Will overlap cause a false rich or false lean condition?
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Will overlap cause a false rich or false lean condition? A lot of information I have seen says that a lot of overlap could cause a false rich condition (actually lean). Is that the case? Or would it depend on your cam specifications?
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Originally Posted by SmokingWS6
False lean, especially at idle. The overlap allows some fresh air to enter the exhaust and fools the o2's into thinking the combustion is lean.
Depends on cam specs. It used to be on an high duration/overlap cam that if you stood behind while idling it would burn your eyes from the unburned fuel. With the o2's doesn't the computer lean it out to compansate?
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Originally Posted by Robert56
Depends on cam specs. It used to be on an high duration/overlap cam that if you stood behind while idling it would burn your eyes from the unburned fuel. With the o2's doesn't the computer lean it out to compansate?
Of course the O2 levels can be adjusted via the MAF table - adding more fuel.
FWIW.
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Wait a minute, something dosen't add up, or I'm missing something. O2's may read oxygen levels but it wants to see lean not rich, for emmissions correct. Now with heavy overlap what's happening is when intake is pulling in fresh oxygen/fuel the exhaust valve stays open longer in order to displace the leftover burn from last cylce, read: pollution. Good for power bad the lungs. In this partialy burned/unburned mix the o2's read the oxy level in order to make adjustments for pollution control and if it's allready rich reading why would the computer be set to add more fuel.
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Now you understand my confusion. In reality, with a bunch of overlap, wouldn't you end up with a ton of wasted fuel going into the exhaust, even on a fuel injected car? A lot of things did not add up for me when I was researching this issue. I know the O2 sensors "see" air, but is that really the direction everything is getting fooled? Is the air going from the intake to the exhaust, or is the exhaust going back into the intake? Would it depend on your cam split and/or centerline? Or even cam timing?
Last edited by Another_User; Apr 30, 2005 at 10:40 AM.
Definitely going to follow this one. Were are all the cam engineers when you need one? This is such a complex issue and depending on which cam and specs we are talking about diff things taking place. It really comes down to VE and that is the key to having more overlap, so you get rid of the crap that's left over from the last cycle and have a clean mix. The more duration and overlap the higher in the rpm band goes the power and this can pose some tunning issues because of the pollution being created (lower rpm's) to make the extra power up top. I do know you can ruin your O2's by pumping raw fuel past them and that will give you a code. Also reversion can take place with added overlap, the mix going back into the manifold.
Its really simple. With a huge overlaping cam
exhaust = very rich o2= reading very lean
Thats why people get confused. They think because it smells so bad that it must be a false rich condition, but thats the exact opposite. The only thing that would make the PCM add fuel would be a reported lean condition(pcm trying to correct the problem)
As far as tuning, maf isn't the proper way to tune a rich idle. the VE table needs to be scaled back in the idle rpm range. This tells the pcm that not as much fuel is requred to maintain a 0 LTFT.
exhaust = very rich o2= reading very lean
Thats why people get confused. They think because it smells so bad that it must be a false rich condition, but thats the exact opposite. The only thing that would make the PCM add fuel would be a reported lean condition(pcm trying to correct the problem)
As far as tuning, maf isn't the proper way to tune a rich idle. the VE table needs to be scaled back in the idle rpm range. This tells the pcm that not as much fuel is requred to maintain a 0 LTFT.
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Originally Posted by white00ws6
use the ASA cam as an example, false rich idle?
specs = 226/236 .525/.525 110 lsa
specs = 226/236 .525/.525 110 lsa
false lean 
yes with these big cams they run horribly rich at idle. why, because the pcm is reading a lean condition. This is why when we VE tune for cams at idle we multiply the idle tables bye 80-90%. This tells the pcm that we don't need as much at idle.
A false rich condition would make the motor run very lean.
yes with these big cams they run horribly rich at idle. why, because the pcm is reading a lean condition. This is why when we VE tune for cams at idle we multiply the idle tables bye 80-90%. This tells the pcm that we don't need as much at idle.
A false rich condition would make the motor run very lean.
ok, it gives false lean, there is no way to fix this is there? or what about a proper way to tune at idle with cam and no false info? or is that just not possible? i wanna have my car run safe with no false info
there are two options. PE Idle tuning or VE idle tuning
VE: If you idle below 1000RPM open up the primary(secondary if you have it) VE table and hilight all the tables in the 400 and 800 RPM section. Multiply that section by 90%, this reduces the fueling in those cells. If this helps clean up the smell you can keep it how it is or Mutiply by another 90% or so. a 20% reduction in idle fueling should help a lot
PE: usually reserved for monstor cams only. if you idle over 1000rpm. from 400-1200 RPM enable PE at 0% TPS. and set the PE AFR at 14.7. This helps with monstor cams and overly cool o2 sensors at idle.
Try the VE method first
VE: If you idle below 1000RPM open up the primary(secondary if you have it) VE table and hilight all the tables in the 400 and 800 RPM section. Multiply that section by 90%, this reduces the fueling in those cells. If this helps clean up the smell you can keep it how it is or Mutiply by another 90% or so. a 20% reduction in idle fueling should help a lot
PE: usually reserved for monstor cams only. if you idle over 1000rpm. from 400-1200 RPM enable PE at 0% TPS. and set the PE AFR at 14.7. This helps with monstor cams and overly cool o2 sensors at idle.
Try the VE method first
Originally Posted by WS6FirebirdTA00
well ill only be using the F13 cam 230/232. ill try that VE meathod. but woudlnt that still throw the o2 reading off?