JAKEJR

1996 LT1

I'm looking to increase the size of the idle air bleed hole in my son's stock 48 mm TB to help improve the idle after installing his new cam. I have another stock 48mm TB also off a 96 LT1 and measured the existing hole as 10/64" (5/32). so far, I've cranked open the butterflies using the minimum airflow screw but I'm not happy with that as a solution.

I want to move in increments of 64ths so as to creep up on the best size and not make it too large by over-shooting the best size. I read two web articles on how other guys have done theirs. Some of you have probably already read them too.

One guy fabricated a piece of aluminum and the other used hi-temp epoxy after drilling a SECOND hole at the TOP of th TB which allows air directly into the intake manifold and NOT through the IAC circuit. I didn't see in the articles why they did that though.

I'm wondering if others have gone this same route or merely enlarged the stock idle air hole (the lower one).

I'd also like to know what size hole(s) you drilled and the cam specs.

Thanks for any feed-back.

Jake

West Point ROCKS!

joeSS97

First of all I would make sure you have a good tune for that cam. You may need to clean the IAC. That should make it idle fine. I wouldn't go drilling any holes. JMHO.

Sk8phreak88

Yeah, I'd go get a tune... I've got a 236/242 and .555/.576 on a 112lsa and before the tune it wouldn't idle for beans even at 900-1000 RPMS..

After a tune It idles away at 700-800 just fine.

JAKEJR

PCM's been tuned and IAC and ALL throttle body passages have been cleaned also. It's the tight 109 LSA on the cam. I suspected that coming in.

Let's talk hole sizes.

Jake

West Point ROCKS!

95Blackhawk

The solution is simple. Drill the lower hole only. Why? Because the intent is to get ALL idle air to go into the idle plenum and not bypass through the blades. FWIW, my 48mm was drilled out to about .25, if I remember correctly. As you see my cam is decent and have perfect BLM's at idle with my IAC at 40 or so. I did the same thing to my 58MM when I put it in and my blades are completely shut at idle. Again, with the 58, BLM's are perfect.

BTW, you will see progressively smaller gains (lower number) on the IAC count going up by a 64th at a time. Also, you MUST have your car up to operating temperatures before you evaluate if the final hole size is correct for idle.

Ben

JAKEJR

Thanks, Ben. That's pointed me in the right direction. I'll double the 64ths to 32nds and deal with the lower hole only.

So since the stock hole is 10/64ths (5/32nds), 12/64ths (3/16ths) would be my first stop, then 14/64ths (7/32nds) next, and, hopefully 16/64ths (1/4") last.

I appreciate the input.

Jake

West Point ROCKS!

JAKEJR

Just an UpDate:

Over the past few days I've been removing, drilling and reinstalling the TB on my son's 96 LT1 Vette.. Drilling to enlarge the hole in the TB that is fed by the IAC.

Following advice fro 95BlackHawk (many thanks again for that), I moved in increments of 32nds, moving from the stock 5/32" hole size - finally to 8/32".

Each incremental change made an improvement and at 8/32 (1/4") it became perfect. Car idles in Park at 950 and 750 in gear which is perfect for the new cam's tightish LSA and stock converter. Idle sounds awesome and guys passing by can't help but stop and comment on how good it sounds.

I modified the stock minimum airflow screw arrangement to make minimum airflow adjustments much easier. I removed the stock airflow screw, then drilled and tapped the hole to accept a 5/16" allen head stud, installed from the front. Then I didn't have to wrestle with finding the head of the stock, tiny screw from the rear or deal with the manifold interference issue. Adjusting the minimum airflow setting is now a snap.

So we're happy campers.

My thanks to all.

Jake

West Point ROCKS!

95Blackhawk

I like this idea. However, you should be closing those blades all the way and never having to mess with it again... I do remember a time when I wish I had this setup though.

What is your IAC count when the car is warm?

JAKEJR

I removed the stock TB Min Airflow screw and drilled out the "tunnel" using a 17/64th drill bit. I then tapped it with 5/16"/18 bit and bought a matching allen head stud from Home Depot. Screwed it in from the front of the TB.

I don't plan on having to make any more adjustments to the Min Airflow but during the time that I was doing the TB mods I had to make a lot, so working from the front saved me a lot of aggravation.

I tried completely closing the butterflies but the blades began to stick slightly when closed in that position. So I had open them a tad to eliminate that.

