aBrotze

I just hooked up my Wide Band 02 setup from Tech Edge (nifty gadget by the way) and noticed that my A/F ratio is around 12.0 at WOT on motor. I have read that an A/F ratio of 12.7 to 13.2 should be targeted. I was just wondering what A/F ratio at WOT you guys target. Just wanted some more opinions before I start leaning the A/F on my 396. Another thing..... I have LT1 edit and was wondering how much to adjust the PE table ( percentage wise) in order to lean my A/F by a full point. I have 42.5 injectors so I guess I could also overreport the size of these injectors (lets say 44.0) also. Thanks....

Soma07

Cool, so you have one one of the TE WB O2 setups as well? How do you like it? I just finished building my v1.5 kit but I'm still waiting for the sensor to arrive so I can test it.

Anyways most people shoot for ~12.5:1 for NA motors, maybe a little leaner. Under boost or N20 I think 12.0:1 is the prefered number just to keep things on the safe side. IMHO the only way to know for sure is dyno testing to see what you motor likes best but those are safe numbers to shoot for.

I havent played with the PE vs RPM table much but it appears to be fairly insensitive to small changes. In other words you need big changes numerically to see a small change in the A/F ratio. Its always best to start of small though.

Raising the injector constant would lean things up but it would do so everywhere, not just at WOT. Thats fine if you're rich across the board but if you just want to tweak the WOT stuff stick with the PE vs. RPM table.

Injuneer

You put it on a dyno and adjust the A/F ratio and timing until you get the torque curve you want. That should be the reason you got the wide-band. Alternatively, adjust the target A/F ratio, and see what setting gives you the best (corrected) 1/4-mile MPH.

You also tune by adjusting the PE tables.... not by giving the PCM false information about the injectors. The objective is accuracy, not "fooling" the PCM.

Eric Bryant

Beware that the Tech Edge design is not a lab-grade instrument and may be off by a half-point or more. If it reads a bit low, this could be a big problem if you're shooting for a particular AFR value.

I'd recommend tuning on the dyno, as opposed to simply trying to hit some arbitrary AFR value. You may find that the optimum AFR for your particular setup varies from what works best for others, depending on compression, timing, etc.

Dr.Mudge

Tune for what makes power, richer for a heavier car - I went for 13.0:1, maybe next time I'll try richer and see how that works.

number77

i always thought that they just used a stoichiometry problem between the air and fuel and the number they came up with was the best for making resonable power with good emissions

rskrause

No, stoichiometric or stoich plus some standard correction factor is not the ideal AF ratio. Stoichiometric AF ratio is the ratio that would give two atoms of oxygen to react with each carbon atom in the fuel. If the fuel were completely combusted, there would be no left over oxygen or carbon atoms, just CO2. Stoichiometric AF ratios for common fuels are methanol 6.4, ethanol 9.0, MTBE 11.7,
gasoline is ~14.6-14.7:1.

The problem is that combustion is never perfect and it occurs over time. Since airflow is limited, what you want for max power is to be sure that each fuel molecule is sure to meet up with an oxygen molecule in time to combust. If there were infinite time for combustion to take place, a much leaner mixture would be ok. But you want an excess of fuel compared to O2 (ie a rich mixture) to be sure each fuel molecule meets the right number of oxygen molecules in time to combust, preferably while the piston is close to TDC.

So even on an NA car, richer than 14.6 is desirable. On a blower car, an even richer mixture is desired as the excess fuel helps cool the intake charge. Only experimentation can show the optimum AF ratio for a given combo. And the optimal ratio will vary at different points along the rpm range. This also must be determined by experimentation. In general, for an LT1, we start with ~12.5:1 for blower cars and ~13:1 or a bit leaner for NA.

You also must consider the test conditions. Tune on a day with dense air, and the car will run richer under most conditions. Tune on a "bad" day, with thinner air (warmer, more humid, lower barometric pressure) and the car will run lean when the weather improves. You want to give a safety margin when tuning under these conditions. And you should keep in mind that running a little rich costs a few hp, it has to be WAY rich to really drop off much. Run too lean (especially with nitrous or a blower) and you can say "bye-bye" to your pistons!

If you ever wondered why a carbureted car runs better when the air is cold and dry, I will now explain why. Dense air means the motor is ingesting more oxygen. Since most calibrations are rich, there is plenty of excess fuel and denser air will bring the mixture closer to ideal. Of course, if your car is fuel injected and has a mass air or other method to measure the air flow, it will compensate. Even though the mixture may not change, it will provide more fuel for the incoming air. And more fuel combusting = more hp. When a car (especially carbureted) is set up "on the edge", with an optimal AF ratio, the jets must be changed when the density altitude changes, otherwise it will be too rich or lean. There is some more leeway with most fuel injection setups, as they can compenstate for changes in air density.

The best thing to do is put your car on a dyno. But no matter where you do your tuning, aim a little on the rich side of where it makes max hp. It's safer and only costs a few hp. I run 12.5:1 on blower and ~12:1 on the juice. And my pistons look good after ~8,000miles with 14lbs of boost and after many tanks of nitrous!

