I had a friend ask for advice on using the dyno tune AFR calculations charts I made (with John L's help), as well as for advice on dyno tuning.  I thought a few people might be interested in this, since I've never really seen it spelled out anywhere. If people want supercharged advice I'll add that in if people want it.

This is mirrored at:

http://carprogrammer.com/Z28/PCM/Dyno/Dyno%20Tuning%20Advice.htm

 

Disclaimer: This is a rant from my twisted brain, and I was nice enough to spell check it, but I didn't check for typos.

If it doesn't make sense, read it again. If it still doesn't make sense try translating it into another language, then back to english- that might help. 

If something is just downright wrong then correct me, my friends always tell me when I'm smoking crack.

 

Hope this helps,

-Christian

 

 I've got them on my website, although without an explanation no one would know what they're looking at.

I've tested and used them to nail my AFR from 13.5:1 to average 12.5:1 (probably leaner on the street)

http://carprogrammer.com/Z28/PCM/Dyno/AFR13.5.gif

http://carprogrammer.com/Z28/PCM/Dyno/AFR12.5.gif

 

Here's the chart to use for 12.5:1

http://carprogrammer.com/Z28/PCM/Dyno/dyno%20tune%2012.5%20to%201%20AFR.htm

and here's 13:1

http://carprogrammer.com/Z28/PCM/Dyno/DynoTune%2013%20to%201%20AFR.htm

 

It takes a lot of change to make a difference of just 1/2 a point AFR ( like 6-7 % PEvsRpm in my experience.)

 

To use the charts, I set my PEvsCool to 16 in all operating temps, this takes out the variable there.

Run the car on the dyno with a wideband and examine each of the data points at the rpm for the PEvsRPM table.

 

i.e..  at 3600 rpm you have a value of 1.0 (wild example)  and the dyno wideband said your real world afr is 14:1, so you're running a bit lean.

Find 1.0 in the leftmost column and go over to the 14.0 column, the value to use is 18, this should give you the change needed to have a 13:1 AFR.

 

If running dual widebands and you see a disparity between the two sides I recommend looking at the average of the two banks.

 

Now the guts:

I saw very little power difference between 12.5:1 and 13.5:1, so naturally I was disappointed.  In higher power cars it makes more of a difference, and in supercharged cars it naturally makes a huge difference. I expected to see something, but I only saw 1 ft-lb of torque between the two. I rather run 12.5:1 on the dyno for two reasons:

1.) Since the dyno is 3000 lbs and my car is 3800 lbs, the car will be leaner on the street, maybe 13.1:1

2.) Since I'm running 11.4:1 compression I prefer to keep the combustion chambers cool, so I don't see knock.

 

What does make a big difference on the dyno is timing, and don't assume more timing is better.

If you're running a higher compression motor, you in fact may need less timing, since the charge is more dense and the flame spreads quicker.

 

On the other side, if you're running a higher overlap cam, with longer duration, then it may take more timing to get the best performance.

This is again because the charge is not as dense, so the flame speed spreads slower. The whole idea is to get the charge to ignite at about 15 degrees After Top Dead Center, it take different amounts of lead time for the flame to spread throughout the chamber, so finding the optimum timing is the trick.

 

For a naturally aspirated car (non-supercharged):

You'll adjust the timing by modifying the 95 and 100 Kpa rows in the Load vs. Adv. 400-4000 and Rpm over 4000 tables.

 

Note:

The F-body stock tables max at what I like to call "total timing" at 2,800 rpm with 33 degrees advance.

The Y-body and most tuner programs for slightly larger cams max "total timing" at 3,200 rpm with 33 degrees advance.

Below total timing the stock advance decreases at a rate of 3 degrees per 200 rpm, so you may want to follow the same format.

Several tuner programs decrease the advance at 2 degrees per 200, some even 1 degree. Most dyno runs don't have data below 2,000 rpm because people usually aren't WOT that low.

You may want to run less timing at low there to retard the WOT so you don't have major traction problems when you floor it on street tires, then again you may want to leave the traction control up to your right foot.

 

Here’s the stock y- body for example:

 

            400   600   800   1000  1200  1400  1600  1800  2000  2200  2400  2800  3200      3600  4000 

90 KPA      7.0   9.0   12.0  15.0  19.0  22.0  24.0  27.0  32.0  34.0  35.0  35.0  35.0      35.0  35.0 

95 KPA      5.0   6.0   9.0   14.0  17.0  20.0  22.0  25.0  30.0  32.0  33.0  34.0  35.0      35.0  35.0 

100 KPA     2.0   3.0   6.0   12.0  15.0  18.0  21.0  24.0  28.0  30.0  31.0  32.0  33.0      33.0  33.0 

 

How to experiment (after the Air to Fuel is calibrated):

1. Make two base runs with just the stock total timing, realizing the second run on a dyno is always higher, since the carbon gets blown out of the ports and chambers.  Keep an eye on knock, you don't want any detonation, since the knock sensors pull major timing and gives it back very slowly.  You should be able to make max power without being within 2 degrees of knock, otherwise the sensor is getting "false knock"

 

2. Try a run with 1-2 degrees more total timing, see if horsepower increases/ decreases.

Try it also with 1-2 degrees less timing, see what areas like a certain timing- again - avoid knock, your power should start to decrease before the knock occurs, if not it may be false knock.

If false knock, set the knock vs. rpm table to 0, and tune 1 degree at a time, again watching for where the torque starts to decrease- and it will decrease (since premature ignition causes increased pressure in the head of the chamber, which makes it harder for the piston to rise on the combustion stroke.) Knock sensors do go bad, so take that into account, but don’t assume all knock retard is false, verify it by seeing if the torque increases with 1 degree less timing, it should. There is a window, like Ed said. 

 

3. You may find the stock timing is perfect for your car, many have. Others have had to add or decrease timing depending on charge quality. If you end up with stock total timing is dead on, then do what I do - nod your head and say "Damn those GM engineers are good."

 

Hope this helps, always share your findings/results

-Christian