To save a bit of typing I will copy and paste something I wrote to a guy on another
forum who was asking much the same question...
The primary issue with putting ITB's on an otherwise stock single throttle 16V
engine is that opening the throttles with ITB's introduces a LOT more air into
the engine compared to a single throttle. On an aftermarket ECU you can adjust
the fuel map and acceleration enrichment to add a LOT more fuel at lower rpm's
when you open the throttles. The factory ECU can't do this, so you get a lean
condition (stumble) when opening the throttles quickly followed by an over rich
condition as the MAP reading goes high before the engine revs build up.
Higher up the rev range the situation is not so bad as the amount of air going into
the engine is not so different between a single throttle and ITB's. But the fuel map
will not be optimum at any rpm. In other words, it will run but may not run as well
as a single throttle engine.
The second reason is to do with idle air. On an AE101 or AE111 ALL
the idle air
is introduced via the ISCV. This is a rotary solenoid valve driven by a PWM signal
from the ECU. On the stock TVIS engine extra idle air for startup is handled by the
auxiliary air valve under the throttle body. Warm idle is set by the idle bypass screw
Whe converting to ITB's and using an aftermarket ECU it is usual to either use the
ST/BT factory ISCV or a Bosch ISCV and drive it with two PWM capable outputs
from the ECU. Ideally, the idle air should be introduced equally into all four of the
The vast majority of ITB conversions are tuned using alpha-numeric (TPS tuning)
rather than speed-density (MAP tuning). This is because with ITB's the manifold
absolute pressure reading will hit atmospheric with a very small throttle opening.
However, single throttle engines are invariably tuned using a MAP sensor or airflow
measuring device for the load input.
When used with ITB's a MAP sensor has good definition at small throttle openings,
but poor definition at larger openings as once the MAP is at atmospheric opening
the throttle further has no affect on fueling. The TPS has poor definition at low
opening but much better definition at large openings as the ECU will increase fuel
according to the throttle opening. .
As said above, if you want your engine to run well with ITb's you should look at an
aftermarket ECU like the Haltech so that you have full control over your fuel and
ignition maps, and you will also have the ability to control a PWM idle air valve.
This is a pic of the setup I had running on a smallport engine. This ran with the
factory ISCV from a 20V silvertop. Although it was a TPS tune my ECU used a MAP
signal input as part of it's fueling calculations, so you can see a small vacuum
accumulator hooked up to each inlet runner. I used the smallest available tubes
to dampen the pressure fluctuations and get a stable MAP signal.