Part of what I was dealing with was the stock converter which caused the RPMS to drop too low when the trans was shifted into gear. I wanted the in-gear R's to hit at around 750 to give the nice lope my son wants but without causing the R's to be too high once the trans is shifted into Neutral or Park. So it called for a lot of trial and error adjusting to find the sweet spot.

Also, the effect of the 109 LSA of the cam I installed created more overlap than I had anticipated. Before ordering it, I calculated the numbers, but numbers are just that; numbers. Hard to translate numbers with any degree of accuracy into how the engine will idle and sound. As it turned out, the sound was GREAT, but, at first, the idle was uneven. I didn't like that.

After the first round of TB hole mods, he took the car for a long drive and returned telling me that he had to two-foot it at stop lights. The idle was dropping to the 500/550 range. Engine never cut off but felt and sounded raggedy. So more mods were in order.

After the second round, he returned saying it was a little better but still not quite right, so round three began; etc., etc.

I don't know the IAC counts; my scanner bit the dust; literally. A friend ran over it with his truck. My fault for leaving it laying beside the car while I was working on it. Said he'd buy me a new one, but I told him "No"; wasn't his fault. I'm hoping the counts are in the low to mid 20s, but right now, there's no way for me to tell.

You were a GREAT help. THANKS AGAIN!

Jake

West Point ROCKS!

RACEON

Any pics as to where you drilled? I am not seeing this, little challenged here.

JAKEJR

No I didn't take any photos.

If the TB is installed on the car, remove the accordian, look directly into the front of the TB. At the lower, center of the TB, buried deep within the TB is a hole. Mine originally measured 5/32. That hole intersects the IAC air chambers.

If off the car, lay the TB on it's backside (the side that mates with the intake manifold (LT1). Look in the same area; lower center.


Jake

West Point ROCKS!

RACEON

Thanks. Mine is a AS&M and was modified. Idles well, but I am sure could be better at 800 in gear and 850 out of gear

Joe B

I Just saw this thread now.....Sounds like you've got it figured out just about perfect for your combo.......When I had my factory GM Throttle body (which was bored to a 52mm unit).. Like you, I ended up "needing" to drill my IAC hole in small increments (trial and error) up to just a hair over 1/4 inch diameter (IE: 1/4 inch drill bit fit a bit loose after I was done).....(Old LE-1HL Cam was on a 108, and the newer solid roller is also).........Here is my car domain page for my 95 Corvette.....I just recently switched to a Arizona Speed & Marine 58MM unit, and put up some pictures of the process I went thru with mine (IE: Stock Throttle Body , and with the new AS&M 58mm unit) ....It was interesting, my stock throttle body "needed" the 1/4 inch IAC hole to be right, and the new AS&M 58mm unit "on the same exact combo" (after the IAC was sealed in & drilled out) only needed a bit over a 1/8th inch size IAC hole for the same good idle & drivability charateristics (and similar IAC counts...which were tuned to be between 32-35 when warmed up) http://www.cardomain.com/ride/2467023 ..... PS: Page's 6, 7, & 8 on my car domain site pertains to the throttlebodies...............Joe

Joe B

Also, another thing that's worth mentioning that some still don't know about.... "IF" your interested in getting the nicest possible idle out of a cammed LT-X set-up (if you haven't done it already).......Next time you have the LT intake manifold off the car.....flip it over, and remove the bottom (metal cover with the 4 bolts)......then remove ALL of the large Allen type pipe plugs that are under it......By removing the large pipe plugs, you'll gain access to the individual IAC port holes that go out to "each" manifold runner (there are two under "each" plug)..........From the factory, these holes are not perfectly even (size wise) from hole to hole like they should be.....And (very commonly) one of them is usually REALLY bad (small)......Many of them also have thick aluminum burrs partially blocking the passages due to a sloppy factory drill job......Take your largest hole out of the bunch (verified by sliding drill bits in them), then once you get the drill bit for the largest one....Drill the rest out to match.....Then RE-install your pipe plugs with some permatex #2 (the non-hardening sealant), and ofcourse loc-tite your metal splash cover back on.........This process also made a noticeable difference on mine, and is something that I always do on ALL of the LT-X engine's that I do Heads & Cam packages on.......The end-result is Not as drastic as the IAC hole being sized properly, but it's a nice difference none the less..............Joe

JAKEJR

Thanks for that additional info, Joe. I had no idea those idle holes were accessible after removing the Allen head plugs.

Next time I have his intake off - or mine for that matter - I plan to do as you suggested.

Jake

West Point ROCKS!