LS1-Inside

I'm curious if there's a connection between stoichiometric AFR 14.7 and Atmospheric Pressure at sea level 14.7?

Any link between those two?

Thanks,
Dan

3.4camaro

iTrader: (1)

I'm pretty sure it's a coincidence. You need 14.7 parts of air for every 1 part of gasoline, doesn't matter how close together(pressure) the air molecules are.

Anyone care to add anything else?

LS1-Inside

That was my initial thought too... trying to tie the ratio of Fuel & Air to the weight of Atmospheric pressure is boggling my mind! LOL

More input would be appreciated - i'd love to see someone bust out their PhD in Physics or something... LOL



Dan

JohnnyC

Purely coincidental; The pressure has units of pounds per square inch (psi) and the stoic 14.7 is a ratio of air and fuel masses. No relation. You could use 101 Kpa for your pressure units and you would still need a 14.7 AF mass ratio (use any mass unit that you want).

Pressure is force per unit area

Stoic A/F is a ratio of the mass of air to the mass of fuel

3.4camaro

iTrader: (1)

YAY I was right!

LS1-Inside

Great - cause thats what i told my friend who asked me... I figured they're completely unrelated units of measure, and there can't really be any tie there... but i figured I'd ask on here just to make sure... (He asked on his favorite forum too... heh curious to see what his fellow forum members say)

Thanks,
Dan

LS1-Inside

Wait - isn't stoic impacted by altitude? (Atmospheric Pressure)

-Dan

3.4camaro

iTrader: (1)

No. But the amount of fuel you put in for a given volume will change. You still want the same RATIO, but maybe not the same AMOUNT.

gun5l1ng3r

No, stioch is not changed, ever. It is just the ideal perfect chemical ratio of air to fuel paticles. I don't think a combustion motor will ever have a perfect 14.7:1 ratio at all points in the combustion chamber. I could be wrong.

When you go up in altitude you A/F ratio goes down, but you would still ideally want 14.7:1 for perfect chemical mixing...

Manic Mechanic

iTrader: (3)

Also different fuels, alcohol, nitro-methane, propane, etc. reach stoich at different ratios. The 14.7 is for standard Gasoline.

Vernon

1 FMF

iTrader: (1)

what is standard gasoline today?
here in corripticut it's 10% ethanol, and that has a low AF ratio by itself so having gasoline with 10% of it has to make today's gas have a slightly lower A/F ratio than the traditional gasoline of old, however it may have been blended which yielded the 14.7 A/F?

just checked wikipedia, the more accurate stoich ratio is around 14.1. The number 14.7 is for "pure" gasoline containing n-heptane and iso-octane. All the oxygenates and other crap blended in like MTBE, ethanol, methanol, drive the stoich number downward.

'JustDreamin'

1 FMF makes an interesting point....

We shouldn't be using 14.7 as stoich for all of these new blends of fuel...Because of the ethanol and everything else, the fuel suppliers (in response to legislation) has changed the stoich ratio.

If the stoich ratio for the "new" gasolines are around 14.1, where does peak power now occur? Is it still around the 12.5 range? Maybe lower? And where is peak economy now? Somewhere around 16.5 to 17 or so?

'JustDreamin'

1 FMF

iTrader: (1)

when narrowing in on peak power, the a/f ratio is less dependent on stoich than it is on combustion chamber and cylinder head design, at least that what the trend i've seen seems to be.
All the dyno results i've seen in chevy magazines tend to show A/F around 11:1 to 12:1 for the traditional carb'd 350, but when you look at the LS1/LSx's that tends to be higher at around 13:1. But peak economy on the other hand is always going to be the highest A/F ratio you can run without hurting anything, because all that amounts to is using as little fuel as possible. And I think it can be as high as 17:1 on the LS1 under favorable conditions, but usually around 15:1 I think, the guys in the tuning section would probably know.

3.4camaro

iTrader: (1)

I thought anything over 15:1 on a LSX would cause major pinging and bad things. Heard this over in the tuning section.

1 FMF

iTrader: (1)

pretty sure it would depend on engine load whether major pinging and bad things would happen. I'm going from memory, but I'm pretty sure that under light throttle, high vacuum, and light engine load the pcm under factory settings will run the engine with a high air/fuel ratio, in the neighborhood of 15 to 17. And it would make sense to do so since it would yield greater fuel economy and lower emissions. As soon as you increase throttle though or engine load increases, the PCM lowers the a/f ratio, and may reduce timing. I get the impression that aftermarket tuning is still in the medieval stages, so the reason bad things happen over 15:1 is probably their programming isn't 100% correct and the pcm isn't catching the right parameters to know when to add more fuel. Correct me if i'm wrong, but most aftermarket tuning is to correct for power adders and the goal of the tuning is max power and performance: when you stomp the pedal the car goes, and not so much light throttle cruising with high fuel mileage and low emissions.

3.4camaro

iTrader: (1)

I dont think tuners touch much of that stuff. Most of the things they mess with have to do with idle, VE, MAF, and PE ratio.

That doesn't really have anything to do with "when vacuum is this much, pull this much fuel" things that you're talking about.

1 FMF

iTrader: (1)

and that's my point, they don't touch much of that stuff or know how to correctly and therefore bad things happen. things like idle, VE, PE and especially MAF are all interrelated. If you don't understand the algorithms using all that input, you're not going to handle all operating conditions of the motor anywhere near close to optimal.

3.4camaro

iTrader: (1)

I disagree.

The tuning of LSXs is getting all the sensors calibrated as accurately as possible, so the algorithms can do their job better. ex:

A stock tune on a stock car will work pretty well. Swap out some headers, and you've changed the VE at every rpm a LITTLE bit. This change to the VE is corrected by the fuel trims, but in order for the algorithms to work properly, you have to change the VE so the fuel trims get back to where they were.

A stock tune is something that will work ok for everybody, but won't work perfectly for anyone. It's designed to work ok in Alaska and in Florida and everywhere in between. Tuning is getting your tune working specifically in your area and weather conditions.

hammertime

iTrader: (1)

IIRC, what you have described is known as lean-burn, and has been Illegal in the US for some time. Australia does allow for lean-burn, and it can be found in the PCM on GTO's, though I believe it is supposed to be locked.

3.4camaro

iTrader: (1)

Lean burn will make your NOx levels go through the roof, IIRC. When you go lean, the engine runs hotter, because there is no unburned fuel left to take heat out. When they get above 2500F, NOx will be created, which is sposed to be some nasty stuff for the environment.