ITB & idle changing - Question about stability

[Yves]

each individual EGO settles around a certain number, but it's not steady at that point.

[lutorm]

each individual EGO settles around a certain number, but it's not steady at that point.

Are we talking +- tens of percent or a percent or two?

The part that settles at nonzero you can trim out with the trim tables (assuming you have one trim table per injector, which I assume) and might represent real VE differences between the cylinders. The random part might be MAP noise. You could try cranking up the MAP smoothing to see if it goes away. This won't work for actual driving since then MAP will actually change, but if you're just idle testing it would at least tell you if the EGO random part is from MAP sampling error.

[Yves]

10% and more in some cases. Like I said it's strange given that the flow meter says they flow the same.

[kaeman]

I have an old set of hilborn itb's, my idle was smooth and easy to tune, I was not using any iac setup. I used a flow meter on the stacks and balanced them at idle. After getting them well balanced I went for a test drive and found that from 2% throttle opening to 7% throttle opening one bank had about 9% more airflow than the other bank, above 10% throttle the airflow balanced out again, upon contacting kinsler and hilborn, I was informed that was very much normal for that style intake manifold, both banks of throttle blades rotate the same direction, and at low throttle the air path to the pistons on one bank was more direct than the other and therefore had better flow than the other bank at the throttle values noted earlier. If you search youtube for videos from hilborn and kinsler they address adjusting the butterflys to get even flow. I removed my itb's because most of my driving was done in the 2% to 7% throttle position(cruising and easy highway driving speeds up to 70 mph) and that is where the banks are out of balance. Medium to hard acceleration (above 10% throttle opening to wot) banks ran very even afr's. My camshaft is in the 254,260 duration @.050 with a 110 lobe center, it idles smooth and both banks run pretty close afr's with the single plane manifold at all speeds and throttle openings.
I ran the old hilborns for about 8 years before changing to the single plane manifold. I believe that you will get your system close, but the nature of the itb beast is that with individual runners for each cylinder you will always have some minor differences in airflow just because castings are not perfect and you might just have to settle for great looks and good running, with excellent acceleration and be happy.

[Yves]

Maybe it's just me, but if the flow meter indicates the same airflow going into the cylinder and the AFR's showing an imbalance, where does the air stay ? I don't quite see this.

On the other hand, in my case the front cyls are running noticably leaner than the other ones.
Based on this I tend to believe one of these factors could be involved :
- IAC air which enters the vacuum chamber at the rear of the manifold giving more air to the rear cyls than the front ones.
- uneven burn speeds, because of which not all fuel is burned showing up as a lean condition.
- uneven exhaust speeds/imbalances in the exhaust system, causing one system to have a noticably higher effect of valve overlap than another cylinder.

The problem though is that this cannot be easily be distinguished with the equipment at hand.

I also have to point out that all of my valves were lashed before balancing the TB's.

Before going to an ITB system, I ran on a singly plane, although with a Holley DP. This was a real PIA to setup with the camshaft involved. I do think that an EFI system will be better suited to manage it, but it will remain a PIA nonetheless. The ITB system is much smoother all around with more power at low to mid range compared to for example a Victor Jr.

[billr]

"This has worked so far, but it has one draw back and that is that not every throttle seems to flow the same amount making it difficult to maintain a stable idle speed and AFR. "

That is from your first post in this thread. Is a "stable idle" still the only real problem? I tend to ignore AFR and pay more attention to how the engine runs. Post a recent MSL so we can get focused back on the idle. An MSQ, too, if the pertinent one isn't in this thread already.

If, however, your "real interest" is actually combustion theory and characteristics of different intake systems, then I apologize for my intrusion and will bow-out.

[Yves]

Bill, the more even the throttles are the more stable the engine runs at idle. Since I balanced them and I got nonetheless an uneven fuel requirement, I'm wondering what is happening.

I have logs, but the log file size allowed here is stupidly small. I deleted most of my log and am still above 1 MB in size.

[Yves]

Attached a shortened log at idle. Note that it has parts where I plugged the iac causing rpm to drop.

Turns out that the iac is causing the unbalances I've seen that the fuel trim corrected.

I will add the msq in a next post.

[Yves]

Attached the msq.

[billr]

What are you using for IAC? Does the stepper turn the idle-stop screw, common to all throttles?

Your last post said you "plugged the port", so I'm kind of confused.

[Yves]

Sorry for that.

The IAC has 2 inlet lines connected to my airboxes that I plugged. Those are tee'd and connected to the DIYautotune block with Jeep IAC. The outlet of that block goes to the vacuum chamber which is underneath the runners. Every runner connects to the vacuum chamber with a drilled hole in the runner.
So the iac is a stepper type valve that is run by MS based on closed loop.

I have been thinking about opening the throttles more, but in order to have the effect of the iac disappear it would have to be completly closed, rendering it unuseable...

[billr]

With the drilled holes between each runner and the plenum(s), all that exact balancing of airflow into each runner is rather meaningless for the idle. The drilled holes are obviously providing a significant portion of the idle air, and those are not balanced. I gotta read through the thread again...

[Yves]

A motor pushing on the throttle to open it a little more or less would be ideal, but I haven't found one that is strong enough to push agains the springs acting on the mechanism.

[billr]

I believe you have posted quite a bit about problems getting all those throttles to move exactly the same; things like backlash in coupling and stiffness of shafts. If so, trying to move the throttle-stop may not be all that much better. However, I'm sure we can come up with a suitable actuator. It may well require a different stepper driver, or PWM control, and finding one readily available in your area will take extra research. How serious are you in wanting to try that?

After browsing through the thread again, my understanding is your question is simply why the idle is still a bit unsteady, even after careful balancing of airflow. Hasn't that already been answered? Besides the drilled holes, there are many other factors that affect idle. Not the least of which is that compression pressure is quite low, so the combustion process is inherently poor; I think that makes it a bit variable from event-to-event even if you could ensure exact same airflow and mixture distribution. Which is also difficult, since mixture velocity is at its lowest, with least turbulence. Ah, d***... I'm wandering into the "combustion theory" area where I don't belong!

[Yves]

I believe you have posted quite a bit about problems getting all those throttles to move exactly the same; things like backlash in coupling and stiffness of shafts. If so, trying to move the throttle-stop may not be all that much better. However, I'm sure we can come up with a suitable actuator. It may well require a different stepper driver, or PWM control, and finding one readily available in your area will take extra research. How serious are you in wanting to try that?

After browsing through the thread again, my understanding is your question is simply why the idle is still a bit unsteady, even after careful balancing of airflow. Hasn't that already been answered? Besides the drilled holes, there are many other factors that affect idle. Not the least of which is that compression pressure is quite low, so the combustion process is inherently poor; I think that makes it a bit variable from event-to-event even if you could ensure exact same airflow and mixture distribution. Which is also difficult, since mixture velocity is at its lowest, with least turbulence. Ah, d***... I'm wandering into the "combustion theory" area where I don't belong!

I think you're confusing me with someone else here.

And, my last test shows that the AFR's and stability is better when I plugged the IAC. So there's a 1/1 relation to the iac not with everything else you have mentioned.

[billr]

Yes, it is quite possible I am confusing you with somebody else. Balancing ITBs is a problem, and is discussed fairly often here. Sorry.

As to the rest, I'll wish you "good luck" and bow out. I hope I have answered your wonderment about why the drilled holes between the runners and vacuum plenum negate the careful throttle balancing, but kind of doubt you believe my opinion.