What exactly does the BAC do?
04-26-05, 04:35 PM
#26
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74trk- the BAC is there to keep your engine from dying with loads on the car at idle. Power steering and A/C are taken care of by the air bypass valve (ABPV), with the BAC helping out a bit as needed. Some guys that remove their BAC's regret it later. Just a thought...
Hailers- OK, I think we've arrived at something we can both agree on (after just two pages at that)...
1) On a cold start, both the ABPV and BAC help to maintain an approx 3000 rpm for 17 seconds, everything being controlled by the ECU.
2) On your car, with no ABPV, your BAC goes wide open and takes your idle to only 2300 rpm on cold start. For 17 seconds. So you have a 2.3K warmup (snicker)...
3) On my car, with no ABPV, my BAC goes wide open and takes my idle to 2800 to 3200 rpm on cold start. For 17 seconds. So I have a textbook 3K warmup. And yet you say MY car is busted...
4) from the above, we can infer that even though the ABPV opens "on" on cold starts, it's the BAC that does the brunt of the work (passes the mostest air, if you will)...AND, with the ABPV off of the car, the BAC is perfectly capable of getting to a 3K idle on it's own. At least on my car.
Are you sure your mechanical idle is set correctly? Why can't you get to 3K with your BAC valve? Is your car in need of repair? Need a hand? I'm in the same state, you know...Just trying to stir the pot again, lol
Hailers- OK, I think we've arrived at something we can both agree on (after just two pages at that)...
1) On a cold start, both the ABPV and BAC help to maintain an approx 3000 rpm for 17 seconds, everything being controlled by the ECU.
2) On your car, with no ABPV, your BAC goes wide open and takes your idle to only 2300 rpm on cold start. For 17 seconds. So you have a 2.3K warmup (snicker)...
3) On my car, with no ABPV, my BAC goes wide open and takes my idle to 2800 to 3200 rpm on cold start. For 17 seconds. So I have a textbook 3K warmup. And yet you say MY car is busted...
4) from the above, we can infer that even though the ABPV opens "on" on cold starts, it's the BAC that does the brunt of the work (passes the mostest air, if you will)...AND, with the ABPV off of the car, the BAC is perfectly capable of getting to a 3K idle on it's own. At least on my car.
Are you sure your mechanical idle is set correctly? Why can't you get to 3K with your BAC valve? Is your car in need of repair? Need a hand? I'm in the same state, you know...Just trying to stir the pot again, lol
07-19-05, 12:03 PM
#27
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don't mean to bring it up again, but this thread is the best i have read in awhile 
it's quite surprising actually that a straight explanation on how these two components, the ABPV and BAC, is hard to get.
what has been posted so far does make sense though...
got a question though guys, what would cause the accelerated warm up to go to 4k instead of around 3k?
once it warms up i can get the idle down to about 750, although i am having some idle "hunting" issues sometimes (trying to determine the cause still).
- Aaron
PS: wouldn't you love to have the phone # to the Mazda engineer in charge of designing these systems. i would have so many damn questions! lol
it's quite surprising actually that a straight explanation on how these two components, the ABPV and BAC, is hard to get.
what has been posted so far does make sense though...
got a question though guys, what would cause the accelerated warm up to go to 4k instead of around 3k?
once it warms up i can get the idle down to about 750, although i am having some idle "hunting" issues sometimes (trying to determine the cause still).
- Aaron
PS: wouldn't you love to have the phone # to the Mazda engineer in charge of designing these systems. i would have so many damn questions! lol
07-19-05, 01:51 PM
#28
coldfire thank you for bringing this up again 
I have a GSL-SE with a full S5 intake manifold that's about ready to go on. I'd like to maintain near stock functions though. Everything just about attaches with some modification except the 1st gen has no Air Bypass Solenoid Valve or Relay for that matter.
With reading the above quote, Hailers and Wayne, your cars do not have the ABSV? Did you simply remove it and reconnect the air hoses?
I have a GSL-SE with a full S5 intake manifold that's about ready to go on. I'd like to maintain near stock functions though. Everything just about attaches with some modification except the 1st gen has no Air Bypass Solenoid Valve or Relay for that matter.
4) from the above, we can infer that even though the ABPV opens "on" on cold starts, it's the BAC that does the brunt of the work (passes the mostest air, if you will)...AND, with the ABPV off of the car, the BAC is perfectly capable of getting to a 3K idle on it's own. At least on my car.
07-19-05, 02:23 PM
#29
HAILERS
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Just remove the air bypass valves connector. Done. It cah't open then.
A Non turbo series four has a relay. The turbo series four didn't.
Wayne's car is messup IMHO. I've a couple of these cars and with the air bypass valve (AWS on a series five) connector off/disconnected, the rise in rpms is only to about 2100rpms vs the approx 3000 rpm with it in play.
The AWS should only last for seventeen seconds per the fsm. The high idle after that is NOT a part of the AWS. That is part of the fast idle operation. The parts involved in that are the thermowax, fast idle cam, fast idle screw and cam roller. It keeps a cold engine idling around 1100 rpm and slowly falls down to approx 750rpm as the water warms up and the thermowax's piston extends and the fast idle cam comes off the roller. It's described in the Intake Air System.
I've no real idea why some go to 4000. Actually I have to agree with some people that disabling the AWS by either holding the car in a gear for seventeen seconds or disabling the air bypass solenoid by pulling its plug off is preferable to reving a cold engine to three grand or so.
Tell you what. If yours goes to 4000, pull the plug off the air bypass solenoid and see how high it goes the next time you start cold.
Again, Wayne's car is busted and he needs someone to set his mechanical idle and most likely needs to buy some Navel Jelly to get the rust off his rust bucket of a car.
A Non turbo series four has a relay. The turbo series four didn't.
Wayne's car is messup IMHO. I've a couple of these cars and with the air bypass valve (AWS on a series five) connector off/disconnected, the rise in rpms is only to about 2100rpms vs the approx 3000 rpm with it in play.
The AWS should only last for seventeen seconds per the fsm. The high idle after that is NOT a part of the AWS. That is part of the fast idle operation. The parts involved in that are the thermowax, fast idle cam, fast idle screw and cam roller. It keeps a cold engine idling around 1100 rpm and slowly falls down to approx 750rpm as the water warms up and the thermowax's piston extends and the fast idle cam comes off the roller. It's described in the Intake Air System.
I've no real idea why some go to 4000. Actually I have to agree with some people that disabling the AWS by either holding the car in a gear for seventeen seconds or disabling the air bypass solenoid by pulling its plug off is preferable to reving a cold engine to three grand or so.
Tell you what. If yours goes to 4000, pull the plug off the air bypass solenoid and see how high it goes the next time you start cold.
Again, Wayne's car is busted and he needs someone to set his mechanical idle and most likely needs to buy some Navel Jelly to get the rust off his rust bucket of a car.
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