If your vehicle has been converted to CNG or LPG and you’ve noticed sluggish acceleration, engine knock, or higher fuel bills, ignition timing is likely the culprit. That’s exactly the problem a timing advance processor (TAP) is built to fix.
A TAP is a compact electronic device installed between your engine’s sensors and ignition system. It intercepts the timing signal and adjusts when the spark plug fires — ensuring complete combustion regardless of which fuel is in the tank. In this guide, you’ll learn what a timing advance processor is, how it works, which sensors it relies on, and how to tell if your engine needs one.
What Is a Timing Advance Processor?
A timing advance processor is an electronic control unit that modifies your engine’s ignition timing signal in real time. It sits between the engine’s position sensors and the ignition coil, intercepting the signal and shifting it earlier or later depending on the fuel being burned. It doesn’t replace the ECU — it works alongside it.
Most engines are factory-calibrated for petrol. When you switch to CNG or LPG, the combustion characteristics change and the original timing no longer works efficiently. A TAP corrects for this automatically, without any ECU reprogramming.
The device itself is small — roughly the size of a relay box — and mounts under the bonnet near the ignition wiring. Once installed, it operates invisibly in the background every time you drive.
Why Ignition Timing Matters for Engine Performance
Ignition timing determines exactly when the spark plug fires inside each cylinder. The goal is for peak combustion pressure to arrive just after the piston reaches Top Dead Centre (TDC) — roughly 10 to 15 degrees past it. To achieve that, the spark must fire before TDC, giving the flame front time to develop fully.
Fire too early and combustion pressure pushes against the still-rising piston, causing knock and long-term damage to pistons, bearings, and cylinder walls. Fire too late and peak pressure is wasted — power drops, fuel efficiency falls, and exhaust temperatures rise.
The ideal timing angle also isn’t fixed. It shifts constantly based on engine RPM, load, fuel type, and air temperature. A timing advance processor monitors these variables continuously and adjusts the spark timing accordingly, hundreds of times per second.
How a Timing Advance Processor Works
The TAP intercepts the low-voltage timing signal from the crankshaft or camshaft position sensor before it reaches the ignition coil. It analyses the signal in real time — checking RPM, engine load, and whether the vehicle is running on petrol or gas — then applies a programmed timing offset.
For CNG and LPG, the spark is typically advanced by 6 to 15 degrees of crankshaft rotation compared to the factory petrol calibration. This is necessary because alternative fuels have a slower flame propagation speed and need more time to burn completely before the piston starts its downward power stroke.
During idle and deceleration, most TAPs disengage the advance automatically to prevent rough running. They also include an emergency bypass connector: if the device fails, it reverts the vehicle to standard factory timing so you can continue driving safely.
Engine Sensors That Feed the Timing Advance Processor
The TAP relies on live data from several sensors to calculate the correct timing at any given moment:
- Crankshaft Position Sensor (CKP): Tracks piston position and engine speed — the primary input for all timing decisions
- Manifold Absolute Pressure (MAP) Sensor: Measures engine load; high load requires more conservative timing to prevent knock
- Throttle Position Sensor (TPS): Detects rapid acceleration, which requires a different timing curve than steady cruising
- Knock Sensor: Listens for detonation vibrations; triggers immediate timing retard if knock is detected
- Coolant Temperature Sensor (CTS): Adjusts timing during cold starts when the engine behaves differently
The TAP cross-references this sensor data against a pre-programmed ignition map — a lookup table of ideal timing values across different RPM and load conditions — and outputs the corrected signal to the ignition coil in milliseconds.
Why CNG and LPG Engines Need Timing Correction
Petrol, LPG, and CNG all have different combustion characteristics. CNG and LPG burn more slowly than petrol and have significantly higher octane ratings. When a petrol-tuned engine switches to these fuels without timing correction, the spark fires at the wrong point in the combustion cycle.
| Property | Petrol | LPG | CNG |
|---|---|---|---|
| Octane Rating | ~95 | ~105 | ~120–130 |
| Flame Speed | Moderate | Slower | Slowest |
| Needs Earlier Ignition? | No | Yes | Yes |
Without a timing advance processor, a converted engine typically loses 10–20% of its power output, consumes more fuel, idles roughly, backfires through the intake, and builds carbon deposits from incomplete combustion. A properly calibrated TAP eliminates all of these issues by ensuring the spark matches the fuel actually in the cylinder.
Real-World Benefits of a Timing Advance Processor
The most immediate improvement drivers notice after fitting a TAP is restored throttle response. By aligning peak cylinder pressure with the power stroke, acceleration feels sharper and more predictable — much closer to how the vehicle performed on petrol.
Fuel efficiency also improves noticeably. Optimised combustion extracts more energy from each unit of fuel, and CNG/LPG conversion users consistently report efficiency gains of 5 to 10 percent compared to running without timing correction.
Long-term, a TAP protects the engine. Complete combustion means less carbon buildup, lower exhaust temperatures, and no knock-related wear on piston rings, bearings, and cylinder walls. For the cost of the device, it’s one of the most practical investments in any gas conversion.
