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Fast6
Garage Guru
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128 Posts
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 Posted - Apr 22 2003 : 14:06:12
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So...
This will make a few of you cringe. I don't have an A/F ratio meter. I'm also trying to learn how to tune this beast. I know that clockwise at the bypass screw and counterclockwise at the AFM are both for richening and that CCW at bypass and CW at AFM are for leaning out.
What's the difference between the adjustments at the AFM and the bypass screw? How does each one affect driveability?
If you're trying to make an adjustment, how do you know to adjust one or the other or both and how much?
In light of my miserable failure attempting to tune this weekend, I'll forget everything I think I know and just listen to what you guys have to say.
Thanks!
Eliot
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'83 645
Red w/ rockchips... |
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Jon 86 745 MS
Over 745 Posts!
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Posted - Apr 22 2003 : 20:18:57
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Eliot, I have tuned lots of AFM's and I can tell you that you will be spinning your wheels without an A/R meter. Turning the AFM and making it idle smoother is not the same as seeing the action with the A/R meter. Ask any of the guys like Don or John Stein. They will tell you just like me that it's impossible to set it without it.
Just look at what you said that your mechanic did on your AFM. Where did he get 20 clicks from? A SWAG (scientific wild ass guess) from just listning? Twenty clicks on the AFM is just far to much even for the Altitude your at. The barametric sensor is good up to 3200 feet ASL. Anything above or below that should be minor adjustments. Maybe just may one or two clicks.
Stop guessing. You will just continue to work you ass off and get no where without it. It just makes life so much easier. If I ever get the chance to get the 745i.com site under some control I am going to remove the section on tuning the AFM. Hell now even Jarmo has it on his site now. I wrote that 3 or 4 years and it does have some validity to it but for the most part it's just dead wrong.
Once you get an A/R meter I would gladly burn up lots of cell phone minutes walking you through the adjustment.
Sorry for ranting just hate to see you keep working and getting no where. |
Jon Jacobs
85 Korman 745i (AKA Das Borg) R.I.P.
86 745i (Die Rakete)(no mas :(
1.2 bar + Boost with
Team WWW.745i.com Performance Parts
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Fast6
Garage Guru
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Posted - Apr 23 2003 : 07:11:37
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I completely understand your frustration. I know it's dumb for me to be stumbling around without an A/F meter.
HOWEVER, I guess my real intent for this post (I gave it some thought overnight) was to understand the theory behind the two adjustments. I've decided to restate the question twice, once from a tinkering mechanic's point of view and again from an engineer's point of view.
1:
Problem=??? Solution=AFM
Problem=??? Solution=Bypass screw
2:
Is the AFM a proportional adjustment? Is the bypass screw a static adjustment?
Is there any validity to
Total Air to Engine = AFM x (throttle or RPM?) + Bypass screw (+ Pesky vacuum leaks  )? |
'83 645
Red w/ rockchips... |
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Jon 86 745 MS
Over 745 Posts!
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Posted - Apr 23 2003 : 08:26:52
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| Eliot, I am not a good typeist. Give me a call later tonight on my cell phone and I will explane how it works. I think Don gave you my number. If not email me and I will send it to you. |
Jon Jacobs
85 Korman 745i (AKA Das Borg) R.I.P.
86 745i (Die Rakete)(no mas :(
1.2 bar + Boost with
Team WWW.745i.com Performance Parts
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turbo535im
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Posted - Apr 23 2003 : 15:57:47
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| Bypass screw generally affects the calibrated vacuum leak at idle and afm spring will afect mixture accross the full range. |
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turbo535im
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Posted - Apr 23 2003 : 16:03:06
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| You could try the signal from the o2 sensor substitute this for air fuel ratio meter thats what it does anyway. 450 mv average is normal for normal aspirated little higher for turbo less would be too lean. |
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Jon 86 745 MS
Over 745 Posts!
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Posted - Apr 23 2003 : 16:35:44
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The bypass screw will also effect the entire range also. Thats just what it is a bypass. This is why you want to get it setup to the point that the bypass screw is almost closed. Think of the AFM as Hover Dam at Lake Meade. The bypass going around the dam is still flowing water (air in the case of the AFM) even if the dam door is not open. As the dam door opens (AFM flap) then the baypass is still flowing air up to it's maximum amount. Hence the mixture is affected across the whole opening range.
The whole trick is to achive a balance between the Spring Pressure, Contact Arm postion and the Bypass Screw. Proper spring pressure does not always mean that the Contact arm is in the right postion to achive the proper A/F ratio.
Turbo535im is right in saying that you can use a Volt Ohm meter to read the O2 sensor. Only problem with this is calculating in your head what voltage value is the proper A/F ratio while your driving down the road a different throttle settings. Examples would be WOT, Cruise settings at different speeds, Idle, throttle blip. Much easier to use a properly calibrated gauge and a heated O2 sensor the proper distance from the Turbo. |
Jon Jacobs
85 Korman 745i (AKA Das Borg) R.I.P.
86 745i (Die Rakete)(no mas :(
1.2 bar + Boost with
Team WWW.745i.com Performance Parts
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AFM vs. bypass screw