AFR Target question
 

Guys,

I'm starting to tune my NA. '89-'91 block with a header, cats and cat-back. I have been told to target 13-13.5 for the WOT AFR across the RPM range. Makes sense to me and pretty easy with the RTek - I'm adjusting the correction factors in the low (-6 to 0 inHg) MAP cells across the RPM range.

But what AFR targets should I have for the lower MAP values? Should I observe some leaning out of the AFR as the MAP values go more negative?

Appreciate any input as I'm fairly new to this. Thanks.

John

For low load and lower map values I think your going to want to shoot for stoich 14.7:1 or around 14.

Not 100% sure about that tho.

I'm no way a tuner or a pro, but that's what I am shooting for on my TII.

Side note: my palm read 16in/hg, but the afr's changed when I adjusted the 10in/hg values. Kinda strange, its like their -psi opposed to in/hg...

Good luck!

I'm seeing very good correlation between what I change in the fuel table and where the AFR changes in the log.

I wrote a piece of MATLAB code that reads in the log files, discards the off-throttle (super lean) datapoints and then maps the Min, Max and average of every point in the fuel table (MAP versus RPM) to a cell. When I go make changes and go logging again I can see that those cells that I wanted to change are indeed changing. My car is an NA, though, so I'm sure the MAP sensors behave somewhat differently.

See the attached picture of average AFRs before and after tuning. The bottom table is before the latest round of tuning and is a sample of about 10 minutes of logged data from all kinds of driving. The table on the top is after tuning. Obviously lots of work needs to be done and it's interesting to see that not all of the cells are populated (I didn't explore all the areas of the fuel map on my drive). I was mostly focusing on WOT fueling at all RPMs, so anywhere from 2,048 RPM to redline and from 0 to -6 inHg. Notice how rich it was, especially near redline at WOT.

They run that rich to protect the cat. It's not surprising. On OEM calibrations the AFR is going to steadily get richer as rpms go up to keep exhaust temps from getting too high.

Your method of populating the cells is one technique. Using the logviewer is another way to do it.

You wrote your own program to do that? That's pretty awesome.

I think this is what arghx is referring to: http://www.pocketlogger.com/index.php?pid=utilities

I use the PL viewer a lot!

I use the PL viewer as well, but I don't like the way it visualizes the data.

So the question is what should your AFR's be in the low vacuum areas that are not WOT. And to that I answer that you should just have a smooth progression at every MAP point for each RPM row. So if your -2 WOT AFR is 13:1, make your -4 AFR equal or leaner.

Check out this factory AFR target table on a nonturbo Subaru Outback:

https://www.rx7club.com/attachment.p...1&d=1271986635

it is based on the AFM signal, not the MAP signal (although Subarus do have both sensors), but you see what I mean by smoothing out the AFR's. You don't necessarily want those exact values but you want AFR to richen or stay the same as load/MAP increases. Then as RPM increases you either richen the AFR (as Mazda has done from the factory) or keep it flat at some particular AFR.

Hm. Maybe the next question I should ask is - should I be running richer than 13-13.5:1 at WOT and redline? From the research I had done it seemed like I should be leaning the car out up there for best performance. I am only doing autocross and a little street driving, no "real" track racing. Is there risk of damaging the engine by running leaner up top?

John

What does your timing look like? That will also play a role in how lean/rich you wanna be? Do you have an egt gauge hooked up?

Remember that as you lean out the afr, your flame speed will start to slow down as well. I noticed for driving around town my car likes 14 to high 13's, even on the freeway it felt strong. For lean cruise I am around mid 15's to 16, but you can feel a lack of torque. Cruising on flat ground is great though.

Haven't touched the stock timing tables and don't plan to.

^ You can pick up some power by advancing the timing a bit. back in 2004 I actually rotated the CAS to pick up power on my nonturbo. This was long before the 2.1 was available. I wouldn't be very concerned about detonating. I never had any problem with that and I always ran 87 octane.

as for AFR, you can definitely try shooting for 13:1 or so

I have been advised by a few people that since the S5 timing was more aggressive from the factory, there isn't much to be gained by advancing it any more.

Fair enough. I had an s4, and I've never looked at the s5 n/a timing maps.