Well, my Subaru wagon has been wonderful for the winter, but it's getting warm, I want my AC back, and I'm sick of not having the power to merge into traffic.

My current plan to kick the fuel economy up on my emissions-less SE is to wire in a SAFC, disconnect the O2 sensor (so the ECU never goes into closed loop mode), and lean it out in the low-load highway cruising band as far as possible. I don't have any cats left to burn up, and the front part of the exhaust is a Racing Beat presilencer & cat replacement pipe, so I'm not worried about them holding up.

I'll replace the stock O2 sensor with a wideband, so I can see what's coming out and tune it better in the other ranges as well (lean it out slightly where the stock ECU goes pig rich).

Will a SAFC let me do this, and if so, what kind of mileage can I expect?

-=Russ=-

 

Consider this:

Wideband $200.

Difference in fuel economy ~6mpg. Average driving per year: 10,000 miles.
10,000mi @ 30mpg : 333 gal/yr * 2.50/gal = $833
10,000mi @ 24mpg : 416 gal/yr * 2.50/gal = $1040

1040-833~ $200.

The wideband would be paid in the first year.

 

... right. How does just a wideband tie into leaning out the mixture? Or is there some sort of "fake O2 sensor" mode that will feed the ECU a signal based around whatever mixture I want? I thought a wideband was just for reporting/tuning.

-=Russ=-

 

^ exactly, its for tuning, which you will need to do to lean out the mixture...correct?

 

... right. I'd need a SAFC as well (I don't have one wired in).

Lacking a wideband, I was planning to (have a passenger) just fiddle with the SAFC at normal highway cruise until I got lean stumble, then richen it up a bit.

-=Russ=-

 

gotcha

i wonder what a shop would charge to tune that for you? More than 200? May be worth looking into that as well

 

i'm looking to do the exact same thing, with a s-afc (either with professional tuning or a wideband) to make up for the damage that a possible streetport might do to fuel economy.
is there a way to easily switch between basic settings, one optimized for performance and one for fuel economy? sometimes i just want to get places without using so much gas (which hit 2.20 for 85 this week).

 

I was debating doing the "multiple fuel map" thing, but realized (with the help of some other people who had gone through the same thing) that you really don't need it.

At low throttle settings (20-30% throttle, cruising down the highway), you can run pretty much as lean as the engine will go without causing any damage. Detonation occurs under high pressure/load settings, and isn't a problem at a light cruise. As the throttle (sensor or manifold pressure) goes up, you switch over to a richer mixture for power/detonation prevention.

On a SAFC, this looks like having different low throttle & high throttle corrections, and the appropriate setting of low/high throttle points. On a standalone, you can pretty much do whatever you want in the lookup table.

-=Russ=-

 

holy i got 8/12 mpg ..... so i m paying 2k/3k on gas 1 year !!! omg

 

The only chassis dyno in my entire state doesn't do any tuning.

-=Russ=-

 

Leaning the S-AFC's low map will net you maybe 2-3 MPG savings.
Most of that is from leaning the idle, and during mild acceleration.
What happens when you're cruising the ECU bypass the AFM & goes closed loop with the oxygen sensor.
During deceleration fuel is cut completely by the ECU.

 

I'm planning to completely disconnect the O2 sensor, preventing the ECU from going into closed loop. At this point, I should be able to lean the ECU out significantly at cruise - I don't have any cats to take care of.

-=Russ=-

 

I got about 27MPG driving from Texas to Colorado. I was kind of surprised, but i was using the SAFC to lean out about 10%. I didn't know that the ECU bypasses all of that mess during cruising though.

 

I suspect those long upgrades took just enough throttle to drop out of cruise/closed loop & fall back on your lean map. - Cool..

 

Very likely. Having watched the O2 test lamp on highway, it's *very* easy to drop out of closed loop mode with anything but a feather touch. If you know what you're doing and have feedback on it, you can keep it in closed loop mode, but "normal" driving will drop it out on most slopes. Given that people are seeing significant economy improvements just by leaning out the maps that are used for a small part of the trip, I'm thinking I could get some serious improvements by staying in the mapped region constantly and leaning it for all cruising.

14.7:1 is great for babysitting cats. I don't have any, so I want to run 16:1 or 16.5:1.

-=Russ=-

 

I thought 14.7 was optimal burn? How can you get more than out of an optimal burn? Is there proven facts behind that?

 

Not optimal. Complete. In theory. A 14.7:1 mixture will, under ideal conditions, completely burn all the gas & all the air. Reality, try running 14.7:1 in your engine & you'll still shoot flames.

Leaning it out will generally produce roughly the same power for less gas. There are tradeoffs in emissions & such, but you can get better economy.

Also, as pointed to earlier, whenever the ECU is *not* in closed loop mode, it's running on the rather rich side of stoich - 12:1 or so. Leaning this out gains you power & efficiency as well.

-=Russ=-

 

Well, some updates, I got 24mpg on the last tank of mostly highway stuff - up from the 20 I was previously getting.

I've found that leaning it out too far leads to increased fuel consumption (which matches some charts I later found). I'm pulling out around 18-20% at light throttle cruise, and it seems to be sucking a bit less fuel.

I'm tuning for lean on the main O2 sensor, but not "0.00v" lean - around 0.05 - 0.1v at cruise. This should be right around the correct point for fuel efficiency.

I've also discovered that the airflow required to run at higher RPM isn't significantly higher than at low RPM - there's very little difference between 4th & 5th on the highway, though 4th is slightly higher airflow. However, fuel flow is SIGNIFICANTLY higher, because it's in "stupid-rich open loop mode" at the 4500 RPM point.

It also appears that at higher throttle settings, the ECU is "more rich" than at lower settings - I have to pull out a lot of fuel at high throttle to get it close to stoich.

I'll see what kind of mileage I get on the next tank - I just cleaned my plugs. I'm also debating putting a CD ignition module in for the leading coil to help at lower RPM.

-=Russ=-

 

a picture is worth a thousand words:


You can see the optimal fuel economy afr is just a bit lean of stoich, and like Russ experienced, if you go too far int othe lean end fuel consumption starts climbing again.