What Is Injector End Angle?

Injector end angle is the crankshaft angle, measured in degrees, where fuel injection stops. It is usually shown as degrees Before Top Dead Center (BTDC).

In simple terms:

• The engine rotates 720° in one full 4-stroke cycle.
• Injection must happen within a limited window.
• The timing of when injection ends affects:
  • Air-fuel mixing
  • Combustion efficiency
  • Power output
  • Emissions
  • Injector duty cycle

Ending injection too late can cause poor combustion or fuel escaping during valve overlap. Ending too early may reduce cooling effects or mixture quality in some setups.

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Why Injector End Angle Matters

Fuel injection timing affects:

• Power under load
• Throttle response
• Idle quality
• High RPM safety
• Injector sizing

For example:

• At high RPM, available time per cycle shrinks.
• If injectors stay open too long, duty cycle rises.
• Above 85% duty cycle, injectors risk running static.

This calculator helps prevent those problems.

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How the Injector End Angle Calculator Works

The calculator uses your engine inputs to estimate:

• Required fuel mass
• Injector pulse width
• Crank angle duration
• Injection start angle
• Target injection end angle
• Injector duty cycle

It also provides warnings if your injectors are undersized or oversized.

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Key Inputs Explained

1. Engine Speed (RPM)

Higher RPM means:

• Less time per engine cycle
• Shorter injection window
• Higher injector duty cycle

Formula used:
Cycle Time (ms) = 120,000 / RPM

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2. Injector Flow Rate (cc/min)

This determines how much fuel the injector can deliver.

Typical ranges:

• Stock 4-cylinder: 200–350 cc/min
• Performance builds: 550–1000 cc/min
• High power turbo: 1000+ cc/min

If flow rate is too small, duty cycle increases quickly at high RPM.

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3. Cylinder Displacement (cc)

Fuel demand depends on cylinder size.

Examples:

• 2.0L 4-cylinder → 500cc per cylinder
• 6.2L V8 → 775cc per cylinder

Larger cylinders need more fuel per cycle.

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4. Target Air-Fuel Ratio (AFR)

AFR determines how rich or lean the mixture is.

Common AFR targets:

• Power: 11.5–12.5
• Cruise: 14.7–15.5
• Lean cruise: 16–17

Lower AFR (richer mixture) requires more fuel and increases pulse width.

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5. Fuel Pressure (psi)

Injector flow changes with pressure.

The calculator adjusts pulse width using:

Pressure correction factor = √(Fuel Pressure / 43.5)

If pressure increases, required pulse width decreases.

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6. Injection Strategy

You can select:

• Port Fuel Injection (PFI)
• Direct Injection (DI)
• Dual Injection (PFI + DI)

Each strategy calculates end angle differently.

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Injection Strategy Explained

Port Fuel Injection (PFI)

Fuel is injected into the intake port before the intake valve.

Best practice:
Injection should end before intake valve closing (IVC).

The calculator:

• Accounts for intake runner length
• Estimates fuel travel time
• Adjusts end angle relative to IVC

If injection ends after valve closing, fuel may escape during overlap.

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Direct Injection (GDI / DI)

Fuel is injected directly into the combustion chamber.

DI allows:

• Late injection for charge cooling
• Stratified charge operation
• Higher compression ratios

The calculator adjusts end angle based on RPM:

• < 4000 RPM → Later injection
• 6000 RPM → Earlier end angle

At high RPM, the window becomes very small.

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Dual Injection (PFI + DI)

Dual systems use both injection types.

Typical strategy:

• PFI for low load and emissions
• DI for high load and power

The calculator blends timing to prevent fuel film interference.

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Understanding the Output

After calculation, you receive:

1. Target Injection End Angle (° BTDC)

This is the main result. It shows when injection should stop.

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2. Injection Start Angle

Calculated as:

Start Angle = End Angle − Adjusted Pulse Width

If negative, it wraps within the 720° cycle.

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3. Pulse Width (ms)

Time injector stays open.

Higher pulse width means:

• More fuel
• Higher duty cycle

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4. Injector Duty Cycle (%)

Duty cycle = (Pulse Width / Available Injection Time) × 100

Guidelines:

• Below 30% → Injectors may be too large
• 40–80% → Healthy range
• Above 85% → Risk of lean condition

The calculator provides automatic warnings.

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Example Scenario

Let’s say you enter:

• 6000 RPM
• 550 cc injectors
• 500cc cylinder
• 12.5 AFR
• 43.5 psi fuel pressure
• Port injection
• 250mm intake runner
• 40° IVC

The calculator will:

• Compute fuel mass
• Convert to volume
• Determine pulse width
• Convert pulse width to crank degrees
• Adjust for pressure
• Calculate end angle relative to valve closing

You also receive a small RPM comparison table to see how timing and duty cycle change from 1000 to 8000 RPM.

That helps you spot problems before they happen.

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Common Tuning Warnings Explained

High Duty Cycle Warning

If duty cycle exceeds 85%:

• Injector may run static
• Engine may lean out at high RPM
• Larger injectors are recommended

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Low Duty Cycle Warning

If below 30%:

• Idle may become unstable
• Poor transient response
• Resolution issues at low pulse widths

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Intake Valve Conflict (PFI Only)

If injection ends too late:

• Fuel may exit during valve overlap
• Hydrocarbon emissions increase
• Combustion stability suffers

The calculator flags this condition.

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Benefits of Using an Injector End Angle Calculator

Using this tool helps you:

• Size injectors properly
• Avoid lean high-RPM conditions
• Optimize combustion timing
• Improve throttle response
• Reduce tuning guesswork
• Protect engine under boost

It removes trial and error from fuel timing decisions.

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Best Practices for Accurate Results

1. Enter realistic fuel pressure values.
2. Use actual injector flow ratings.
3. Verify camshaft intake valve closing specs.
4. Monitor real-world AFR with a wideband sensor.
5. Stay below 85% injector duty cycle.

This calculator is a planning tool. Final tuning should always be validated on a dyno or with proper data logging.