|Top Previous Next|
A different technique is required for tuning ITBs (individual throttle bodies) than tuning for a conventional plenum.
Some ECU background is necessary. One of the primary tasks of the ECU is to meter fuel for the engine. To do this, it much accurately determine the mass of air in the cylinder and injector the appropriate amount of fuel. There are two primary techniques used by the ECU to do this - speed-density and alpha-n.
This is the stock Honda method of metering fuel. It is called 'speed-density' after the table indices - engine speed and manifold air pressure.
In order to calculate the air density the ECU needs to know three things: the manifold air pressure (from the MAP sensor), the air temperature (from the IAT sensor) and the engine volumetric efficiency (how much air ends up in the cylinder at a certain engine speed and manifold pressure). Thus the fuel maps are actually volumetric efficiency tables (adjusted for manifold air pressure).
An alternative method of metering fuel is to use the throttle position sensor as the column index. Thus the name 'alpha-n' - after throttle angle (alpha) and engine speed (n).
ITBs has specific characteristics which means that they do not provide an air pressure signal representative of the actual air entering the engine. Thus using speed-density fuel metering does not work very well, as there often is a range of cylinder filling which is represented by the same manifold pressure. An example would be at low rpm - the MAP reading will go to almost atmospheric pressure at light throttle, and will not change as the throttle is opened.
A fuel table for ITBs tuned using the MAP sensor. Note how the table is very steep as the MAP nears atmospheric pressure. In this table, just 10% change in throttle from 90% to 100% resulted in the fuel value doubling.
While the engine will run correctly tuned using the MAP sensor, it would be very sensitive to small changes, and would very lean with a change in elevation or ambient air pressure. For this reason it is not recommended to tune ITBs using the MAP sensor.
The same engine tuned using the TPS sensor. Note the shape of the fuel curve - at low rpm, a certain throttle opening will result in maximum air flow, and opening the throttle further will not increase volumetric efficiency. Hence the flattened shape at low rpm / high throttle opening. This effects becomes less at high rpm where the throttle / fuel relationship is more linear. There is an anomaly around the idle area, which is characteristic of ITBs. Normally they need 2 - 4 times the fuel of a regular plenum at idle.
Note that there is now no inherent correction for changes in air pressure or elevation (provided with the MAP sensor when using speed-density). For this reason it is recommended to use atmospheric pressure correction which using alpha-n.
ITB Dyno Tuning
The most time effective way of tuning for ITBs is to perform dyno runs at a constant throttle opening, matching the column index TPS values. Note that the tuning range of throttle opening and rpm will depend on the dyno - but generally you can tune the bulk of the fuel table this way.
The datalog above shows TPS overlaid on the fuel table.
Lambda recorded from a dyno run. From here it is easy to adjust the column fuel in order to achieve the desired lambda.