Another_User

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?

SmokingWS6

iTrader: (1)

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.

Another_User

See, I don't get it. Just about everything I can find says the exact opposite. That's what I thought, but everyone else seems to think that is not the case. It's not actually the air slipping from the intake to the exhaust, but instead the exhaust left behind in the cylinder (EGR effect) fooling the engine into thinking it is too rich because of the diluted mixture.

Robert56

iTrader: (2)

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?

Bink

O2s sense oxygen - not fuel. With high overlap there is O2 present in the exhaust discharge and the PCM then adds fuel to reduce the O2 levels.

Of course the O2 levels can be adjusted via the MAF table - adding more fuel.

FWIW.

wrencher

iTrader: (2)

Yup it will see lean. Just like a misfire, it will see as lean , a cylinder full of O2 goin into the exhaust. Same thing but a big overlap not near as bad as a misfire.

ChucksZ06

iTrader: (9)

False lean...The cpu is being told the mixture is lean and trys to richen mixture which causes engine to run rich.

Robert56

iTrader: (2)

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.

Another_User

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?

Rays C5

Definitely going to follow this one.

Robert56

iTrader: (2)

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.

SmokingWS6

iTrader: (1)

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.

Casperws6

use the ASA cam as an example, false rich idle?
specs = 226/236 .525/.525 110 lsa

Another_User

I am thinking we will have a false rich. Probst's Corvette fuel injection book, and some info off DYI-EFI seem to back it up.

WS6FirebirdTA00

iTrader: (33)

SmokingWS6

iTrader: (1)

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.

WS6FirebirdTA00

iTrader: (33)

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

SmokingWS6

iTrader: (1)

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

WS6FirebirdTA00

iTrader: (33)

well ill only be using the F13 cam 230/232. ill try that VE meathod. but woudlnt that still throw the o2 reading off?

mr2guru

iTrader: (16)

Yeah, Wouldn't the MAF just compensate over time with reading the false O2s and adjusting LTFTs/LTITs Irregardless of what the PE table says. Or does turning off the Idle proportional take care of this? This would all be moot if you're running SD, which I may go with btw in the end. So much easier to deal with, I think.