Hey just want to know that if i install aftermarket ecu would be safe to remove the bac. see my bac is all corroded and broken (cap fell off and been glued on by PO and the water line snapped off). I have read that you can take it off and the car may run and it may not run. but i cant find anywhere that says it will be more successful with a aftermarket ecu.

So can i just get rid of it and use and ecu to tune

 

yes you can remove bac you will need a block off plate and have to reroute the coolant line that goes to it. other then that instead of changing the idle on the bac you will now do it on the throttle position screw . It will run you will just have to adjust the idle so it idles a little higher around 900-1000 because the bac wont be there to adjust idle under draw and load

 

If you are installing an aftermarket standalone, it can run the BAC anyway.

 

see the problem is that its broken beyond repair and if i dont need it they i will get rid of it. One more question if i have aftermarket ecu and i wanna put the ac back on will the ecu be able to compensate for the extra load

 

so will it or should i not worry about ac

 

I have both year bac for sale. $25 shipped and it's yours. N326 and one for N350. Is the only reason why you want to remove it is because it's broken?

 

How? The BAC is the way that the stock ECU adjusts the idle, so how do you propose that an aftermarket ECU does it when it doesn't have anything to control it with?

 

lol good question yeah your right didnt really think about it

 

I got an 88 na I removed the bac with the stock ecu . I just had to bump up the idle to 900. Runs fines, idles fine. If you are going to a standalone you should be fine.

 

Any decent aftermarket ECU can do it if you know how to set it up. It uses a fixed frequency PWM output, same as a boost control solenoid. Haltech can do it, Power FC can do it, etc.

There is nothing special about the BAC valve. Its basic design is the same as used in the FD, in 5.0 Mustangs, in Hondas, in SR20's. I have actually cut an s4 BAC valve open to look at it and I've done extensive testing on controlling it with a Power FC.



Here's how it works. It is a spring loaded plunger that is normally closed. There is a magnetic coil inside. Ignition power is supplied to it (through the main relay etc) on the postive side. The ground side is rapidly triggered by the ECU, in a manner not too different from a fuel injector. When the ground is cycled on, the magnet is engaged. The magnet pulls the valve open against the spring force, and air is delivered into the intake manifold, bypassing the throttle plates (hence the name Bypass Air Control).

But the magnet can only be ON or OFF; there's no "in between" setting--although other designs of idle control valves do work that way, like a stepper motor used on 4G63's, LS1's, etc. When the BAC is active, the ECU always cycles the valve on and off at a fixed frequency of 244hz (that frequency value comes from the FD service highlights document but they are the same valve functionally speaking). So every second the ECU cycles the ground ON and OFF 244 times. What controls the amount of idle air is the ratio of ON time vs OFF time within those 244 pulses. This ratio is also known as the duty cycle, and the control method is called pulsewidth modulation (PWM). The signal sent from the ECU is called a "square wave" because the signal is either completely ON or completely OFF, instead of a wavy signal you would get from a crank angle sensor or vehicle speed sensor.

OEM's use idle speed control valves for multiple reasons. It supplies extra air when an electric fan or the headlights come on, or when the power steering pump is engaged. It also has a dashpot function. That means that when you let off the throttle quickly, the BAC valve adds a little extra air so that the idle does not drop dangerously low from the throttle valve closing.

 

I think you misunderstood what I meant to say:

What I should have said: "How? The BAC is the way that the stock ECU adjusts the idle, so how do you propose that an aftermarket ECU controls the idle when it doesn't have a BAC valve to control the idle with?"

 

The factory idle is 750 RPM +- 50 RPM, so 900 RPM isn't idling fine.

I'd suggest anyone (standalone or not) keep their BAC valve. Removing it has zero advantage.

If you are oh so worried about the coolant line to the valve breaking, just remove the coolant line. It will have no negative effect. In very cold weather, the valve could possibly freeze up, but I've never seen that happen.

 

ok so keep on sounds the best option

 

what is the screw towards the rear of the bac, it sits just above the water passage and its a flat head screw what does this do. i have removed my water passage so i should now be able to keep the bac

 

That's your bypass air adjusting screw which is on USDM (and I guess Aus spec) turbo BAC valves. It sets a fixed amount of air that always flows from the BAC hose through a passageway in the UIM and into the primary ports. Even if the BAC valve is off air always flows through it. I have cut open a BAC valve and physically tested this. Most cars with that style of idle air control valve has a screw like this (FD, most 90s Hondas, etc) It is the main idle adjuster screw besides adjusting the throttle plate itself.

 

so by adjusting it i am basically adjusting the idle. i thought that was what the one on the front did ( the one with the needle)