Throttle body coolant lines.
Question: Why does the EVO have coolant lines to the throttle body, and can I remove them as a free mod for more power?
The DSM and EVO throttle bodies have two mechanical and one electric control of idle speed.
The electric control is the stepper motor controlled by the ECU that opens up a bit to raise idle, and closes to lower idle.
The first mechanical idle control is the Base Idle Set Screw (BISS) The BISS is a screw that blocks off a passage that allows airflow to bypass the throttle plate. You turn the screw out and more air bypasses the throttle plate raising your idle, turn the screw in and less air is bypassing the throttle plate and your idle goes down.
The second mechanical idle control is the Fast Idle Air Valve (FIAV) The FIAV is a plastic/wax plug that blocks off a passage that allows airflow to bypass the throttle plate. When this plug is cold is shrinks and blocks less of the air passage and more air bypasses the throttle plate raising your idle, as the coolant heats up, the plug gets warm and expands blocking more of the air passage so less air is bypassing the throttle plate and your idle goes down.
If you get ride of the coolant lines to the throttle body, the car will remain in "high idle mode" because the FIAV never warms up enough to expand blocking its air passage. If you live in a warm climate where a fast idle to warm up the engine is not that big of a deal you can bypass the lines if you like and the electric idle control combined with a one time BISS adjustment will have you idling fine. If you live in a cold climate you will need to make seasonal adjustments of the BISS for the car to idle correctly in each season because the electronic idle control will be out of its range of adjustment if you donít compensate with the BISS.
The throttle body is heated by a combination of the coolant running through it, and by heat conducted up through the intake manifold from the head and into the throttle body. Any power gain from not heating up the throttle body with coolant is theoretical, and the throttle body may in fact be cooled by the coolant depending on how hot the intake manifold gets on our cars... I do not have data on that issue. No matter weather the coolant has any cooling affect after the intake manifold heats up or not, the air spends so little time in the throttle body at WOT that in my opinion the affect on power would be negligible to the point of being non-existent.
EVO 11.53 @ 123.37, Black WRX Wagon 13.62 @ 99.24