Idle A/F ratio

[jgkurz]

Hi all,

For some reason I have never been able to get my car to idle at an A/F of 14.7:1. It seems to idle smoothly and have better off idle throttle response at 12.5 to 13:1. I'm using the wideband O2 sensor in my Tec3, but I'm not sure how accurate it is until I get my car dynoed with a Horiba.

 

Is this common with an L6? My cam is stock and I'm pulling almost 20 inches of vacuum at 700RPM idle.

 

Thanks

[Guest Anonymous]

Yes, most engines like to idle at slightly rich conditions. The reason for this is that fuel dispersion is poor at low intake velocities. This leaves much of the fuel in liquid form stuck to the intake walls and valves with less atomized in the air stream. The result is less efficient combustion which must be compensated for. Some of the raw fuel is then dumped into the exhaust and reads as a rich mixture. If you try to run Stoich at idle, you end up running the mix lean and the idle suffers as a result. The same effect is present at low engine temperatures. This is why you need to choke a carborated engine when you first start it. Reducing the air intake volume by choking effectively richens the air/fuel ratio back up.

 

Hope this helps

[jgkurz]

Originally posted by jt240zt:

Yes, most engines like to idle at slightly rich conditions.

What I'm puzzled about is that some engines are able to idle well at Stoich. The below link has a response from BLKMGK that claims an idle at 14.7 or leaner if desired. I believe he runs a newer Toyota engine.

 

http://www.hybridz.org/ubb/ultimatebb.php?ubb=get_topic;f=16;t=000980

 

I've also talked to some folks running late technology engines and they seem to have no problem setting up the EFI to idle at Stoich. Is this because of variable cam timing, head design or some other factor?

 

I'd like to take a poll. Those with programmable EFI's and L6's, what A/F ratio do you idle your engine at?