tb idle air
11-08-2008, 11:22 AM
#4
When I first installed it I was having some idle issues. TPS/IAC seemed to be fine. That's when I posted a thread on here about the issue and a few guys on here said to drill that portion out. The stock T.B. has the same hole through it..so to be completely honest I really don't know the main reason for the hole being there in the first place. All I can tell ya is it ran perfect after I drilled it out. Maybe someone with more knowledge then myself on the topic will chime in 
11-08-2008, 01:13 PM
#5
IF the hole is too small the IAC counts will be high. The car will shut off occasionally after driving it and then you bring it back down to idle. It better to do with a scan tool or something that can tell you iac counts while it is running. MY efi truck conversion I had to fix mine i ended up drilling a .300 hole to get it right.
11-08-2008, 06:07 PM
#7
If you still are a bit confused, here it the long answer...
Aftermarket TB's route the air for the engine at idle(0% TPS)behind the blades, so with big cams the front cylinders hog all the air which can lead to idle problems and surging. GM deals with idle air distribution by routing a small amount of air from the throttle bodies IAC plate into dedicated passages in the stock intake manifold - that big hole in the front that seemingly leads to nothing.
There are small passages leading to each runner to give an equal amount of air to each cylinder at idle. Aftermarket LT1 TB's just block this port off and run the IAC air through a little track right behind the TB blades where all the other air goes when you give it throttle. Drilling the hole and isolating it forces the air to go where it is supposed to, into the IAC passages in the intake.
This also leads to fact that the factory IAC passages to each runner are not always all even, and in some extreme cases finding the largest one and drilling them out to all be the same size can help a radical cam idle as well. I did not even bother to measure them out with my last modest custom grind, but I will with my much larger one for my stroker just to be sure. A couple minutes to at least double check may save me a lot of frustration down the road.
Aftermarket TB's route the air for the engine at idle(0% TPS)behind the blades, so with big cams the front cylinders hog all the air which can lead to idle problems and surging. GM deals with idle air distribution by routing a small amount of air from the throttle bodies IAC plate into dedicated passages in the stock intake manifold - that big hole in the front that seemingly leads to nothing.
There are small passages leading to each runner to give an equal amount of air to each cylinder at idle. Aftermarket LT1 TB's just block this port off and run the IAC air through a little track right behind the TB blades where all the other air goes when you give it throttle. Drilling the hole and isolating it forces the air to go where it is supposed to, into the IAC passages in the intake.
This also leads to fact that the factory IAC passages to each runner are not always all even, and in some extreme cases finding the largest one and drilling them out to all be the same size can help a radical cam idle as well. I did not even bother to measure them out with my last modest custom grind, but I will with my much larger one for my stroker just to be sure. A couple minutes to at least double check may save me a lot of frustration down the road.
