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Air Fuel Ratio

Engine, Fuel and Exhaust

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Old May 7th, 2008, 12:18 AM     #1
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Ride: 2007 softail fxst
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Air Fuel Ratio

I'm looking for some opinons on air fuel ratios. I'm now running a Thundermax Auto Tuner and I really like the way the bike runs, but I'm looking for that perfect ballance of power and Economy if you know what I mean. With the thundermax auto tuner it's very easy to adjust the target ratios but it's a little time consuming to do it right. there are 32 throttle positon point for every 256 rpm's from 0 to 8000 rpm's the map I loaded had the afr set to 13.8:1 in the cruizing rpms up to about 1/4 throttle. after that the ratio's start heading to 12.9:1. With those setting the bike seems to run good..........110 mph but even at 70 mph I only get about 31-32 mph. Yesterday I moved my cruizing rpm afr to 14.2:1 to see if it makes a difference. I'm still running my running my lower rpms a little richer to make the bike run a little cool when Idling. So far it seems to be running the same, if not may be a little better better but I haven't got to check the milage yet.
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Old May 14th, 2008, 05:09 PM     #2
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Ride: 1996 Road King
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Re: Air Fuel Ratio

FWIW, I run mine in the high 14's from 20% up to about 40% throttle and 2000 to 3500 (or so) RPM. I got 45-49 MPG out of a 90+ horse 80-incher with a 3.36 final drive, cruising generally around 65-70 MPH. (I have noticed a big hit in mileage when I cruise above 70 MPH, probably due to aerodynamic effects).

We'll see what happens when I get it running, but I expect similar results from my new motor.

My experience has been that people (dyno people) set the AFR in the max-power ranges at all settings--probably because it's just less time consuming to do it that way, but as you found out, your mileage goes down the tubes.

In my opinion, you do not need max power until you get to 80-100% throttle. Everywhere else you're probably looking for better mileage. I have run mine as lean as 15:1 at highway speeds/loads, but I don't recommend that to anyone unless they have really good cylinder heads, fairly cool weather, and they are willing to watch/listen very attentively to what the engine is telling you as you're going down the road. Keep in mind that mine is an MM unit, and I can control the mixture distribution pretty well. May not be so true on a Delphi bike.

Not sure you're doing yourself any favors by running rich down low. All of mine have tended to be sorta okay there anyway, and not really leaning out until they get up above 2500 or more.
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