Hi all,

Searched the forum for this and couldn't find much.

A few months ago I finished putting together my 383 with a decent sized cam (244/252, .612/.612, 110 LSA) and have been trying to get my idle tune right ever since. Warm IAC counts are in the mid 60s (and that's about as low as they will go). I have seen and read that people drill their throttle bodies to allow for more air to enter the intake to accomodate for the extra cubes and larger cam to get the IAC counts to come down. My car is idling pig rich and I can only assume that this IAC problem has a large role to play

I have a 52mm Holley TB. What do you all think is the best route to go about dealing with this problem? Should I drill small bleed holes directly into the throttle blades or should I be drilling a hole in the IAC passage on the TB (as shown here: http://www.hashmarks.com/techtips/throttle_body_mod.htm)? What's a ballpark on the size of the hole(s)? Thanks in advance

 

Drill the iac passage first, not the blades. I think the size is 11/64, but double check that...I'm on my cell. You also need to isolate the hole you drill, to send it into the iac passage, or else it just mixes with the Plenum air.

 

If the IAC passage is not isolated then the front cylinders will be lean and the rear rich, if all cylinders are rich then I would think it is a tuning issue. though resolving the IAC troubles will improve idle.

 

Since the IAC is not maxed out and still in control of idle air, I would say that the big cam and the tune contribute the most to your richness. The computer is letting in what it thinks is enough air and that happens to be at a count of 60 right now (~20-40 is normal, but your engine is not "normal"). If it maxes out at some other operating conditions, then it might be helpful to increase the passage. IOW, it might not even make a difference if you increase the size of the passage and change nothing else.

 

Just a little off topic but can the 52mm TB move enough air for that big cam?

 

Thanks for the replies. As for the question about the 52mm...TB should be enough. I have seen only marginal gains from stepping up to a 58, although there might be some who feel that the car would run better, be more responsive, etc.

 

the stock hole in a 48mm is 1/8". On my 383 I now have a stock 48 bored to 52. Smaller cam than you.

I drilled 1/64" bigger and scanned again, then 1/64" larger. IIRC 11-13/64 is where many wind up but I caution you really need to scan after each size incriment as 1/64" makes a big diffrence/ My TB is stock rebore but on aftermarket ones you may need to mod/block as shown in the link to direct the idle air passage the way stock goes.

I got my IAC to 32 (30-35) is typically what "desired" IAC count is in program.

When you turn on AC, turn wheel hard over while stopped (basically adding load to motor) does the car want to idle very low, stall?

if so you are maxing out IAC with a base of 60 so you need more "room" which is why 30-35 is the "normal" setting....althoug like Rob says your motor is no longer "normal"...like many of ours :-)

 

Thanks for the reply. The car surely doesn't like heavy amounts of braking. As I slow to a stop (with heavier brake pedal pressure applied) the rpms dip, car throws a high MAP code but then gets right back to idling again (set at 1150). I am going to try by starting with an 1/8" hole and work my way up. I will see how it goes...