ITB & idle control question

[Yves]

I'm running a Chevy SB with ITB's. The manifold is a crossram setup.
The manifold was equiped with a vacuum chamber blow the runners and each runner has a 5mm hole connecting it to the vacuum chamber. The map sensor, brake booster, pcv and IAC is also connected to this chamber. So far it has been working quite well. All vacuum lines are connected to the vacuum chamber at the rear (so where the distributor normally is on a SBC) except the map line.

Recently I went to a setup with one wideband per cyl. I'm seeing some cyls make erratic jumps. Especially the rear ones (7&8) are doing this with number 7 being the most offending one.

Could the iac be causing the rear cyls to run more lean at certain moments ?
The reason why I have been thinking this is that the rear cyls seem to run really lean when starting up, gradually improving as the engine heats up and thus the iac is closing more.

[Matt Cramer]

[Six_Shooter]

Yes, your thinking is correct.

I'm putting a similar intake on my SBC (Crower mechanical injection modified for EFI injectors), and when I was going to run IAC, MAP, etc I planned to go with two independent manifolds, one for IAC and one for MAP so that the IAC would not effect the MAP readings. ALl of my reserach suggested this was the best approach when using IAC and MAP.

I have since decided that since I will be using a Megasquirt (original planned on using a Delco '7730) that I will be running with no IAC and use Alpha-N. If it doesn't work as well as I'd like I'll add MAP and/or IAC.

Is there a port at the front of the chamber? You might want to try moving the IAC or MAP connections to see if the problem cylinders move, and would be a way to verify your theory.

[Yves]

six-shooter,

It's not so much the iac influencing map. I run an itb setup btw.

However, what I noticed was the misfiring, jumps in AFR coming on whenever the IAC would start to engage.

[Six_Shooter]

Have you tried adjusting MAP enrichment to compensate? I don't see an IAC enrichment ability is any of the MS firmware, unless I missed it. If there is an IAC enrichment setting try using that. ITB intakes seem to be very touchy when it comes to any change in airflow through them.

FYI, The crower intake I'm speaking of is ITB, just originally mechanical injection, first used back in the '60's.

Mine is very similar to this (before the EFI injector conversion):

[Brian]

I'm doing something very similar. SBC 383 with a chinese ITB, but it didn't include a vacuum chamber. I added 2 chambers. One for PCV and IAC, and one for MAP. The PCV/IAC one has 8 -3/8 tubes, and the smaller MAP has 4 - 1/4" tubes, but they Siamese the intake ports.

So far so good.The larger tubes for the larger chamber also help hide mis-adjustment in the sync.

If the back 2 cylinders are idleing lean, maybe you should check and adjust the synchronization of the throttles. If the back 2 blades are not closing as much, they will idle lean as each cylinder is getting the same fuel. I am experiencing some throttle hang. sometimes the TPS (on back drivers cylinder, I think #8) reads -5% sometimes 3%, depending how fast you let the throttle slam shut. You may be having the same issue letting in too mach air. The crappy chinese linkage is a little flexible.

[Yves]

I know about the cylinder synchronisation. I adjusted them. With my setup the throttles are siamesed per 2, so 1+3, 5+7, 2+4, 6+8. This makes it hard to synchronize them one by one, but they are pretty close.

Anyway, I'm not having any issues with map/PCV and brakes being connected to the same vacuum chamber. I'm just having the issue that the rear cyls seems to draw more air from the iac than the front ones. If I would add another connection in the front of the chamber the front ones would draw more air, but the middle ones would still be at disadvantage.
In essence the tubes that I see you guys using are a vacuum chamber but will give the same consequences imo.

[suberimakuri]

Hmm. Sounds somewhat mechanical.
Wonder if perhaps ditching the idle valve and opening all throttles a little more plus use of idle advance would work.
This is all I do on our itb setups, 0 deg Celsius to mid 30s or even 40 deg range. Couple hundred rpm variance cold to hot no biggie ?

[Yves]

Hm, a couple of hundred rpm difference with a big cam can make the engine run really difficult (better than with a plenum setup, but still not good). The effect is reinforced by the lengtening of the rods and other items that make up the linkage.

It's not a mechanical issue imo, however to note is that setting idle rpm with the idle screws is subject to a balancing act in itself. I mean set it to high and the ability for the closed loop to control the idle rpm diminishes. Add to this that the throttle blades are more angled at that point and the engine gets more leverage to pull them open against the springs that shut the throttles.
One issue I have on my setup is that the idle screws (2 of them - one for each bank) are kinda hard to reach in fully mounted condition (caused by the airboxes, but that's another story)

Also the engine is extremely sensitive to AFR. I mean anything above 14 is not liked and increases the number of misfires taking place. Above 14.5-15 is virtually impossible.