ULTM8Z

I ended up doing a separate IAC manifold similar to a NOS under-manifold system.

Here it is. It's a little prototype-ish in appearance, but I'll probably go back and do something a little more professional later on. I just wanted to quickly prove out the concept rather than do something really elaborate and have it not work at all.

ULTM8Z

Here's the underside view....

The black box is an off the shelf manifold that McMaster Carr sells. It comes pre-tapped for various sizes that you can select. I used 1/4 NPT for the inlet and 1/8 for the outlets. The number of holes is fixed (unless you make your own).

The brass fitting in the upper right protrudes through the manifold valley cover.

ULTM8Z

From the top side, here's the fitting protruding from below...

ULTM8Z

I then put a 90° fitting with a hose barb to run a hose up to the front of the manifold where the inlet is...

ULTM8Z

I had to drill and tap some 1/8 NPT holes at the port exits...

ULTM8Z

Hmmm....I thought I had the rest of hte pictures uploaded to Yahoo (showing the air inlet). I'll have to post those later.

But a quick description: the air inlet is completely divorced from the throttle body. It's a fixed orifice which I "tuned" the diamter for the idle speed through trial and error such that I need less than 10 steps to achieve 675 rpm. At first, there was a huge vacuum cleaner type sound (you can imagine the engine sucking all that air through a hole the size of a drinking straw. But I solved that by hooking up a small lawn mower muffler to the inlet. Overall, with most of the hardware hidden under the manifold and/or plenum, it's a fairly non-descript modification that I'm sure I can refine to make it virtually undetectable.

But the important thing is that with this set up my BLM's are only off by 2 at any point in the RPM range!! Woo Hoo!!! Before, at idle, they were off by as much as 15-20 points.

Jaysz28

iTrader: (30)

Very nice work. Wish I had the skills and time to come up with solutions like that

-jason

Grumpy

So the IAC just feeds through this external plumbing?.
What do you set the min idle speed at, using the butterfly?.

It's interesting......

ULTM8Z

Grumpy, the throttle body is unmodified and the factory IAC passage in the throttle body works just like stock. During cold starts when the IAC steps are higher than 10, the IAC motor will supply the additional air in the conventional manner. What's now changed is that the main throttle blades are always closed during idle. The air that used to go through the main throttle blades is now rerouted through this auxilliary intake tube. At normal operating temperature, practically all the air is coming through my idle air tube (minus the <10 steps worth)

liquidh8

Very cool. Good work!

ULTM8Z

Ok, here are the rest of the photos...

Here's the inlet. It's right behind the A/C delete pulley.

ULTM8Z

Here's another side view. That long black hose under the plenum is barely visible when the manifold is fully assembled.

ULTM8Z

Here's the overall shot of the engine. Like I said, the modification is somewhat non-descript.

SBNova

So your using the muffler as an air cleaner? You made a dedicated idle air setup so you can have the throttle blades closed, right? I dont know anything about miniram problems, I was just curious.

RednGold86Z

iTrader: (1)

Maybe there's a valve with a **** or screw (and safety wire it) that you could use to set your minimum air for that setup, rather than calibrated orifices. Actually, you could rig an external IAC (and computer control it), and plug the original TB hole. I know I've seen remote IACs available (forget if you already mentioned wanting to try it).

I'm still a little surprised (but I don't doubt your results) that it was soo bad at idle before, and that it's so sensitive. I mean really, at idle, there's so little flow altogether, and why is it better (as opposed to worse) with the custom-manifolded IAC (basically why is the small manifold better than the big plenum? and why split with the banks from such a distant TB?). Wish we had someone with some good flow software. But, as mentioned in your other post I think, LT1s had something like this, right? So maybe the work has already been done...

ULTM8Z

I looked at some flow control valves, but I thought it would be a simpler set up with a simple orifice. Now that I have it where I want it, I don't have to mess with the idle speed anymore.

The split BLM phenomenon is apparently fairly widespread, particularly with LT1 guys and aftermarket throttle bodies that do not employ the IAC passages in the LT1 manifold.

To be honest, I still don't have the idle quality where I want it- even with all cylinders running equally. It's quite baffling actually, that I still have some roughness considering my latest emissions numbers and nearly uniform header temps. Right after it goes to closed loop, it idles smooth as glass for about 10 minutes. It then progressively gets worse to where it's rough enough to annoy me.

Remember when I had my misfire and the #2 cylinder wouldn't react to anything (removing spark plug wire, disconnecting injector, etc). Well now, every cylinder reacts the same way if I disconnect an injector or a plug wire. That's a very good indication that whatever the problem it's system-wide and not isolated to any one cylinder.

