What things affect AF ratio? How does AF ratio affect the performance of the engine? When should it be above stoiciometric, and when below. What are good target ratios for various typs of performance levels. Don't assume anything when throwing your knowledge into the pot here. It may seem really obvious to you, but someone reading this forum may be helped by your comment. Let me give you an example. Until I started tuning on this EFI setup with Lou, I thought light acceleration required a richer mixture. I assumed that the squirt of gas provided by the accelerator pump was to make it richer than 14.8:1. But apparently I was wrong. The extra gas under light acceleration is to keep the AF ratio at 18.8:1, not to make it richer. It is things like this that would be useful to many of us here, because it is these underlying principals that guide us in our tuning.
Sam

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Check here:

http://www.mummbrothers.com/SRF_Stuff/S ... r_Fuel.htm

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I'll throw out some of what I know and see what sticks.

O2 sensors or lambda sensors were designed specifically for determining the stoichiometric air/fuel mixture. This is the theoretical point at which all the oxygen reacts with (burns/oxidizes) all the hydrocarbons to reduce them to CO2. This is not the mixture ratio that produces best economy nor does it produce best power. Because the air/fuel mixture is not homogeneous excess oxygen (leaner) is required to completely combust all the available fuel and excess fuel is required to completely consume all the available oxygen. So a maximum economy mixture is lean of stoichiometric and a maximum power mixture is richer. How rich and how lean depends upon the engine in question. Let's just say for now that 18:1 might not be your engines ideal economy mixture and 12.5:1 might no be the ideal power mixture.

So who cares about lambda if we want to operate lean for economy and rich for power? The catalytic converter cares. The catalytic converter cannot do it's job well in the presence of extra oxygen. And obviously without enough oxygen the catalytic converter cannot oxidize unburned hydrocarbons and carbon monoxide. Emissions controlled engines are run at stoichiometric for this reason alone.

Ideally we want an in cylinder mixture that is homogeneous; that is without rich and lean areas. Achieving and maintaining a homogeneous mixture requires great care from at least the place and time fuel is first mixed with air all the way to the point of ignition. This ensures complete combustion and the most power for the fuel consumed. Of course I'm ignoring stratified mixtures, but I feel we should leave that to another discussion. There is much more to know and learn of course, but I'm not going to write nor am I really qualified to write a book on this.

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Joshua

Josh, that was really interesting, and something I did not know before. Stoiciometric is for the catalytic converter to work. . Since most of us dont; have catalytic converters, we can be free to set the mixture up more ideally. What is a good user friendly, engine frinedly, economical cruise AF ratio, that is not too deadly to the planet? When setting up a fuel map, what guidelines would you offer for allowable jumps from cell to cell? If yoiu can get your car to cruise, and drive around town on 16:1 ratios, is that OK? How about 17:1?
Tht would mean taking it out of closed loop operation, which most tuners have said is not really needed if you get the fuel map tuned out well. Any thoughts on that?
Sam

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I'd say the 15.4:1 mentioned in the link I posted.....


With a wide-band, you can have it in closed-loop mode, but with an air/fuel ratio of your choice (15,14,13......) not the 14.7:1 of the narrow band sensor

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Sam,

The air/fuel ratio your engine will tolerate at cruise might be 20:1, but it's likely richer. If the intake port had a lot of swirl and mixture activity and a good quench combustion chamber you could run it leaner than with an average intake port and the slant's open chamber. You can play with the air/fuel ratio at cruise and see what gets you minimum injector pulsewidth and maximum economy. Be warned that your throttle position and MAP values will both increase as you lean the mixture so the cell you thought you wanted to change is going to be too low in kPa or however manifold pressure is expressed in Calmap. Idle will most likely need to be richer than 14.7:1 and you'll gradually taper to a leaner mixture for cruise and then rich again for power. You can run pretty lean with a lot of manifold pressure (but not boost) at lower RPM, but richen it up as the RPM goes up. I don't see the need to run richer than 14.7:1 under 2500 RPM give or take except for idle. In all cases though the changes from cell to cell should be smooth and gradual. I would not try and use closed loop until you have a very good base calibration map and then only if you want to run stoichiometric.

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Joshua

That mummsbrothers web site is packed with usefull information. I printed out the AF ratio section and it is in the back of my Accel manual. It will become bed time reading for awhile. I'm not all that bright, but I make up for it by being slow. This is great fundamental information for what ever kind of EFI you are trying to set up. Thanks Man,
Sam

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How about warm up AF ratio targets? Anybody had experience with specific AF ratios that seemed to work at different coolant temperatures? It would be nice to let guys know where to start when trying to get the cold start drivability working.
Sam

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Warmup is a pita because once you turn the key on first attempt to start its not in a truly cold start condition anymore, so you have to wait until engine is truly cold again.

I don't think theres a rule of thumb for warmup. "Rich enough to start in the coldest condition" maybe? That balanced with idle air.

I'm talking about that area between after it has started, and has idled maybe 20 seconds or so, and until it is 185 or higher. Let's say you are driving down the road and the engine is getting progressively warmer. Are there any ideas out there for target AF ratios for when you are hitting 150 for instance? Should it be more like `13.5, or are most people finding that 14.7 will run OK even when the coolant is still only 150 degrees. If you are tuning with a wide band o2 sensor, then you can look at the ratio, you can use the warm up VE modifiers as you drive, and fine tune for a specific ratio if you want to, and not have to settle for , "just doesn't die". Just doesn't t die" would be better than nothing I suppose, but having a ball park target would be helpfull. Until I learned better from Lou, I didn;t know I should attempt to keep it at 14.7 under light acceleration, and that info was a good underlying principal to help me tune.

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You won't be able to run stoich with the car cold and still be driveable. Its hard to ballpark because the variables not only involve engine but local ambient temp and conditions. The balpark for someone that lives in southern cali will be differnet then antartica. If you want a number, 12-14. But your not going to get much more precise then that.

That'a still a help. Any ballpark is a good place to start. It seems as if the process involves first getting your fuel map really dialed in so O2 feedback is maybe not really needed to make it run well. The idle and cruise modes are most important for drivability. Only after you really have tht done can you go after the cold start drivability since that involves applying modifiers to the VE map at various speeds. I know when I first set up Mega Squirt, it would not run when it was cold, and then it was way too rich. The O2 gauge was returning numbers in the 10-11 range.

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Eli, at Accel instructed me to lean the top two rows of the fuel map, which would be the highest vacuum, very lean, for all RPMs above 2500 RPM or so. This is to keep excess fuel from blowing out the spark, and kill the engine. I did what Eli suggested, and set the VE at .2 all the across the top two rows. This seemed to remedy the problem I was having, but perhaps I made it too lean. My AF gauge tops out at 25:1, and that is where things go when I let off the gas. Does this seem too lean.

Does anybody out there have an AF ratio they shoot for in those ultra high vacuum cells where the engine goes when you let your foot off the gas at speed. At seems as if 20:1 might be a better target, but I don;t want to re-enter the dying and then back firing scenario again. I guess I can just try it and see, but suggestions and experience are always helpful. Thanks,
Sam

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