The purpose of the Idle-Air Bypass, is so that you can restore functionality of the low-speed circuit, which is the SUM OF Transfer-slot fuel and mixture screw fuel.
Any time you add more air than the engine needs at idle, if the engine simultaneously gets more fuel, the idle-rpm HAS to go up.
The biggest problem I see,
time and time again, is guys running more idle-timing than is necessary. EVERY time you add Idle Timing, up to around as much as 30 degrees, the Idling engine will pick up idle-rpm. I repeat; EVERY TIME. this is because, the engine is picking up Idle-power, as you get closer to developing peak cylinder pressure at the optimum point in the cranks decent on the power-stroke.
That's fine if you need the power, and/or you have a hi-stall convertor. But if you have a street-car, you neither need nor want that. Most of the time, it's just big pita.
Therefore,
in a street car, with an automatic, you only need to run just enough Idle-power, so it doesn't stall when you put it into gear.
So then
The order of operations is to set the T-slot to mixture screw synchronization, with the Curb-idle screw; and then, set the idle-speed with Ignition timing. That's all.
If the engine wants more air,
it will tell you, by having a burn-your-eyes exhaust stink, and NO MIXTURE SCREW setting will make it go away. Also, it is possible at very low engine vacuum, for your PCV to switch from being a simple fixed-orifice , to some larger proportional air leak cuz the low vacuum signals it to respond with a larger opening.
Also, It will tell you by idling rough and it cannot be smoothed with mixture screw adjustment.
The biggest cam I have tuned, in a 360, was the DC 292/292/108, 248@050. That said, the smallest was a 270/276/110, 223/230@050 ..
The biggest in a BB was a little bigger.
I've never seen a cammed-up engine NOT respond positively to Idle-Air bypass. The amount will vary, depending on the combo, the idle-timing, and the idle-speed. I can tell you that one hole in each primary, of .125, is too much for the 292 cam.
I can also tell you that, the 223 cam doesn't actually want any bypass air, until you run too much Idle-timing. and
I can tell you that, properly tuned, either cam will idle just fine down to 550 rpm in Neutral, if you get the T-slot sync set right, and the Idle-Timing rolled off....... unless yur rings are seriously dragging, or your valves are not getting enough seat time.
The point is this;
Automatic guys with hi-stalls can get away with the worst tunes imaginable, cuz it all gets lost in the convertor.
So, what do you do?
Well,
with a street engine running a streetable cam;
I suggest you begin your IDLE-tune by setting your Idle-timing in the window of 12 to 14 degrees. Set your T-slot sync, which I'm assuming that you already know how to do. And then go find your real problem.
Which could be anything here listed;
Incorrect WET fuel level,
Percolating fuel,
Incorrectly working PCV system,
Air getting into the engine from an unauthorized source, NOT past the throttle-blades.
Fuel getting into the engine from an unauthorized source, NOT from the low-speed system.
High friction on the crank, stealing Idle-power.
Too tight a Valve lash, stealing Idle-Power
Engine coolant temp not under control; you cannot tune an engine in the which the coolant system is running amok.
And all the usual stuff nobody remembers like;
Fouled plugs, bad wires, a weak coil, a heated intake, a restricted exhaust, leaking header flanges, changes in elevation, float-bowl venting, fuel-tank venting, etcetera,
Here's a clue;
You will know when your T-slot sync is not adequate, by the engine having a tip-in sag, at gentle driveaway, assuming a decently low idle-rpm. A tip-in sag is caused by a momentary failure of the T-slot to respond, which usually means one of three things; the throttle is too far closed, or the fuel level in the Idle-well is too low, or the fuel has a problem; which could be that it's stale or it has not been properly emulsified in the idle-well. Not enough Idle-timing is seldom a contributor.
In other words, you can use the Tip-in sag for tuning purposes. You can use it to find the lowest limit of your throttle-opening. Which will help determine your Idle-power, so that you don't get that huge discrepancy between Idle-speed in Neutral verses idle speed in Drive, and the huge BANG!, that it usually makes upon engagement.
Lets back up the bus a sec.
Very few guys will install a 292 cam, cuz that cam does not have very good street manners. and, if a guy is gunna spend money on a cam, you can bet money, it'll be bigger than a 262. What that means is that, for a streeter, yur looking at a range of about 30 degrees. One cam size is generally agreed to be about 7 degrees, so, 30Degrees is 5 sizes, namely, 262/268/276/284/and 292.
My 292 cam liked two bypass holes total, each one being 7/64ths
My 270 was happy with 2/64ths to none. Therefore, I'll guess that each cam size smaller than the 292, will want at least 1/64th less.
Since you do not know what cam is in your engine, this makes it a lil tough.
Many years ago, when I did my first tune, I Tee'd into the PCV system, and drilled holes in my Plastic Tee, until the engine liked it. Just remember to get your idle-rpm down first. I've never had a street engine, that wanted more than 800rpm, and more often than not, they will idle down to 600/650 in gear. Keep your eye on the oil-pressure, When it gets down to under 10psi, I start to worry.
When you get this worked out;
now you have another job, which is hitting three or four other Timing targets, namely;
1) Power-Timing
2) Stall-Timing
3) Part-Throttle Timing, and
4) Cruise timing.
Are we having fun yet?
Happy HotRodding.
You asked;
"Can someone here help me understand IF and HOW drilling carburetor throttle blades for an aggressive cam idle/T slot adjustment/optimization might also or not help in gear and out (park/neutral) idle balance on an automatic?"
The holes only make a difference at idle, with the throttles nearly closed. I put mine on the front no closer than .25 to the edge, and I spot them between the T-slots and the Idle-discharge ports.
When the throttles are vertical, obviously, those holes do nothing. In between, as the throttles are opened, they contribute less and less. Around town, I've never even thought about them.
If you drill, drill small and work with them for awhile.
If you drill too big, yes you can solder them shut, move over, and try again.
How do you know when the holes are too big?
The engine will not idle down with a correct T-slot sync, no matter how far you retard the timing. Well I mean within reason. I set mine to run down to 500/550 rpm at 5* advance, so that with a manual trans, I can parade it at 4mph. IF I had too much Idle bypass air, this would not be possible.
