Air fuel question HELP
07-11-2003 | 09:47 AM
#1
Thread Starter
Racer
Joined: Jun 2003
Posts: 497
Likes: 0
From: CO
Air fuel question HELP
Does anyone know what the difference if any between measuring the A/F before the turbo vs. at the tail pipe? I'm questioning the accuracy of my O2 readings. I'm using the ground connection of the car for the return path. I checked the volts at the O2 right at the header and then in the car and there the same(10mv diff). I then verified the cal. of the meter vs. a known good source and it's dead on.
The reason is it seems I'm running too lean 13.5:1(at the dyno)when I need to be 12.5:1 yet my O2 reading was .90-.93v or no worse than 13.2:1? I only ran 7psi of boost an no higher than 5k rpm's in case the A/F wasn't good. I have to believe what the high dollar A/F reader at the dyno says over my cheap O2 setup. BUT, it doesn't seem to respond like an O2 should and read correctly at idle of 14.7:1 or around .5v? I have tried 2 different O2 sensors and they seem to act the same. All of my fuel pressures are correct and I just had my injectors cleaned and balanced 98-102cc each. Anyone know what might be going on here?
Thanks
The reason is it seems I'm running too lean 13.5:1(at the dyno)when I need to be 12.5:1 yet my O2 reading was .90-.93v or no worse than 13.2:1? I only ran 7psi of boost an no higher than 5k rpm's in case the A/F wasn't good. I have to believe what the high dollar A/F reader at the dyno says over my cheap O2 setup. BUT, it doesn't seem to respond like an O2 should and read correctly at idle of 14.7:1 or around .5v? I have tried 2 different O2 sensors and they seem to act the same. All of my fuel pressures are correct and I just had my injectors cleaned and balanced 98-102cc each. Anyone know what might be going on here?
Thanks
07-11-2003 | 09:56 AM
#2
Nordschleife Master
Joined: Dec 2001
Posts: 8,305
Likes: 12
From: Kingston, NY
I have a MoTec wide band meter which uses a BOSCH 5 wire LSU-4 sensor which is a very fast O2 sensor. The sensor is placed directly after the turbo which is where MoTec suggests to place it. Dialing in AFRs on a CIS car is difficult, especially if it has been modified because you most often have a rich condition under idle/cruise to make up for having enough fuel under boost.
For an accurate reading, the sensor must be placed a sufficient distance from the exhaust port and from the exhaust outlet, usually a minimum of 1 foot. Placing an O2 sensor directly at the outlet of the tailpipe may give leaner readings than one placed properly. If you think you are going to do any dyno tuning, I suggest an O2 bung welded on your exhaust directly after the turbo so you can plug the dyno in there.
Additionally, a narrow band sensor which is what I assume you are using will read between 0 and 1v and is really only a good gauge for light throttle applications. It is not accurate or fast enough when the car starts to build boost.
With an EFI system, I run them 15.2:1-15.5:1 under cruise and 12.2:1 under boost. What you are dealing with are the limitations of CIS.
For an accurate reading, the sensor must be placed a sufficient distance from the exhaust port and from the exhaust outlet, usually a minimum of 1 foot. Placing an O2 sensor directly at the outlet of the tailpipe may give leaner readings than one placed properly. If you think you are going to do any dyno tuning, I suggest an O2 bung welded on your exhaust directly after the turbo so you can plug the dyno in there.
Additionally, a narrow band sensor which is what I assume you are using will read between 0 and 1v and is really only a good gauge for light throttle applications. It is not accurate or fast enough when the car starts to build boost.
With an EFI system, I run them 15.2:1-15.5:1 under cruise and 12.2:1 under boost. What you are dealing with are the limitations of CIS.