Even though it's WAY better than before, the way it's running, it's almost like there's some bad connection somewhere, such that once I find it and correct it, it's going to immediately run perfectly! The problem is, every connection checks out! Every ground (voltage drops on grounds are all below 5 mV), every signal wire, every power-wire, every injector, every sensor, every spark plug, every plug wire, everything! All scanner data looks good (BLMs all at 128), no anomalous readings, no codes.

I think I really need to get the O2 sensor fixed on my gas analyzer to see what this car is really doing at idle. I have a hunch it's running rich because I can smell some sulfur out the exhaust.

mike1986fyrbird

Neat idea.

Couldn't you just hook it into the air intake tube instead of having the muffler?

69 Ghost

iTrader: (1)

Good to see that you solved this problem. As mentioned before I believe that this is an issue with the earlier TPI's and nobody knows about it. The later TPI manifolds have IAC holes in the intake like a LT1. Guys with other aftermarket intakes probably have the same problems but don't know it. I was wondering how you logged both sides? There is a switch in the 727/730 bins for a Right O2 sensor. I think I looked it up and it looks like they are in parallel. Either way it would be nice to see if that switch could be used for a second O2 sensor even if it could be used for datalogging only. I think your solution is fine except I would look into just hooking the hose to the intake if possible and it would be even more stealth. I never would have thought about doing that to a Mini-Ram

ULTM8Z

I originally had it hooked to the main air intake duct. But it was still loud and you could hear it in the car.

Try this experiment , take a drinking straw and suck air through it as hard as you can. Then imagine and 350 cubic inch engine sucking air through it!!

It really needs that muffler to silence it.


69Ghost, I have a connector in the O2 wire on the ECM harness. I simply change back and forth to monitor both sides during diagnostic procedures. When I change from one O2 to the other, the BLM's don't move. Before, they used to move up to 20 points.

Grumpy

Is there some on line pics of the 2 TB's (with the different IAC channels) you talk of?.

I *think* on of the things your seeing, with the warmer it gets problem, is due to the fuel being more gaseous, ie better vaporized, as oppsed to atomized at the lower *MAT* temps.. Once, ya get to a MT (Manifold Temp) of ~130dF, you're at the lower end aromatics boiling temps.. Running above or below this temp., is the *advantage* of the air gap type manifolds, to a certain degree. I've talked about this in the past with my dropped floor plenum manifold, where any raw fuel, has to drop below runner level, and then has to be vaporized to get off the floor, and into the runner.

Juss adding some fuel to chew on......
Not dismissin what you've found by any means....

69 Ghost

iTrader: (1)

Here is a pic of a earlier TB that does not seal up the IAC passages. I have a ASM modified TB like this and I JB welded it to seal it off instead of a pipe.

http://www.noid.org/~lj/PCM%20Tutorial/SplitBLMinfo.htm

91L98Z28

very interesting.

It might be painful overkill, but you might be able to use an individual orifice at each cylinder (perhaps using nitrous jets?) to keep the orifice restriction right at the runner, which might quiet it down and let you run the main air feed back into the main air intake duct.

It would be interesting to build an intake with a variable size plenum to use on a dyno mule and see if the small plenum size of this intake contributes to the problem.

It would also be interesting if the TB could be mounted in the middle of the intake (between the front 4 and the rear 4 cylinders) and if that would smooth things out. Ford's 5.0 intake comes to mind, with the throttle body faacing the passenger side of the car and feeding the intake plenum from the center.

I also seem to recall a GM SBC intake that feeds the TB off from the side, but I can't remember if it's some ancient thing from the past or a modern intake for fuel injection...

DAVECS1

iTrader: (2)

I think what you have done is great. I guess my only question is why the need to idle so low. I guess I am not to picky. I will let it eat at 1000 RPM if that is were it smooths out. I usually find my off idle is much more responsive when I take this approach. But I am also getting lazy in my old age. I use a C950 for crying out loud

I just read the "split BLM curse" for the record out of the three Minirams I have tuned on a 730, this split BLM goes away after 2000 grand or so depending on stall and cam. If your worried from a dead stop thats what line locks or two footing it is for. Clear it out and bring the RPMs up. Anything over 230 duration and .5 lift I usually do not do much closed loop control at idle an low RPMs anyhow, because in order for the car to stay lit it is going to be rich and grumpy for the most part. I think it is kinda fun, but I could see how someone who drives their car everyday may want to tackle the issue.

