Need input on some comments I got

[Yves]

The following is an excerpt of a discussion I had going on another engine related forum. I opened a subject about my engine expelling a light cloud of oil on high rpm decel. The discussion was mainly oriented towards localizing the root cause (in effect it turns out to be the guides).

Some member said the following :
Newer run richer than stoich at light load, it will ruin your engine. Unburnt gas will made excessive carbon built and rich afr will keep piston temperature lower than optimal which open up piston to wall and ring end clearances. When tuned ok you want to be much leaner than stoich at cruise and have plenty of accelerator shot to keep acceleration crisp, much much harder to tune right than rich-everywhere but if engine sees any mileage at road it's the only way to get any life of it without sticking oil rings into pistons.

And with closed-loop correction it's even more crucial to aim at stoich or leaner. With light part-throttle load rich afr will lower EGT to point where lambda voltage drops -> false lean condition and ecu will richen afr even more.

I commented that I had difficulty running my engine at stoich at light load and ran slightly richer (14:1). At stoich the engine AFR varies greatly and even starts missing. IMO this was mainly the effect of the large cam I'm running.

Then I got the following comment :


Get rid of closed-loop autotune, if you have problems with stoich AFR closed-loop is not usable either. Even with very mild stock cams std engines have almost zero ignition advance at light load to keep EGT high enough to keep lambdas alive. With high-overlap cam EGT recirculation with rich afr will certainly lower egt to point where false lean happens, and ecu corrects to very rich afr, which ruins engine incredibly fast.


This would seem counterintuitive for me. But other might have a better clue.

[jesse408]

The only platform I have experience with is old school American V8 engines.
To the comments about near zero Advance at light load Cruise I would ask what he feels a vacuum advance distributor was designed to do. It's not a vacuum Retard, that's for sure.
Wouldn't retarding to near zero advance during cruise make your exhaust valve glow red and eventually burn up your guides/seals?
Just some thoughts here. Take them with a grain of salt.
If your long overlap cammed engine seems to be surging lean at stoich, and it smooths out nicely when run a little richer than stoich at low load, IMHO, it was not a false lean from being too rich...It's internal EGR causing some exhaust diluted cylinders to actually be Leaner than indicated or False Rich. I mean perhaps some Some cylinders are actually at 16:1 when an Average of All the cylinders is indicating stoich.
I could be totally wrong here I'm just rambling at this point.
I've always been a give it what it wants type of guy.

[Yves]

I'm with you as well.

I have individual wb's so I know what each cyl is doing. And yes this is a SBC.

[racingmini_mtl]

A low EGT will not affect the AFR reading of wideband sensors because they have their own heater. That is only true of narrow band sensors. The wideband controller will keep sensor temperature at a precise value regardless of EGT (unless EGT is too high and then the controller cannot cool the sensor).

Jean

[EspeNS]

There may be several reasons OEMs run near to 0 deg. ignition advance at light load.
One may be engine design. High CR with a fast burning combustionchamber design, and henche low advance to make MBT.
Another may be to keep the catalytic conveter(s) in fuction.
To keep a catalytic converter, maybe mountet to distant from the engine heated, less than MBT may be needed, even if the milage get worse.
A engine with agressive cam would want mutch advance at light load and low to medium rpm because of low VE and dynamic CR, and internal EGR as mentioned above, making this engine needing a lot more ignition advance to make MBT.

AFR. OEMs without direct injection will want to run at stoich or richer, because of emissions.
Leaner AFR than stoich = high NOX, that a regulat catalytic converter can not handle.
A little rich AFR = a bit more CO than wantet, but a regular catalytic converter is made to convert it to CO2.
Combustion temperature is highest at stoich, and anyting leaner makes for lower temperature as much as richer, maybe requiring lesser than MBT to keep up EGT at low load, to keep NB lambda and catalytic converter hot.

This is my understanding of how a engine works. Maybe not completely correct, but the statements in what you have been told do not make all sense to me.
A lot of engines that run richer than stoich on low load have been made.

[Matt Cramer]

Running zero advance at cruise sounds like an attempt to drive up EGTs and light off a catalytic converter. It doesn't sound like a good tune for economy or performance.

Sometimes abruptly snapping the throttle closed can get the EGTs to drop so quickly the O2 sensor heater doesn't keep up, but this is more likely to just give a brief off reading, not dramatically shorten sensor life.