I know lean running is bad under heavy loads, due to detonation.

I also know lean running tends to generate insanely high exhaust temperatures.

However, running at a steady speed on the highway, not under any sort of insane load, it should be fine for a NA engine to run stupid-lean, right? As long as the exhaust system can handle it.

If one were to monitor the EGT to prevent the exhaust system from melting, and make sure that the mixture richened before any heavy load happened, would any harm come from running insanely lean on the highway? I'm mostly interested in it for the fuel economy benefits on long highway trips. I'm not exactly how to set this up without going to a standalone fuel computer, but that's something I'd be willing to do.

People with standalone systems are frequently reporting 30mpg on the highway when properly tuned - running lean could lead to significantly higher mileage numbers.

Or am I just nuts?

-=Russ=-

 

The 02 sensor does that automatically for you
When you're crusing, your ECU goes into 'closed loop' mode.

Where it bounces between insanely lean and as close to stoceometric (sp!) as it can get.

 

PS:

Get an A/F gauge and you can see when it is in closed loop mode, and adjust your driving habits to try and stay in closed loop as much as possable for ideal fuel economy.

The ECU uses the pressure sensor and the TPS (I THINK) to chose when to to in and out of closed loop. it usually drops out of closed loop for me below around 7-5mmhg or above 3800RPM.


EDIT: New Bosch O2 sensors are $7 at Olympic Auto Parts. Most people reccomend replacing them anywhere from 50,000Miles to 80,000Miles. if you havent changed yours, I reccomend it. Your MPG will thank you. (watch out for the sheilding in the existing wire)

 

Funny thing... I have a new O2 sensor... and can't get the old one off. This weekend I'm going to take off the UIM for some work, I'll probably pull the LIM & exhaust off as well, try to get a better angle on the O2 sensor.

-=Russ=-

 

I have my car running at 14.7-15 afr on the freeway. I get 30mpg.

 

Do you have a stand-alone EMS?
If you don't, there is really no way to control it.

Yes, you can keep leaning it out as long as it doesn't hesitate. You'll feel the car start to buck if you go too lean. As RPM's goes up, the leaner you can go. I've seen as hot as 1150°C which is about 2000°F! I didn't want to melt anything, so I actually put back fuel just to keep the temps from melting anything.


-Ted

 

I don't have a standalone, but I'm considering getting one at some point (and realize I would have to get one to run ultra-lean).

Would water injection help keep combustion temperatures down when running ultra-lean?

-=Russ=-

 

stupid question: can you pass emessions with a stand alone, assuming they don't look under the hood?

 

how much can you lean out a turbo 2 on the higway? is 14.7 to lean? assuming your not under boost at all

thanx

 

The problem I'd see with leaning a turbo out is that the turbo is still pumping heat into the intake air, so the intake charge will be hotter than in a NA. It shouldn't be a big problem, but it's something to consider.

And yes, a properly tuned standalone should pass emissions without a problem.

-=Russ=-

 

The only time you really have to worry about lean (i.e. engine damage) is under load - i.e. under boost or WOT.

Running water injection under cruising is a waste of time.


-Ted

 

Yes, you still need a cat.


-Ted

 

It can go past 14.7:1...
I've seen over 15.0:1.


-Ted

 

No, unless there is something seriously wrong with your oil and / or coolant (cooling) systems.

The oil (and coolant) cooled center sections should be removing most of this heat.



-Ted