Hey guys Im installing 58mm TB this weekend, Im putting it on my 350 right now, but in the next couple of months its going on my 383. What is all this about drilling the IAC out? Open my eyes a bit about not just installing the TPS and IAC like normal. What do I have to look out for?

 

When I installed the BBK 58 mm TB on my 388, other than transfering the parts, (bottom plate, etc., ) all I had to do is make a small adjustment to the minimum idle screw.

I didn't have to do anything else, no drilling, etc. I, to, have read posts about drilling the IAC, but if you have to do anything like drilling, I'd suspect it's the PCM tune.

Jake

West Point ROCKS!

 

I did not drill any thing.

even before the tune it ran fine

 

So what were your IAC counts according to your datalogger?

 

there's your problem no data logger

unfortunately I got my 58mm before I found this sight,so I didn't know anything was or is wrong


I'm trying to get in the mind set of getting everything to tune my own car but, damn there is so much to know

 

You may have no problems...or you may have a high idle / low idle when coming to a quick stop because the IAC not in an acceptable range, either too far open or too far closed, irradic idle, may die often...several different things. Sometimes just adjusting the IAC screw can let enough air in when closed to operate properly, other times this can cause a high idle when stopping.
If you look closely at the stock TB you'll notice a tiny hole in the lower passage that allows air to pass into the smaller set of holes on the front of the plenum. Those small holes on the plenum face go to tiny passages that come out in each of the runners, thus allowing air to pass from the TB directly to the runners without going into the open plenum. This allows for more even air distribution into each cyclinder when at idle. Most after market TB do not have this provision for some reason.
Some folks drill an extra hole in the butterfly, that is not correct and has the same effect as cranking open the IAC screw. Some TBs there is no way to get isolated air into those small passages, the Holley does but requires drilling the hole. There are several different models of the BBK over the years so it depends on those...
Or you may find you don't have any issues the way it is. It seems like those with bigger cams have the most issues.

 

http://members.***.net/chipsbyal/page/idle/index.html

Drilling the IAC bleed is simply copying a factory setup. Look at your stock TB, it has a bleed. Not doing anything wild. The L99 and LT1 actually had different sized bleed holes even, the L99 being smaller displacement they gave it a smaller bleed.
By cracking the blades open with the throttle stop screw you partially defeat the IAC circuit.

When I bought my BBK 58mm used the blades were cracked open a hair. I noticed this TB hurt idle quality so I went looking for a cause and found this. A little trial and error drilling out the bleed and closing the throttle blades improved idle quality with the tiny ZZ4 cam I had at that time.

 

This was back in early 2007 right after I finished building the 388. I didn't keep notes on it, but from memory the counts were in the mid 20s.

Jake