It has been reported (by another forum member) that miniram's typically don't idle as well due to the lack of IAC passage (in the plenum). To be honest, it's the first I've heard of real design problems after viewing this forum for over a year. Guess I'm not looking closely enough....

After reading posts (and my FSM), I understand the function of the IAC is to regulate flow of air thru the lower TB idle passage. In my 1989 plenum, there are two holes in the floor of the plenum directly behind the input bores. Actually, they are right behind the small ridge typically thought of as an obstruction (at the base of the bores.) The plenum passages are not referenced in the forums or in the FSM.

Upon further reflection, I'm guessing the ridge (behind the plenum TB bores) may be designed to aid in the deflection of air down those holes. Any air successfully entering those holes travels thru a "special" passage under the plenum, then down the smaller incidental-looking runner. At that point, the air enters the base and likely the EGR chamber. I surmise this as an attempt to dispurse (slower moving idle) air more evenly to the 8 cylinders.

Agree or not?

Even if it is, I'm not sure I see a huge potential for improvement. And, I must question the conclusion about it's necessity/desireability.

When questioning this issue (OT from the point of another thread), it was recommended that I search 3rdGen for add'l detail. Last night, that search was less than satisfactory. I located several threads on the subject of IAC mis-operation -- and even someone who built a custom air divertor (apparently for the purpose presumed above). But, nothing was really stated about the pros/cons of IAC operation in aftermarket intakes where the IAC passage does not exist. IOW, there doesn't seem to be a concensus that the lack of IAC passage causes consistently adverse conditions.

I have to wonder if most don't care, don't understand it, or if the issue is not that big of a deal. I also have to assume the worst possible outcome is a (slightly) rougher idle due to unequal air supply for all cylinders. Any add'l info would be much appreciated....

gp

 

hmm.....that theory makes a little sense actually. My car will not idle dead on with the IAC controlling idle, but....If I unplug the IAC and set the throttle stop to the desired Idle, its spot on.

 

Do a search on "split BLM's" to find your answer. The two holes behind the throttle body on L98's was strickly used for the EGR output into the plenum. The hump was most likely put there to aid in the distribution and prevent a reverse flow back down the EGR passages. On the LT1's with there new dual oxygen sensors (one on each bank) it was found that a split BLM was happening with the new intake style at idle and off idle. Dedicated idle passages were needed to counteract this. The problem still exists for some LT1 enthusiats that upgrade their throttle bodies with ones that aren't machined correctly (idle air passage). A search on "C4 Tech/perf" with the words split BLM will give you some info on the subject.

 

Thanks for your reply.

Ummmm.... If the air passage in the plenum is strickly for EGR "air" distribution, then I fail to see how IAC operation could be adversely affected by swapping to an aftermarket intake.

(Also, split BLM issue for LTx cars wouldn't apply to L98's (like I have), right?)

 

The split BLM problem is stricktly a LT1 issue due to it's manifold design and use of an oxygen sensor in each bank. The problem is dealt with on the factory LT1 and LT4manifolds (dedicated passages) but not on the miniram manifold. I'm not familiar with the Edelbrock style of intake. Some aftermarket throttle bodies have an incorrect IAC passage for use with LT1 style of manifolds. They can be modified for correct use with a LT1 or LT4 manifold.