Camaro_freak

iTrader: (1)

I'm having trouble getting my IAC counts down with my new setup and am looking for suggestions. First, some background on the setup...

The engine is a 383 CI with forged Lunati crank, Eagle H beam rods, SRP forged pistons, ~9.6:1 SCR. AFR 210 competition ported eliminator heads with custom AI cam (~.55 range, and 116 LSA), Comp 1.6 ultra pro mags, LS7 lifters. STS turbo running only 5 psi now until I get tuning done. LT4 intake manifold ported for monoblade TB, that I have UN-modified to restore the idle air port.

I got it running with a Professional Products 58mm TB over the weekend, but IAC's were pegging at 130. I did the "drill mod" in the TB and incrementally went up to 7/32" hole (on the high end from what I've found on this and other forums), but IAC's are still around 85 at startup, and work their way up to 130 as it warms up. I have the idle speed set to 900 rpm, but plan to lower it down to 700-800 later on, which should help. I can get the IACs down if I open the throttle blades slightly, but this is not ideal since it is dumping air into the plenum rather than using the idle air passages. So my question is basically this... should I continue to drill the idle hole larger, or use the idle screw to open the blades slightly?

BALLSS

if you open the blades more your TPS volts will be off so you would then need to slot the 3 holes of the TPS to "clock" it back. TPS vdc should be around .67 vdc closed and 4.5 vdc full open.

stock hole is 1/8" in TB. You are now at 7/32 and IAC is dropping. My TB I wound up at 11/64" to get them to 35 at idle but every set-up is different

Given your IAC hole is now 7/32, maybe doing a "slight" blade adjustment and then the hole slot mod on the TPS will be your formula.

you could try drilling the hole 1/64" larger to see if that gets you from 85 closer to 35 at idle......although there is no "reverse" of the drill mod hole size.

Might want to check your TPS vdc before next step and confirm as a base it is already in spec.

Camaro_freak

iTrader: (1)

My closed throttle voltage is .63, which I fine tuned to match my stock TB. So yes, if I decide to open the blades slightly I will re-adjust the TPS accordingly.

I think I am going to drop my idle RPMs to 700 and put the elbow back on for this process since there is a slight amount of air being pushed in from the turbo that's not accounted for right now. Like you said, once you drill bigger there's no going back smaller. Although I think I could insert a pop rivet or something to shrink it if worse came to worst.

TXsilverado

iTrader: (18)

I had to hog a throttle body out pretty good on an old combo. it was just a heads/cam 408 with a fast intake and 90mm TB, but that motor seemed to want a lot of air at idle. it was a cartoon large hold in my blade, but once the IAC steps were in range you could teach a 15 year old girl how to drive with my 247/255 cam.

BALLSS

when I was drilling mine I did source some brass sleeves about 1/4" long that were flared on one end (saw them at a local hardware store but don't know what they are actually for) I could use if drilling the hole to big

1/64" does make quite a difference in IAC counts.

dropping the idle will affect IAC. If you can make tuning adjustments in your PCM might want to do that first and see what the IAC counts are

Camaro_freak

iTrader: (1)

Yeah, I'd better get the idle speed and induction right before drilling any bigger. The reason I bumped up the idle speed in the first place was due to poor idle quality, which may have been related to the IAC counts being high or dumping in the plenum instead of the idle port.

Also, I get terrible surging/bogging under deceleration. I'm hoping that resolves with the IAC in a better range.

BALLSS

aftermarket & larger TB's are not plug & play, especially on H/C motors. Get your idle speed and intake sorted as the higher RPM you programed was just a band aid not resolving the pegged IAC counts

The IAC needs head room above 35 ish for when power steering, AC or other accessories add load to the motor so it keeps it idling right

your surging/bogging may be a secondary result of the pegged IAC although it could also be programming. I assume you have a higher stall TC

Camaro_freak

iTrader: (1)

Yes, 3200 stall Vig converter. It had the bog/surge issue even with the stock engine and it started right after I installed the converter. I bumped up the idle speed at that time and it was good enough for me. Seems to be worse with the new engine though so I bumped idle speed even higher, but 900 is higher than I want it.

BALLSS

tuning the PCM is above my pay scale....I suspect there are several parameters that need to be adjusted regarding higher stall TC and drivability issues. I have always used mail order tunes as my mods changed. A hands on tune with dyno though would be ideal to get the most out of it IMHO.

Camaro_freak

iTrader: (1)

Thought I'd give a quick update... I looked at the IAC path through my TB and noted it has MUCH smaller passages than the stocker. I suspect that even when the IAC is fully open the path is too restrictive. I slightly ported the path yesterday with a dremel, the the IAC still pegs fully open at startup and runs like crap. I think it needs more porting yet, otherwise I will have to resort to cracking the blades open a bit or drilling the idle bleed hole bigger yet.

BALLSS

are you sure your IAC itself is good?

Camaro_freak

iTrader: (1)

Yes, the IAC worked fine with my stock TB, and when I open up a vacuum port to get more air in (same as cracking open the blades) it runs fine with low IAC counts.

ACE1252

Throw the stock TB on it. See what you get(if you have not already).

I agree that the high IAC counts would suggest some kind of restriction somewhere(if all the electronics are working right).

garage_queen

iTrader: (3)

Try adjusting the TPS down more. I had the same issue with a Holley 58mm TB. With the TPS @ .46 fully closed, the IAC counts came down to normal.

Camaro_freak

iTrader: (1)

I ported the IAC passages in the TB with a mill and dremel and finally got the IAC counts to the normal range. Car idles pretty well and I'm going to try driving it tomorrow. Final bypass hole size ended up being 17/64", which I realize is considerably higher than most. Blades are completely closed, and none of the IAC air goes into the plenum. I used epoxy to fill the side passages that normally feed the plenum.