ULTM8Z

I'm running a factory LT1 Vette torque converter. It gets to be a real chore holding the car at a stop in drive if I idle it much higher than 700 rpm. So I like to keep it fairly low.

91L98Z28

You may find this link of interest if you aren't already familiar with it:

http://www.theherd.com/articles/torque.html

GM's S10 torque converter will drop in place and offers a bit higher stall with factory-stock drivability. I've driven an LT1 Caprice (9c1/cop car) with this converter, and it's very stock like (even the wife/fiance could drive it with no issue) but it does afford you a few hundred extra rpm.

I've also driven a 2400 stall 9.5" converter. In terms of how noticable/slippery they are, On a scale of 1 to 10, with "1" being a stock converter, and "10" being the 2400 stall 9.5" converter, the S10 converter ranks about a 3.

ULTM8Z

The reason I went with the Vette converter was because it was a factory converter that was rated at 400 ft-lbs of torque at 6000 rpm. Do you know what the S10 conveter is rated at?

91L98Z28

I wasn't aware that GM even rated their torque converters for HP/torque, and I'm not aware of any particular ratings on this particular converter.

My (limited) understanding of the S10 converter is that it has internals of equal load handling as any other 700r4/4l60(e) converter. Certainly some of the Impala guys are running 300+hp against it with no issues (in cars much heavier than ours!!). it's no race 9.5", and I doubt it would stand up to trans brake/5000rpm launches/heavy shot of nitrous/whatever, but it should be as good as any other stock GM converter. Main advantage is that it gives you a few hundred extra RPM of stall while still driving like stock.

The LT1 caprice that I spoke of before (the one I've driven) is a daily driver in the family. It's been in there for about 20,000 miles now with no issues. It was intalled along with a rebuild to the 4l60e due to failure of the sun gear at about 100,000.

jsup

What was the problem you were experiencing? I am having an issue on light throttle, around 2400 RPMs. Was this an idle issue or a problem under power?

Kevin91Z

iTrader: (1)

Its my understanding the S10 converter and the Corvette converter are the same, both rated at 2000 rpm stall speed.

ULTM8Z

It was an idle issue. BLMs were way off from pass side to driver side and this modification was an attempt to remedy it.

ULTM8Z

Well I resurrected this IAC manifold. Good thing I didn't remove it... just capped it off on the top side of the intake.

But it seems to make a significant improvement. Even with the inherent idle airflow distribution problems of the miniram, I was able to get the off idle response pretty decent without the IAC manifold. But it would vary from stoplight to stoplight just enough for me to get annoyed. The batch fire plus the distribution problems was always a PITA to try to tune around.

I theorized that the variability was depending on what cylinder was firing at the moment I opened the throttle blades. I.e... if it opened on a "lean" cylinder vs a "rich" one, vs one that was closer to stoich.

There was also just enough variability in the idle quality from startup to startup... again just enough to annoy me. Seemed like depending on what cylinder the injector timing was synced to when the engine first fired up made a difference in idle quality. But again, that was the only thing I could think of that would cause that kind of variability when essentially nothing else changed.

But this little IAC manifold seems to have solved the issues. The only difference this time is I routed the intake hose to the PCV valve instead of that inlet contraption above. Works great with this set up in terms of being quiet.

Now I don't need to do that dual o2 setup anymore (with the voltage averaging circuit). Ended up I solved the issues on the air flow side instead of the fueling side!

Dominic Sorresso

In my recent work on Inj. Bias, the split BLM issue in the LT-5 is significantly mitigated using an accurate set of values. It also has increased gas mileage and reduced exhaust drone at low rpm operation.

ULTM8Z

Interesting... another side effect of the better distribution... the engine is running 5-10°F cooler at idle. The lean cylinders must have been generating significantly more heat.

Bill Chase

Old thread I know, but very cool stuff. Some smart fellers here when the 830/ 727 was the only affordable option for most guys. So yeah, resurrected this one. Makes me wonder how nasty a cam a guy could really get away with using the old batch fire ecu and still have closed loop idle control etc.

TTOP350

iTrader: (8)

I have a gnarly solid roller cam in my 89 formula

ULTM8Z

Hah... 14 years later... still running that IAC manifold. Still doing well too...

badgta

iTrader: (4)

I had somewhat of a issue on that TA i originally had.. Did a intake manifold gasket swap to sure the dreaded china leak from back in the day..

Started it up and it didn't wanna idle good, had to crank on the throttle blade screw, and cold starts you would have to baby the throttle until it held for first 30sec or so..

