Subaru Firing Order: Technical Encyclopedia – 1‑3‑2‑4, 1‑6‑3‑2‑5‑4, Engineering & Safe Practices
Why does the firing order matter for Subaru engines? Because boxer engines have opposing pistons moving horizontally, the firing sequence directly balances inertia forces. Incorrect order leads to catastrophic failure. Understanding this helps mechanics, tuners, and DIY owners maintain reliability and performance.
📐 Subaru H4 Firing Order (1‑3‑2‑4) – Engineering Unpacked
In every Subaru 4‑cylinder boxer (EJ20, EJ25, FA20, FB25, EA71), the firing order is 1‑3‑2‑4. Cylinder numbering: #1 (front left, driver side), #3 (rear left), #2 (front right, passenger side), #4 (rear right). The crankshaft has four throws arranged at 180° intervals, and the 1‑3‑2‑4 sequence provides firing every 180° of crankshaft rotation (since a 4‑stroke needs 720° for a full cycle, each cylinder fires once every two revolutions). This creates even combustion pulses and perfect primary balance without heavy balance shafts.
Watch the exact ignition sequence: each cylinder lights up when its power stroke occurs.
🚗 LEFT BANK (Driver side)
🚙 RIGHT BANK (Passenger side)
💡 Animation insight: The order repeats every 4 firing events, matching Subaru’s crankshaft journal arrangement.
🏎️ Subaru H6 Firing Order (1‑6‑3‑2‑5‑4) – Flat‑6 Excellence
Subaru’s 6‑cylinder boxer engines (EZ30, EZ36, ER27) use firing order 1‑6‑3‑2‑5‑4. Cylinder numbering (left bank front to rear: 1,3,5 ; right bank front to rear: 2,4,6). This order yields even 120° firing intervals (720°/6 = 120°), giving silky smooth power delivery and exceptional balance. It’s one of the most refined firing orders among mass‑production flat engines.
Interactive H6 sequence: see the spread of combustion pulses across both banks.
📌 LEFT BANK (1,3,5)
📌 RIGHT BANK (2,4,6)
⚙️ Why 1-3-2-4? Technical & Comparative Analysis
The Subaru boxer H4 crankshaft has crank pins at 0°, 180°, 180°, 0° (viewed from front). Firing order 1-3-2-4 means cylinders #1 and #2 fire 180° apart, #3 and #4 also 180° apart, avoiding consecutive fires on the same bank which would cause rocking. Advantages: Perfect primary force cancellation, reduced vibration, more uniform intake/exhaust pulsations, and the ability to use equal‑ or unequal‑length headers for tuning. Compared to an inline‑4 firing order 1-3-4-2 (common in many I4s), Subaru’s pattern is optimized for horizontal piston motion.
📊 Full Comparison Table: Subaru Firing Order by Engine Family
| Engine Series | Cylinders | Firing Order | Example Models | Crankshaft Angle (firing intervals) |
|---|---|---|---|---|
| EJ / FA / FB / EA | H4 | 1-3-2-4 | WRX STI, Forester, Outback, Crosstrek, BRZ | 180° – 540° – 180° – 540° (crank degrees) |
| EZ30 / EZ36 / ER27 | H6 | 1-6-3-2-5-4 | Legacy 3.0R, Outback H6, Tribeca, SVX (EG33 uses 1-6-3-2-5-4 as well) | 120° intervals |
| Subaru Diesel (EE20) | H4 diesel | 1-3-2-4 | Boxer Diesel (EU models) | Same as petrol H4 |
🔧 How To Verify / Check Subaru Firing Order (Step‑by‑Step)
- Identify cylinder numbers: Look for embossed marks near spark plug tubes. #1 front left, #2 front right.
- Check ignition coil pack: On distributorless systems, coil towers are often labeled with cylinder numbers. For wasted spark, pairs are (1&2) and (3&4) but actual firing still 1-3-2-4.
- Use a timing light: Clamp to each plug wire, observe flash order. Should match 1→3→2→4.
- ECU scan tool: View misfire counters or cylinder contribution; incorrect order will cause misfire codes on wrong cylinders.
- Reference service manual: Under‑hood sticker usually shows cylinder numbering and order diagram.
⚠️ Is It Safe To Change The Firing Order On A Subaru?
🚨 SAFETY ALERT: NEVER change the firing order on a stock Subaru engine. The crankshaft journal phasing, camshaft lobe positions, and ECU ignition timing maps are hard‑coded to a specific order. Modifying firing order (e.g., rewiring spark plugs to 1-2-3-4) will cause cylinder firing at wrong crank angles, leading to detonation, bent connecting rods, melted pistons, and immediate engine destruction. Even for race builds, altering firing order requires a custom billet crankshaft, standalone ECU, and extensive dyno tuning. For 99.9% of applications, keep OEM firing order 1-3-2-4 or 1-6-3-2-5-4.
✅ Advantages & ❌ Disadvantages of Subaru’s Firing Order Layout
Advantages
- Perfect primary & secondary balance: No need for balance shafts, reduces internal friction.
- Low center of gravity: Boxer layout + even firing = stable handling.
- Signature exhaust note: 1-3-2-4 with unequal headers creates the iconic Subaru rumble.
- Crankshaft durability: Even load distribution across main bearings.
- Scalable to 6 cylinders: H6 retains smoothness with 120° intervals.
Disadvantages
- Wider engine profile: Requires more lateral space, impacting serviceability.
- Spark plug access: Difficult on turbo models; often needs lifting engine or special tools.
- Exhaust complexity: Equal-length headers change firing pulse character (loss of rumble).
- Mistake sensitivity: A single misrouted plug wire can cause severe damage quickly.
🔍 Real‑World Use Cases: Why Mechanics & Tuners Must Know Firing Order
Understanding firing order is crucial for: diagnosing misfires (P030X codes), ignition system repairs, installing performance ECUs (e.g., Haltech, Link), building custom exhaust headers (unequal vs equal length), swap projects (matching engine to chassis wiring), and engine balancing modifications. For example, when installing a Link ECU on an EJ20, you must configure injection timing relative to firing order 1-3-2-4 to avoid cylinder wall washing.
