User guide

Interceptor Manual

Tuning a Haltech Interceptor

Tuning a Haltech Interceptor is quite different to tuning a standalone ECU. Although the

theories for tuning remain the same in terms of tuning each load point for desired fuel mixture

and ignition timing, the response of the factory ECU to Haltech Interceptor changes can be quite

different. This section sets out to detail the recommended methods and potential issues that

should be considered when tuning an interceptor. This section considers basic tuning theories

and methods as assumed knowledge.

Basic Fuel Tuning

The Haltech Interceptor stores corrections in its internal tables as percentages. The actual

percentage change applied is a percentage of the signal level. Therefore +100% corrections will

double the signal level (not increase it to the maximum), and –100% will reduce the signal level

to zero. This gives very high-resolution correction. The actual response of the factory ECU to

these corrections is dependent on two main factors: the load sensor type and the factory ECU

maps.

MAP and TPS type load sensors will generally give a signal level range from the minimum

level to the maximum level at any RPM site. An AFM signal will give the highest signal at the

engines max power (high RPM) and the lowest signal at the engine minimum power (low

RPM). The result of this is that a smaller signal range is used at any RPM site for the range from

minimum load to maximum load. Therefore, when applying corrections to an AFM load sensed

factory ECU, small changes (<5%) can make a large difference to fuel mixtures. In contrast,

MAP and TPS load sensed factory ECUs often require larger percentage changes (>10%) to get

a measurable change in the fuel mixtures.

In addition to the load sensor type affecting the sensitivity, the fuel tables within the factory

ECU can also affect the sensitivity. The actual sensitivity is dependant on the fuel mixture

variation with load of the factory ECU. Some ECUs tend to enrich the mixture quite

aggressively when the load is detected above a normal level as a safety response. Unfortunately

the results can be quite non-linear, and sometimes some trial and error is required. Because the

Haltech Interceptor has a very high resolution of fuel control with variable load and RPM sites,

the desired fuel mixture curves can be achieved with some tuning time.

Closed Loop Mode

Most factory ECUs use closed loop control of injection times at light loads. This means that the

actual injection times are calculated after monitoring the exhaust mixtures with the O2 sensor.

This is generally only done at light loads (cruising, idle) with a stoichiometric fuel mixture.

During closed loop mode, the factory ECU only uses the engine load sensor for the base

injection time, and it then applies corrections to this time to get the mixture to stoichiometric.

The impact of this on the Haltech Interceptor is that the factory ECU will tune around any fuel

corrections made whilst in closed loop mode as it attempts to get back to stoichiometric fuel

mixtures. If large corrections are made in the Haltech Interceptor fuel tables during the factory

ECU closed loop mode, the factory ECU could generate a fault code if the closed loop

corrections it must do fall outside of its normal expected range. It is recommended that fuel

corrections done in closed loop mode should only be done to bring the tune closer to

stoichiometric fuel mixtures. This may even be necessary if mechanical modifications have been