Air/Fuel Ratio
Tourer
Joined: Jan 2011
Posts: 356
Likes: 36
From: Oklahoma
An engine cant be constantly ran at stoich. There has to be fuel introduced into the exhaust to light off the cat. Most engine OEM's use a strategy called perturbation. This runs a cycle or two lean, and then a couple rich (around 13.5/1). Dont ask me how I know this. I might have to kill you if I tell you.
Road Warrior
Joined: Jun 2009
Posts: 1,943
Likes: 3
From: NYC
it depends on the RPM. The ratio is not the same across the entire RPM range. low RPMs the ratio is in the 14.5:1 range. Higher RPMs, the ratios changes to the 13.5:1 area. I saw a map chart recently that did a good job of explaining this. I'll google around and see if I can find it. Here is an example. not the one I was thinking, but it gives you an idea of the ratio change based on rpm.
Last edited by speakerfritz; Jun 16, 2011 at 10:10 PM.
Extreme HDF Member
Joined: Sep 2010
Posts: 14,744
Likes: 402
From: Phoenix '53, '88, '09 Big Twins
Ratio is dependent on conditions, engine rpm, load etc.
Under acceleration or uphill ( higher load) mixture will be richened, to which extent is determined by the ECM by data from sensors ( o2, RPM, manifold vacuum, temperature, air density etc.)
14.7 is theoretically perfect and will be close that when running steady speed at light load, like cruising at 65 mph on a level road with little headwind.
Running an overall richer mixture ( alot) will contribute to less power, higher emissions, cat problems, oil contamination etc.
Kinda like trying to run your lawn mower with the choke on.
mike
Under acceleration or uphill ( higher load) mixture will be richened, to which extent is determined by the ECM by data from sensors ( o2, RPM, manifold vacuum, temperature, air density etc.)
14.7 is theoretically perfect and will be close that when running steady speed at light load, like cruising at 65 mph on a level road with little headwind.
Running an overall richer mixture ( alot) will contribute to less power, higher emissions, cat problems, oil contamination etc.
Kinda like trying to run your lawn mower with the choke on.
mike
Extreme HDF Member
Joined: Apr 2006
Posts: 10,449
Likes: 46
From: Roswell, GA
Yes, you're all wet. As mkguitar explained, there is no one "ideal" AFR. One of the advantages of EFI is it allows a far greater degree of matching AFR to the required conditions. You need lower AFR (more fuel) running up a hill than you do running down a hill. You need lower AFR accelerating from a dead stop and getting up to interstate cruising speed than you do running at a steady speed on a flat highway.
13:1 is extremely rich for virtually any riding condition; and, if you run that regularly you'll probably have a lot of unburned fuel contaminating the oil impairing the oil's lubrication capability.
Another advantage of EFI is the injectior allows the fuel to mist/atomize faster with the intake air compared to carbs which drips fuel into the intake air. So, for a given rpm and load, EFI allows less fuel to be added than a carb due to EFI's more efficient mixing of fuel with a given volume of intake air.
Carl
13:1 is extremely rich for virtually any riding condition; and, if you run that regularly you'll probably have a lot of unburned fuel contaminating the oil impairing the oil's lubrication capability.
Another advantage of EFI is the injectior allows the fuel to mist/atomize faster with the intake air compared to carbs which drips fuel into the intake air. So, for a given rpm and load, EFI allows less fuel to be added than a carb due to EFI's more efficient mixing of fuel with a given volume of intake air.
Carl
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Cruiser
Joined: Aug 2006
Posts: 163
Likes: 0
From: N.W. Iowa
When my last two bikes were dyno tuned, the maps show that at all rpms, the tech tried to keep the AFR at 13/1. The maps also said "Max WBC-AFR =18.00" and "100%_005.drf" and "DAFR 13.0:1" What does all this mean?
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Road Warrior
Joined: Dec 2008
Posts: 1,639
Likes: 2
From: Australia
The 100%_005.drf is the Dyno Run File - 100% throttle and run number 5.
DAFR is the target AFR?
Road Warrior
Joined: Dec 2008
Posts: 1,639
Likes: 2
From: Australia
Naw, you can run 12:1 or even 11:1 at WOT without a drama.