Rich Krause

Injuneer

And to complicate things even further, the numbers quoted above for "stoichiometry" are the mass ratios for "air" and "fuel". Adding nitrous to the mix alters the stoichiometry, because of the higher weight percent of oxygen present in the N2O. The stoichiometric "N/F" ratio for nitrous and fuel is 9.65:1. So you will be looking at a "blended" stoichiometry, depending on the ratio of nitrous to air.

cmillard

Lean is mean, or hotter they should say, but not for long. According to most combustion text 12.6:1 should make the most power for gasoline, but the extra heat increases cylinder pressure, so the leaner number will make the most power until it detonates and blows a hole in something (hyper pistons most likely.)

Experiment and see what your car likes on the dyno power wise, but most likely you'll see the leaner the meaner. This is where you decide how long you want your motor to last and you choose the slightly richer mixture for the long haul (hopefully)

On the LT1 motor it takes a value change of about 10 - 12 AFR on the PE vs Rpm table to move a whole point ( i.e. 13:1 to 12:1, etc) The LS1 isn't quiet as linear in it's enrichment, but that's the darkside for ya.

Timing is critical as well, too much timing you detonate, too little timing and you are leaving power on the table. Earlier spark advance gives the flame more time to process the mixture, again increasing cylinder temperatures, thus pressure, thus torque, thus horsepower.

Make sure to let the car cool slightly between runs to avoid heat soak, it will hurt performance and invite detonation.

Hope this helps, good to see so many friends here,

-Christian

number77

the sad thing is that i learned about stoich. in chemistry class before i realized it was used in cars, well at least now i can do a full stoich. problem in about 5 minutes

hey, rskrause i have seen you post like a billion times and all of its good hard-core info i think the mods should start a new section and put all of your posts in there and give the section a name, i know i would learn more than a little.

Dr.Mudge

rskrause, what he says is correct. It is NOT used for WOT, however it is what makes the CAT do its business, along with fast switching in a minute fashion slightly lean and slightly rich, how many cycles per second I have no idea, but stoich is there for smog purposes and reasonable power, at non WOT conditions.

I have seen a chart that shows how AF affects power at WOT, and if the chart is accurate there is very little if any power to be gained between 12.0:1 and 13.0:1, meaning one is almost no better than another, however the resolution of the chart may not allow for something as fine as 3-5 HP but in general terms its probably somewhat accurate. I can try to post it in a day or two if there is interest, or maybe Christian can host it for us

Dr.Mudge

Well, lets all remember there is a limit to how cold we can go, carbs do not atomize fuel well in cold air (again how cold are we talking), this is what the choke is for (richen the mixture till it runs well enough to back it out).

BTW, I am not at all debating that you know or dont know what your talking about, but you mis-understood the Stoich post intentions I believe, at WOT obviously it is not what we want (so it is confusing when this thread is asking for WOT numbers), but what was stated is exactly the intention of stoich

rskrause

Aw shucks , FWIW, I am working up an "LT1 Cam Primer". It's already 12 pages long! I am not sure where to post it though, as I am not in the mood to set up my own website.

Dr. Mudge: Good point in terms of part throttle, but while I am not real familiar with OEM part throttle tuning I always thought that they aim for a bit leaner than stoich at light load part throttle cruise (and leaner still under lift throttle).

And Christian brought up timing, which is another very important variable. Advancing the timing will, to a point, make the AF ratio appear "richer" by allowing more time for combustion. As a result, when the residual exhaust gas in analyzed, there is less O2, which is read as a "richer" mixture. When timing is retarded, the reverse happens. This will, in turn, effect the EGTs. To do a really good job of tuning, it really helps to measure EGT. Ideally, you want an O2 sensor and an EGT sensor for each cylinder. On my car, I have used EGTs for each cylinder and two wide band O2 sensors (one in each collector).

Just some tuning musings here, not sure what I'm trying to get across exactly.

Rich Krause

cmillard

Will I can host that for you, just shoot me an email.

A 4 or 5 gas analyzer is also key for tuning. While most units are well out of the hobbist price range, for ~$3000 a shop can afford one pretty easily.

The only weakness of an O2 meter is it doesn't tell you much about combustion quality, this is where EGT's become useful as well as gas analyzers. Think of an engine with the wrong timing and the wrong fuel metering, viola both can correct for each other to obtain stoich.

The reason O2 doesn't give the full picture is because it's present before and after combustion, whereas one of the resultant gases such as CO2 and NOx levels that can be used to determine combustion efficiency. Too much NOx indicates high chamber temperatures, which promote knock. CO2 is the resultant product of a good burn and can be used to determine combustion efficiency. HC levels will show a misfire or poor combustion, among many other things.

Here's an example of how the gases relate to each other:

http://carprogrammer.com/Z28/PCM/emi.../4GasChart.gif

for a good article read:

http://carprogrammer.com/Z28/PCM/emi...alysis_hg2.pdf

Just for kicks I'm bring a carbon monoxide meter up to the dyno tomorrow, curious what CO levels we tune and breathe in, I know a misfire will burn your eyes so bad that you'll cry.

Stirring the pot,
-Christian

kmook

Done, Look for the "Rich Krause Knowledge Base" forum comming soon