Air/Fuel Ratio

[Rancher2005]

I know that this is often debated but am still curious. Stoichiometric as far as I understand is a 14.7:1 a/f ratio and only meant for good catalytic converter efficiency. This is for good emissions rather than peak hp. I was wondering what ratio would produce peak hp assuming no changes cam degrees or spark changes other that what a stock cdi may do. I know that at either ends of the spectrum hp/torque would be bad. I'm just wondering what that middle number might be. Let's just assume it would stay consistant across all throttle positions on a carb and i could hook it up to an exhaust sniffer. Finally, will a certain a/f ratio produce better hp while another produce better torque?

[JasonP]

Well this is a pretty standard figure honestly. All cars will try and mantain a 14.7:1 air fuel ratio at a light load cruise and some will even lean out more then that. It goes to show how well engines are running now a days. Under load though all bets are off.. .even todays cars will run a mixture of about 12:1 or so, maybe richer for engine life and cooling.
The best power producing mixture is right around 12.5:1 to 13:1 depending on a bunch of ****... cylinder head materal, compression ratio, burn rate, fuel quality, igntion advance just to name a few. But most good dyno operators know this ratio and will try and achive that to produce the maxium amount of power.

Really, your measuring torque with a dyno.. horsepower is just a calulated figure. So to make the most hp you have to be producing maxium torque.. they go hand and hand and there is no air fuel ratio that would make more HP rather then torque.

[Rancher2005]

I'm glad to know that a slighty richer mixture than stoch. gains more horsepower. As it will also keep the engine cooler. I'm guessing that the only drawback would be more carbon buildup? Would you think that the 12 or 13:1 ratio would achieve the brown paper bag spark plug color? or how far off would it be from this?

[JasonP]

Originally posted by Rancher2005
I'm glad to know that a slighty richer mixture than stoch. gains more horsepower. As it will also keep the engine cooler. I'm guessing that the only drawback would be more carbon buildup? Would you think that the 12 or 13:1 ratio would achieve the brown paper bag spark plug color? or how far off would it be from this?

Oh yeah there isn't any more power to be had going any leaner then 13:1 on todays pump fuel. Plus you'll just burn the engine up. That mixture (14.7:1) stoch is a term used for a light load situation only. Carbon build up is more from how well the combustion chamber is designed and how well the engine is burning the mixture on top of actual fuel mixture. There is a lot that start's coming into play here. There really is no draw back because the engine producing max power off the right mixture will have less carbon buildup then say a richer mixture.
As far as doing a plug check, well I've tuned mine so the it's more light gray then brown... brown sounds a little rich yet. But really it's hard to tell with todays fuel and the actual coloring of the plug. What I like to do is rich the main up until you get a little bit of break up on the top end under full throttle then back off one size. Only way to tell is put it on the dyno and tune run a sniffer to tell what mixture your runing and go from there.

[GPracer2500]

Great topic!

Forgive me if I ramble, but I start getting all giddy when tech oriented topics crop up about combustion..... :D

ETA: Skip to the bottom for a summary--I'm sure lots of folks don't have the time or inclination to read this. The summary is actually just a repeat of some of what JasonP has already said. Read on for more detail!

Keep in mind that stoich is really a "perfect" theoretical figure more than anything else. You can figure out on a piece of paper that 14.7 units of air contains enough oxygen molecules to react with 1 unit of "gasoline". But peak torque is rarely seen at those figures. Seems strange since burning every bit of available fuel and every bit of available oxygen would logically make the most power. But in the real world, 14.7 AFR rarely makes the most power. Here's a few reasons why:

I put gasoline in quotes up there because the 14.7 figure relys on the gas being nothing but pure n-heptane and pure iso-octane. There is no such gasoline outside the laboratory. In the real world, all gasolines I'm aware of (pump, race, avgas, et al.) are actually made up of a mixture of HUNDREDS of different hydrocarbons--not just those two. Plus many fuels have additives and oxygenates that further change the actual stoich ratio of the fuels we use.

My understanding is that "real world" gasoline typically has a lower (richer) stoich than 14.7. 14.7 isn't actually the correct stoich for the fuels we use--some lower figure is. That helps explain why peak torque is rarely seen at 14.7 and instead is usually around 13. I don't know the facts, but if I had to guess I'd say actual stoichs for most pump fuels is around perhaps 14.0 (a guess).

And there are still other reasons that explain why ~13 will actually produce peak torque instead of 14.7. One is you can never get ALL the fuel to react with the available oxygen. Or at least you can't without rigorously controled labrotory conditions. One reason is this:

--Not all of the gasoline will actually make it to a vaporized form and liquid fuel won't burn. If you pour a puddle of gas on the pavement and light it on fire, it sure looks like the liquid gas is burning. But it's only the layer of vaporized gas that is rising off the surface of the puddle that's actually burning. Carbs and fuel injectors only atomize fuel (turn it into itty-bitty droplets of liquid), they don't vaporize it (or at least very little is vaporized that far away from the heat of the combustion chamber). In our engines, most of the fuel doesn't actually vaporize until after entering the combustion chamber. Some of it may not vaporize until after combution has already begun.

Vaprorization characteristics are what make the distillation curves of various gasolines relevent to their ultimate performance. Dist curves show at what temperatures the various hydrocabons in a fuel will actually boil and become a vapor. Some of it must vaporize at lower temps to get the burn started. But in your typical pump fuel, it takes temps of perhaps 350-400 degrees F before 90% of the fuel vaporizes. And that's at ambeint air pressures. Step up to the incredible pressures that occur in a combustion chamber and some bits of the fuel may never reach vapor form. This means you need a little bit less air (richer compared to 14.7, or even my 14.0 guess from above) to make up for the fuel that may never reach burnable form.

Summery: 1) Stoich for the fuels we actually use isn't 14.7--it's some figure that is lower (richer). 14.7 is a figure that can only be realized on paper or in a lab. 2) Combine that with the fact that all the fuel never actually gets to vapor form and 3) the result is that peak power is generally seen at ~13 depending on the fuel, engine, and operating environment.

[wilkin250r]

^^^ Man, what a nerd... :blah:

[JOEX]

Now we need someone to explain suspension physics like GPracer and wilkin explain electrical and combustion topics:D

[Rancher2005]

Thanks GPracer. I can see now why a richer mixture is needed. I had hoped you would give us some insight. Thank you too jason. I know that a plug chop is necessary to do the best tuning but I also get my main set by going one size down from the jet size that would break up at WOT. Would you think that this is about 13:1 or still too rich yet (or perhaps it should be something in between those jet sizes)? From my testing it seems as though the engine pulls pretty hard this way.