Introduction

For any engine to perform it must be fitted with an appropriate management system. As standard, the Honda PGM-Fi system uses the speed-density principle (MAP sensor) and the throttle position sensor in normal operation is used only to provide a degree of correction to the stock maps during certain conditions.

To perform any useful function an engine management system needs an accurate indication of the engine load and speed. With a map sensor setup the load is calculated by measuring the pressure in the inlet manifold and therefore the density of the air. This has a great number of advantages, idle control is easy and you can obtain great resolution during light load cruising, allowing you to tune perfectly for fuel economy and emissions. This works great with a conventional inlet manifold, plenum and throttle body arrangement, with a throttle sensor to cope with transients, like sudden movement of the throttle.

Simplified greatly, as you gradually open the throttle and load the engine the pressure in the manifold decreases until you reach near atmospheric at around full throttle.

With individual throttle bodies there is very little volume within the manifold, this means that the pressure in the manifold (remembering each cylinder is breathing independently and is not connected) reaches near atmospheric long before the throttle is fully open. If you were to run a throttle body setup using a map sensor as the primary load input the ECU would be running on it's final load breakpoint long before the engine was in a fully loaded condition. Basically, as soon as atmospheric was reached you would have no control over fuel and timing in relation to load, only RPM. Obviously this is no good if your engine now hits atmospheric at 35% throttle!

Another important point is that because each cylinder is breathing individually and the volume of air in the system is lower you tend to get wild oscillations in pressure in each inlet tract - there is no common chamber from which they are breathing to smooth everything out. This makes the MAP reading even more unreliable as it gets ragged and an unreliable indication of load.

With a throttle body setup the throttle position is a far closer match to the engine's load than the unreliable MAP reading over the entire operating range, so it is common practise to ditch the MAP sensor entirely and switch to using the throttle.

This presents a few problems, idle control and correction for changes in atmospheric pressure, which are important to consider when looking for perfect driveability.

Many of today's production cars (and especially motorcycles) that are fitted with ITB's from the factory use complex control strategies to operate with ITB's without any of the compromises. Maps based on throttle position and manifold pressure are programmed into the ECU, and the ECU may switch between either map on demand.