[SoonerStang]

Some questions for you

i was reading another forum, and thinking about my engineering experience, and couldnt help but disagree with their analysis.

the stochiometric a/f ratio is roughly 14:1.

they stated that every car runs differently due to "so many variables"

when running richer than 14:1, the hydrocarbons do not break down nearly as well as a lower ratio, leaving hydrocarbons to get sent to the catalyst tubes.

when running leaner, you will burn less carbons, but more oxygen, thus burning hotter, again leaving a less complete burn.

does anyone have any GREAT insight?

i think i am going to have to go back to my Internal Combustion Engine book.

[Grabber523]

So what's your question? Stoich is 14.7 to 1. if you're rich, you're not able to burn all of the fuel completely, it's safer but less efficient to be a little rich...too rich, and there's not enough oxygen in the mix to support burning the fuel at all. If you're lean, there's too much oxygen in the mix, and it causes an unsafe temperature, which damages parts. So there's still oxygen left over after the combustion cycle. So you're right. I don't understand why you disagree with that?

[RStrick0352]

Each car is different, they each have their "sweet spot" as far as what they like best (performance and reliability). A good tuner with a dyno can find this out, my cars spot was around 13.5:1. You just got to tune it man.

[Grabber523]

I've run 11.5 to 12.0 to 1 on every FI car I've ever built...13.5 seems so lean to me...but I've seen other NA'ers with mixes about where you're at...

[SoonerStang]

my question was more of, THEORETICALLY, 14.7 to 1 will produce the best (most efficient, most power) combustion.

they stated that the leaner you get the more power you get.

while i am at it, when i did A/F ratio calculations, i had to measure the mass air flow in, and the EXACT amount of fuel used, using charts to give the mass air flow with respect to the venturis that we used on that particular case.

i assume the MAF meter calculates the air flow, and then something in the gas calculates the fuel over a given time, does the stock computer try and adjust to the stochiometric continually, or does it just keep you running where you are?

so many questions moving from carbs to fuel injection

[RStrick0352]

Yeah, thats the impression I'm under is forced induction and nitrous cars like it richer, its almost a security blanket i guess.

[Grabber523]

Stoich is NOT the best power or efficient ratio to burn fuel...however, it ensures a clean burn, no fuel or oxygen left over after combustion. when adjusting the mixture while flying a piston aircraft, 50 degrees celcius rich of peak temperature on a pyrometer is best power, 50 degrees celcius lean of peak is best economy...and that's only at cruise...no idea what the a/f ratio is. Why do I tell you this? Because stoich is not the best power, and any leaner is dangerous, and any richer is for high r.p.m. or high load acceleration, like a blanket to prevent detonation...rich of peak, if you want to call it that...for best safe power, but it's not for cruising under part throttle/low load.

[Grabber523]

wow...okay...the computer has a table that the tuner will adjust that tells it how it will run based on ideal conditions, based on the volumetric efficincy of the engine, how much air it can ingest and burn, how much fuel and spark SHOULD be there to get a decent burn...all in theory and calculations, all while referencing the MAF in and the O2s out, and based on the info it gets, it can adjust the a/f ratio based on what it's ACTUALLY reading from the MAF and the O2 sensors, along with several other sensors and it checks this several hundred times a second, as temp, density, load, humidity, ambient pressure are constantly changing...does that help?

if you are cruising, the computer rechecks all the air inputs and exhaust output to ensure that you are as close to stoich as mechanically possible...under part throtle/low load...if your load increases or you open the throttle to accelerate, you computer knows it needs more power and will richen the mixture for more power, and the spark advance will also adjust to get the best power without knock or detonation...

[SoonerStang]

i know where the o2 sensors are, and the MAF, but where the heck are all the rest of the sensors.

it all makes sense, but i feel like i need to take the 07 to the dyno to get a real feel for it.

i guess the stoch was beat so hard into me in coursework as a target, that i have just had it ingrained into my head.

just have to get it tuned.

[Grabber523]

You didn't tune your carb to be stoich unless you were at idle or cruise, though, right?? Did you ever check it under acceleration?

shit, you also have a water temp sensor, knock sensor, crank angle sensor, fuel pressure sensor, throttle position sensor, I can't even name them all off the top of my head...there's a ton of inputs on these cars, that's why everything you do to it as far as induction needs a tune.

[SoonerStang]

when i had my 70, i didnt CARE about stoich, we adjusted the carbs in the lab during idle. but we did check it during 5 or 6 different levels of load. not during accel, but we would set it at 1500rpm, check the dynomometer, set at 2k , check dyno, check fuel, and air mixture. etc.

it stayed pretty consistent.

[Grabber523]

Sweet, man if I could get my hands on a load-based dyno...it would be glorious!! Throw a wide band O2 gauge in that car, it'll give you a feel for what the car is doing under various conditions...it will make a lot more sense...