The Art Of Determining The Best A/F Ratio...

[ZBass28]

How does one go about picking and laying out the best a/f ratio table?

I've been using FAST now for a little over a year and am wondering what are the
deciding factors in choosing where to set a/f at.

I mean I have a good idea about where I want a car to idle, and where I want it
to be at WOT at max rpm, but what about all the stuff in between for normal driving?
What determines whether something should be 13.0:1 vs. 13.4, or 12.7? Is it all trial and
error or is there some sort of formulaic basis for it. I can spend hours beating my head programming
VE tables so that it hits my target A/F, but how do I know for sure that my target A/F is actually where my engine
wants to be?

 

[quickstrike]

I think this is an excellent question. I have been asking alot of questions at the track about this very thing. There is a mustang locally that runs 5.0 in the 1/8th and they believe in adding fuel as well as timing. They like to run 11.5 to 1 which seems rich to me, but they see it as a way to cool the combustion chamber down a bit and take advantage of it with more aggressive timing. I have heard this logic works very well for blower cars but not necessarily supercharged cars(not sure why the difference). I have been playing with numbers between 12-1 and 12.5-1 and have only made minimal improvements. I will be trying 11.5-1 next week.

As far as normal driving, I also wonder where to set my car up to run. Around town less than 4000 rpms at anything under vacuum, it seems I should shoot for a 14.7-1. Isnt that what a factory computer and o2 would be trying to do, to maximize fuel mileage and efficiency. Right now I simply use 12-1 and car runs fine, but I wonder if this is just to rich and will use an excessive amount of gas as well foul plugs. (BTW like most people car is only in closed loop at boost and high RPMs)

I wish I had more of an answer. Hopefully someone will chime in.

Randy

 

[bobc455]

Well this is the $64,000 question. I hope you aren't expecting an easy answer...

As far as I can tell, there are two ways to do it. First way- find a very long, straight, level stretch of highway. Go 50 MPH and pick the A/F ratio that minimizes the injector PW (trial-and-error), as long as you don't get any knocking or stumbling. The lowest PW means that the engine is working most efficiently. After you've found the best setting for 50 MPH, do the same for 60 MPH and 70 MPH. If possible, then you can find a mountain with a long uphill and long downhill to make the same adjustments.

Second way- do the same procedure on a dyno- one capable of providing a fixed HP resistance (to simulate aerodynamic drag). Not all dynos can do this, check with your local dyno shop. You might try simulating a 50HP constant resistance, then 75, then 100 (based on your feel).

I have most of my normal driving at 14.5-14.7, with a few of the light-throttle spots up to 15:1.

Third way- set the whole map for 14.5:1 and just drive it, making an adjustment only if necessary.

For WOT tuning, I like a dyno- this gives you the ability to adjust "parts" of the curve, for example I found that on my car it likes 1 degree less timing above 4000 RPM compared to below 4000 RPM. A dragstrip won't let you see little things like this.
My car gave the best HP #'s at 12.2:1 A/F at all RPMs.

-Bob Cunningham
bobc@gnttype.org

 

[ZBass28]

Ahhh...very interesting Bob.

I'll have to utilize some of those ideas on my next round of tuning. I have even
thought about going as far as trying to contact programmers from the likes of GM or Ford to
see how they go about it. I know their goal isn't quite as 'performance' oriented as mine, but they
seem to get the job done pretty well

My car too seems to prefer right around 12.2 at WOT on the dyno, at idle it seems to idle great at around 14.0.
I think my around town settings are in the 13.0-13.5 range and now I have to wonder if that's a little high.

I get really intrigued with this stuff, but usually after an hour of messing with stuff that intrigue turns into ANGER! HA!

Thanks for the insight, I'll have to try some of these. I guess I have just been afraid to try and run my
car anywhere around 14.0 or higher under normal driving conditions just out of fear of safety margins.

Tom

 

[bobc455]

If you are driving around at light throttle, then things like detonation don't carry the catestrophic consequences that they would at WFO. How many of us have driven a junky old car around for thousands of miles, pinging on every hill? Obviously you wouldn't want to treat your favorite car that way, but if you try 15:1 and get a couple of degrees of knock retard then the world isn't gonna end by the time you get it back to 14.7.

Actually when I've been tuning my car and seeing if it will take 15:1 or leaner, it doesn't ping or knock, it just starts to stumble a bit (under light throttle). Then I richen it up a bit and drive happy.

One other note, if your car gets tested for emmissions, then I highly recommend doing the idle tuning with a good emmissions tester (like at some dynos) to check your CO2, HC, NOx, etc. And, if you have an IM-240, then you should do a full tuning on the dyno *first* to find the best A/F so you pass emmissions. (Best fuel mileage may not necessarily give you optimal emmissions, although they are frequently very very close.)

-Bob Cunningham
bobc@gnttype.org

 

[Otto J]

as far as adriveablity issue and a/f ratio ove found on my car that running it arou13.5 to 13,7 to keeps the engine running a decent coolant temp, when leaning around or above 14.7 my coolant temps are about 10 degrees hotter, although i do have a front mount and 9" convertor,bbut it definlety get beeter mileage the closer it is to 14.7 to 1
Otto

 

[quickstrike]

I tried leaning out to around 14.5 to 1 this afternoon, and I found the same to be true of the coolant temps as the previous post.
About 10 deg hotter. It also had some very light popping/backfire in the exhaust under deceleration at high rpm. Car also stumbled just a touch from a dead stop idle. I think as bobc455 stated earlier some cars are a little different. I think I will back it off tommorow to about 13.5-1 and see if that doesnt work better.

Great topic, this was very helpful.

P.S. do you guys run in closed loop when street driving? Possibly with a street tune and a seperate race tune?

 

[ZBass28]

So far I have always run in closed loop, even at idle.

Never really had much problems til now when I switched from high imped. 50# to lo-imped 65#
injectors. Occasionally I will get a small bounce at idle after a few revs. I may try going to open loop.
I have the car running at 14.0 or so at idle and can smell a lot of excess fuel coming out at idle, which the
car never did with the 50's. My corrections are good too.

I need to get a buddy to go with me so I can do some street driving tuning.

I have noticed that with the new injectors my IPWs at idle went from 3.1 to 2.1, not sure if that's a good thing
or not? Also, my VE numbers around idle are down about 15 points from where they used to be. Is it safe to
assume that I can generally make some changes on the VE table and take out about 10 from things just to get
them closer?

Luckily my car is emissions exempt so I don't have to worry about passing any tests.....except for the butt-o-meter test