AFR vs Fuel mileage. What have you seen?

[Larry Callahan]

AFR vs Fuel mileage. What have you seen?

I should be switching to closed loop mode soon for idle, part throttle and cruising down the freeway. I am REALLY looking forward to cutting down on the smell and the increased fuel economy.

I know it will vary greatly from car to car but I am curious to see what changes other people have seen. Lets say cruising down the freeway at 12:1 vs 13:1. What kind of difference in fuel mileage have you seen?

Eventually I hope that all but moderate throttle and up to run it as lean as safely possible. One of my goals has always been to prove you can make tons of HP with turbos and with an overdrive get decent mileage.

[camcojb]

My twin turbo runs fine at cruise in the 14.7 to 15:1 range, and that's not at all too lean for cruise areas. Your's probably won't like idling lean with that IR setup; my IR setup wanted no leaner than 13:1 at idle, but ran great at 15:1 going down the road. You're not going to lean it out (i.e. hurt the engine) at a cruise in that range.

I've never tried running as fat as 12:1 at a cruise and then switching to 13:1 (which is still pretty fat for a cruise a/f in my opinion), so I don't know what the mileage difference would be. But I've been able to run about 15:1 a/f and 40+ degrees of timing at a cruise in every efi combo I've built. My current twin turbo got 21 mpg average on it's maiden voyage to Hot August Nights last year, and that was with only 20 miles on it, and the mileage-robbing 4L80E trans. I think now it'll do better, and yours should be low to mid 20's at least for a freeway cruise if you get the a/f leaned out to 14.7:1 or leaner.

Jody

[Larry Callahan]

I still have some linkage issues causing one side to lean up for a second and I have the entire range running fat to be safe. In time I will fix that.

I think you are right about having to fatten it up at idle. I have had some lean popping just off idle with my IR setup and had not thought to fatten up the idle then lean it up at cruise load/rpm. Great idea. I will work on that in the morning.

Thanks

[camcojb]

I still have some linkage issues causing one side to lean up for a second and I have the entire range running fat to be safe. In time I will fix that.

I think you are right about having to fatten it up at idle. I have had some lean popping just off idle with my IR setup and had not thought to fatten up the idle then lean it up at cruise load/rpm. Great idea. I will work on that in the morning.

Thanks

give it what it wants Larry. Don't get hung up on the numbers, just use them for a starting point.

My IR setup made the most idle vacuum at an indicated a/f of 12.8:1. I tried leaning it out but the vacuum dropped as well as driveability off idle. On the other hand it was completely happy being much leaner at cruise. Every combo can be different. Have fun and good luck.

Jody

[68Formula]

If it doesn't like to run that lean at idle, try adjusting up the ignition timing and see what happens. A lot of people treat the ignition as a "one size fits all" when in reality component and a/f changes can have an effect on what works best.

[DeltaT]

I am up to 15.75 and 40degrees of timing at light loads and highway rpm, no knock or signs of plug distress. This and fuel cutoff on deccel has helped me get my mileage up to 20mpg on my 4DR Malibu, 383/4L80E/3.73 12-bolt combo. Up from the startup tune mileage of about 13mpg.

Jim