Types of Timing Advance Processors
Analog TAPs are designed for older vehicles with breaker-point ignition systems. They intercept the distributor signal to the coil with fixed or semi-adjustable timing curves. The AEB531 is a common example for mechanical ignition setups.
Electronic Ignition TAPs handle signals from electronic ignition modules rather than mechanical contact breakers. They switch faster and are compatible with systems like AEB526N and AEB549N. Reprogrammable (digital) TAPs such as the AEB515N go a step further, allowing full timing curve customisation via laptop software — useful for unusual engines or performance builds.
CKP-Compatible TAPs are the most common type for modern vehicles. They process digital pulse signals from the crankshaft position sensor rather than analogue distributor signals, and they’re standard on most vehicles manufactured in the last 15–20 years. When choosing a TAP, match it to your ignition system type, fuel type, and LPG/CNG kit brand.
Timing Advance Processor vs ECU Remap
An ECU remap directly overwrites the values inside your engine control unit — timing maps, fuelling parameters, and other calibration data. It’s a permanent change requiring a specialist and is difficult to reverse. A TAP, by contrast, works entirely outside the ECU: it intercepts and modifies the timing signal without touching the ECU’s internal software.
| Factor | Timing Advance Processor | ECU Remap |
|---|---|---|
| Cost | Low to moderate | Moderate to high |
| Reversible | Yes | Rarely |
| Modifies ECU | No | Yes |
| Fuel-switch compatible | Yes (automatic) | Complex |
| DIY installation | Mostly yes | No — specialist required |
| Risk level | Low | Higher if done poorly |
For most CNG and LPG conversions, a TAP is the safer and more practical choice — reversible, ECU-warranty-safe, and automatically aware of fuel switching. An ECU remap is better suited to dedicated single-fuel performance builds with a specialist tuner involved.
Signs Your Engine Needs a Timing Advance Processor
If your vehicle has been converted to CNG or LPG and you’re experiencing any of these symptoms, timing correction is likely needed:
- Power loss or sluggish acceleration on gas compared to petrol
- Engine knock or pinging under load (overtaking, hill climbing)
- Rough idle on gas that disappears when running on petrol
- Intake backfire — a symptom of retarded timing and unburned fuel
- Higher-than-expected fuel consumption despite switching to a cheaper fuel
- Check engine light appearing after the gas conversion
Any one of these on a converted vehicle is worth investigating. Two or more together points strongly to missing or misconfigured timing correction.
What to Know Before Installing a TAP
Before buying a timing advance processor, confirm your ignition system type — mechanical breaker points, electronic ignition module, or CKP sensor. Installing the wrong type won’t work and can cause ignition faults. Check your vehicle’s service manual or ask your gas conversion installer if unsure.
Wiring accuracy is the most common source of installation problems. Incorrect connections can trigger check engine lights, cause misfires, or prevent the timing advance from activating. If your vehicle has a complex modern ignition system, professional installation from a qualified gas technician is the better option.
Also confirm the TAP includes an emergency bypass connector and is compatible with your LPG/CNG kit brand (AEB, BRC, Prins, OMVL, etc.). Once installed and calibrated correctly, a timing advance processor needs no ongoing maintenance and runs quietly for the life of the vehicle.
FAQs
What does a timing advance processor do?
It adjusts the ignition spark timing when an engine runs on CNG or LPG. Because these fuels burn more slowly than petrol, the spark must fire earlier to achieve complete combustion and maintain power and efficiency.
Do all gas-converted vehicles need a TAP?
Most do. Some modern ECUs can partially compensate through knock correction, but that’s rarely enough on its own. A dedicated TAP provides proper timing correction across the full RPM and load range.
Will a TAP damage my engine?
No. When correctly installed and calibrated, it improves combustion and reduces engine wear. Risk only comes from using an incompatible model or poor wiring.
Can I use a TAP on a petrol-only car?
No. A petrol engine is already calibrated for petrol. A TAP is designed for bi-fuel or alternative fuel systems only.
How much does a timing advance processor cost?
Basic analog models cost £20–£50. Digital and reprogrammable TAPs range from £60–£150. Installation is additional but the total outlay is modest compared to the efficiency gains.
Does it improve fuel economy?
Yes. Properly timed combustion extracts more energy from each unit of fuel. Most users see a 5–10% improvement in fuel efficiency compared to an uncorrected gas conversion.
What’s the difference between timing advance and retard?
Advancing means firing the spark earlier; retarding means firing it later. CNG and LPG require advance. Retard is applied automatically when the knock sensor detects detonation.
Can I install it myself?
On older vehicles with simpler ignition systems, yes. On modern vehicles with digital ECUs and CKP-based ignition, professional installation is recommended to avoid fault codes and misfires.
Conclusion
A timing advance processor is one of the most overlooked yet impactful components in any CNG or LPG conversion. It addresses a straightforward problem — factory ignition timing that was never designed for slower-burning alternative fuels — with a low-cost, reversible, and effective solution.
Without one, a converted engine runs inefficiently, loses power, and wears faster than it should. With a correctly specified and installed TAP, you get back the performance, fuel economy, and engine reliability the conversion was supposed to deliver. If your vehicle runs on gas and timing correction isn’t already in place, a timing advance processor is the logical next step.
