to 425 hp at 5,800 rpm. On a 390 with a more aggressive cam (260°+ duration) and 10.5:1 compression, gains can exceed 100 hp, pushing output past 480 hp. For reference, Summit Racing customer reviews often quote similar numbers for AMC applications.

Torque Improvements and Driveability

Unlike some high-rpm heads that sacrifice low-end torque, AFR 225s maintain excellent low- and mid-range torque thanks to optimized port velocities. On a 401, torque increases by 40–60 lb-ft under 4,000 rpm, making the car feel quicker in daily driving. The broad powerband means you don’t need a stall converter or steep gears to enjoy the gains.

Combining with Other Mods

For maximum results, pair AFR 225 heads with a matching camshaft, such as the Comp Cams XR288HR for hydraulic roller, or a Howards Cams 310B11 for mechanical flat tappet. A good set of long-tube headers (1-3/4″ primary) and a dual-plane intake manifold complete the package. Many owners also switch to an electric fuel pump and larger carburetor (750–850 cfm) to feed the extra airflow. With these upgrades, a 401-powered AMX can eclipse 500 horsepower naturally aspirated, putting it firmly in the realm of modern muscle cars.

Conclusion

Upgrading to AFR 225 cylinder heads is one of the most effective performance investments for an AMC AMX. While the initial cost may seem steep, the combination of 50–100+ horsepower gains, improved torque curve, and exceptional build quality make them a favorite among AMC enthusiasts. Proper installation and tuning are critical—do not skip the supporting parts or the dyno session. With the right preparation, your AMC AMX can deliver a thrilling, reliable, and powerful driving experience that honors its muscle car heritage while outperforming many modern vehicles. For more technical data and community feedback, visit the AMC Forum or check out the AFR AMC head test article for in-depth flow comparisons.