One day when cleaning under hood, i notice a wire of the 4 on the IAC Valve wires looked out of place, as if it pulled/popped out somehow... I pushed it back it and it clicked, i thought i broke it so i started it up and it shot up to 2500rpm and held there!!

I realized suddenly that this was the start/idle issue all along, and backed down the throttle blade screw to near closed and it idled (PERFECT) at 1k till warm then 750 when hot, 600-700's when in gear!!!

Also, when you started it cold, hot, blizzard temps, didn't matter after that, first turn, instant hit and perfect idle! Didn't even really "CRANK" it just "HIT/STARTED" as soon as you turned the ignition switch to crank position. That thing was one of the efficient v8's i ever owned. Dusted numerous 5.0 Stangs with the factory 350 tuned port and a kinked 2.25 exhaust.

badgta

iTrader: (4)

One more question...

Why even try to get exactly 128 BLM?

I mean, the computer is programmed to add to or take away....

Like a football field, the perfect seats are at the 50yd line right? But you have all the way to both "END ZONES" each way from the 50yd line. As long as the game is being played within the field I.E BLM 108-160, does it matter so much to try to keep the game being played just at the 50yd line? Does an engine not make best power with a 12.5:1 AFR? That certainly wouldn't be 14.7 AFR like 128 is if i am correct.

If it's rich, it will be below 128 and fuel will be pulled away, same with more than 128, fuel will be added. Sure ya want to get it "CLOSE" but is it really worth it to go all out for exact 128 when the car will probably never be that at all times?

Get what i am saying? Is that right or wrong? The rationale of thinking of how it works i mean?

84Elky

As TTop350 indicated, can run virtually any cam with TPI (despite many post to the contrary), provided time is invested in tuning. If using SD, S_AUJP allows CL idle and OL above idle (which solves many issues), or CL always.

Bill Chase

curious. What would your definition of a rowdy cam in a 350/383 tpi be? 65° overlap on a 112? 🤔
was commenting more on the short runner stuff long runner tpi would be a lot more forgiving in many ways when it comes to tuning. Not so with the mrII and custom efi intakes. The benefits of sequential injection and individual cylinder correction help a lot when you crossed that line.

Bill Chase

To summarize, you shut down the throttle plates to zero, added your plenum and lines, then set iac so it is around 10% or less at idle? And the feed to the iac is pulling from the pcv port so it idles drawing from the crankcase?
I'm getting a list together if you have one, or any other insight, before I start working on it I'd appreciate it very much!
Tha

ULTM8Z

Pretty much, yeah. Though I don't have it routed to the PCV. I routed it over to my air filter. It's a fairly long 3/8" fuel line that I used that comes off the NPT nipple at the back of the intake manifold and routes to the front of the engine and off to where the battery area used to be (which is where my cold air tube routes to the air filter). With the 3/8" hose, I also cut it about half way down the line and inserted smaller ID vacuum line (about 3" long) to tune the idle speed by using a "fixed orifice" approach. The vacuum line is 3/8" OD, so it fits into the main 3/8" line. Even with the 3/8" hose, it can be a pretty big "vacuum" leak, so I basically "regulate" the leak to get the desired idle speed.

Initially when I made this setup, it was pretty noisy having the engine try to suck air through a small tube. It was basically Hoover-vacuum like sucking sound. You can see that funky looking "muffler" gadget I tried to create (picture in post # 13 above). But over time I found that if I used a longer hose and then plumbed it into the air filter, it quieted down A LOT... can't even hear it now. Also, I had the intake off the car just this past year during my 383 swap and all the solder joints and copper lines have held up very well over the last 15 years (I used a high temperature solder with a propane torch to generate the joints). So I never had a reason to go back and rework it as I had indicated I would probably do in the first post.

Lastly, make sure to use locking hardware to attach that black manifold box. I used lock nuts and the non-permanent loctite just to make 100% sure the screws don't come out and fall into the lifter valley.

Also note this only works because it's a MAP car. On a MAF car (I forget what yours is), you'd have to plumb the line to after the MAF sensor, otherwise it's extra air that ECM isn't accounting for.

Bill Chase

I've got some parker 3/8" Id flow control valves for Industrial hydraulics, might take some of the fight out of getting it setup right. As far as the individual lines, probably just use 1/4" compression fittings. Have a stock 94 intake I've got measurements from. The holes are pretty small. Could probably get away with 3/16", and just use pushlock tubing and fittings, make a small baffle to keep hot oil off it, similar to the OEM valley heat shields for the l98 and ltx platforms.