Introduction

If MAF g/s tells you how much air the engine breathes, Injection Quantity (IQ) tells you how much fuel the ECU is feeding that air.

Yet many people see “mg/stroke” in logs and have no idea what it really means. Some tuners even increase injection blindly without understanding how the ECU converts fuel mass into torque. That’s how smoky, inefficient, or unsafe maps are born.

This guide explains:

• What injection quantity actually represents
• How mg/stroke relates to torque
• Why torque limiters matter more than raw fuelling
• How to spot bad fuelling from live data

Once you understand IQ properly, reading diesel logs becomes straightforward instead of mysterious.

What “mg/stroke” Really Means

Injection quantity is shown as:

milligrams of fuel injected per engine stroke per cylinder

So if you see:
• 50 mg/stroke

That means:
Each cylinder receives 50 mg of fuel every power stroke at that moment.

Because engines have multiple cylinders and thousands of strokes per minute, small mg/stroke changes create big power differences.

How the ECU Decides Injection Quantity

Modern ECUs don’t start by choosing fuel. They start by choosing torque.

The process is:

Driver presses throttle
↓
ECU calculates requested torque
↓
Torque request passes through limiters
↓
ECU converts torque to required fuel mass
↓
Injection quantity is commanded

So IQ is the result, not the starting point.

That’s why simply increasing fuel maps without understanding torque structure produces poor results.

Typical Injection Quantity Ranges

Rough real-world figures for common-rail diesels:

• Idle: 3–8 mg/stroke
• Cruising: 10–30 mg/stroke
• Stock full load: 45–60 mg/stroke
• Stage 1 tuned: 60–80 mg/stroke
• High-performance builds: 80–100+ mg/stroke

Exact values vary by engine, injector size, and rail pressure.

How mg/stroke Relates to Torque

More fuel = more combustion energy = more torque.

But only if:
• Enough air is present
• Injection timing is correct
• Rail pressure supports clean atomisation

If fuel exceeds available air:
• Smoke appears
• EGT rises
• Efficiency drops

That’s why IQ must always be viewed together with MAF g/s.

Air first. Fuel second. Always.

The Smoke Limiter Connection

Every diesel ECU has a smoke limiter map.

It says:
“For X airflow, maximum fuel allowed is Y mg/stroke.”

If a bad map ignores this relationship:
• Fuel exceeds air
• Black smoke
• High EGT
• Dirty combustion

A good map keeps IQ matched to airflow.

Spotting Bad Fuelling in Logs

Healthy tuning:

• MAF g/s rises
• IQ rises proportionally
• No sudden fuel spikes
• Lambda stays clean
• No smoke

Bad tuning:

• IQ jumps sharply
• MAF doesn’t match
• Smoke appears
• EGT climbs fast

In logs this looks like:
• IQ = 75 mg/stroke
• MAF = 190 g/s

That fuel level needs ~230–240 g/s air.
Mismatch = smoky or stressed engine.

Rail Pressure’s Role

Injection quantity is delivered by:

• Rail pressure
• Injector open time

If rail pressure is too low:
• ECU extends injector duration
• Combustion becomes less precise
• Smoke increases

That’s why rail pressure logs should always accompany IQ logs in proper diagnostics.

Why Torque Maps Matter More Than Fuel Maps

Modern ECUs are torque-based.

A professional tuner:
• Raises torque targets
• Adjusts torque limiters
• Lets ECU calculate correct fuel
• Then fine-tunes smoke limiter

A poor tuner:
• Directly raises fuel maps
• Fights ECU torque logic
• Creates unstable or smoky behaviour

Understanding IQ helps you instantly see which approach was used.

Practical Example

Stock log:
• MAF: 185 g/s
• IQ: 52 mg/stroke

Tuned log:
• MAF: 235 g/s
• IQ: 68 mg/stroke

Healthy relationship:
• Air increased
• Fuel increased proportionally
• Clean power gain

Bad example:
• MAF: 200 g/s
• IQ: 80 mg/stroke

Diagnosis:
• Over-fuelling
• Smoke likely
• Turbo and DPF stress
• Poor tuning strategy

The Takeaway

Injection quantity is not just “how much fuel.”
It’s the ECU’s translation of torque demand into combustion energy.

When you understand mg/stroke:

• You read logs properly
• You spot bad maps instantly
• You protect engines from abuse
• You tune with precision instead of guesswork

Fuel without understanding is gambling.
Fuel with data is engineering.

Want Your Fuelling Checked?

If you’re unsure whether your current map is clean and safe, we offer log-based fuelling analysis. Send your MAF and IQ logs and we’ll tell you if your tune is balanced or over-fuelled.