Drill the throttle body for a better behaved cam upgraded LS engine

Drill the Throttle Body for a better behaving cam upgraded LS engine

 

LS1 Cable Throttle Body

The LS1 5.7L engine has a cable throttle body which has small hole in the throttle body blade and an Idle Air Control system (IAC) to provide the extra air at idle when the engine is cold.  As the car warms up the extra air provided by the IAC reduces and ends up being little more than the amount making it through the hole and around the blade itself.

 

When a cam is installed extra air is needed at idle cold and also when hot. The throttle can have its blade kicked out so it is more open when at zero throttle and this gives it more air.  This will help it idle better but there are limits of how far this can be pushed before the engine computer will throw an error code reporting that the throttle position sensor is out of range. There is also a negative effect of kicking the blade out further. This is that the throttle will feel jerky. The reason for this is because the throttle blade is permanently at a wider angle, the extra air that comes from the next few degrees which related to the next couple of % of pedal will be far greater than the amount of extra air given if the throttle body was starting from the factory position.  Those first few degrees give a much slower increase in air delivery due to its shape and creates a smoother throttle feel.  This is the one big advantage of drilling the blade so that it can start from the factory position. It just needs to be drilled the right size so that at warm idle there is a low amount of air from the IAC and not enough to fully close the IAC because the engine will idle too fast on deceleration, when clutch is in etc.  When its done right the airflow settings in the tune can be set close to the factory settings and everything behaves itself far better.

 

ETC Drive by wire Throttle Body

In the electronic throttle bodies using on the LS1, LS2, LS3, L76, L77, L98, LSA and LS7 engines there is no IAC system. The extra air at idle that is needed when cold is supplied by the blading staying further open. The tune settings in the engine computer are specified as minimum airflow values and correlate to a minimum angle for the blade. Once again at idle the throttle body is designed to start from an optimal closed position and smoothly add extra air as the blade opens with this amount increasing rapidly every degree for the first few degrees that the blade is opened.

 

When a cam is installed extra air is needed at idle and this can be given to it in the tune without a hole being drilled BUT the throttle will feel jerky off idle because of the rapid increase in air. Also cars done this way tend to dip at idle below their requested idle speed when coasting because sometimes the throttle body blades close to their factory angle. This is more closed than their new wider minimum angle set in the tune for the cam. The better way to make the car idle is to drill precisely the right size hole, this prevents the dipping and the responsiveness is smoother off idle as well. This often gives less of the buckiness that cam upgraded cars can be plagued with at light throttle when driving along at low speeds.

Once again if the hole is too big the car will not want to slow in idle rpm enough and can idle too fast. This is where the tune settings and the hole size need to be balanced perfectly.