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2005-2010 Mustang GT Tech Forum

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Unread 05-13-2005   #1 (permalink)
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Question running rich? running lean?

Can someone explain to me the benefits and drawbacks of running a lean fuel mixture vs. a rich fuel mixture. thanks :shiny:
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Unread 05-13-2005   #2 (permalink)
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Lean vs rich? The V-8 runs on 87 octance(regular). You shouldn't run it on anytihng else cuz that is what it is made for. On occasion you can run some high octane, but I wouldn't do that on a weekly basis. Not good for the engine.
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Unread 05-13-2005   #3 (permalink)
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Quote:
Originally Posted by nonsensez9
Can someone explain to me the benefits and drawbacks of running a lean fuel mixture vs. a rich fuel mixture. thanks :shiny:
Well, you should try to have it PERFECT, not rich nor lean. Hence a professional tune (where they analyse your exhaust etc.)

Higher octane gives some more forgivness regarding the mixture and add some throttle reponse if you tune for it.

Lean might (will?) result in engine damage in the long run as the fuel helps to lube and cool your engine (cylinders and valves). You do have a alu block and the one (lonely) exhaust valve gets it pretty hot as it is!

Rich will waste fuel, make your engine sluggish and in the long run leave residue on valves, sparkplugs etc. reducing effectivness and might lead to repairs (not as expensive ones as running it lean though)...

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Unread 05-15-2005   #4 (permalink)
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Ok, I'm not a gasoline expert, but I gotta ask: What does octane have to do with a/f ratio on an EFI motor? It's my understanding that higher octanes only provided a higher burn temperature on a fuel injected motor.

Running too rich a mixture will mean that all the fuel won't burn during detonation leaving power in the cylinder. Too lean and your engine runs hot, and can cause damage to the internals (pistons) and also leaves power in the cylinder.

To completely burn gas, you need a mixture of 14.7:1. Search for Stoichiometry on Google.

Again, not an expert here, but I think the only time you would want to increase octane is if you had a higher compression ratio, somewhere near 10.5:1, or if you start fiddling with timing. The ECU modifies your a/f ratio.

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Unread 05-15-2005   #5 (permalink)
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actually, running a little on the lean side is more correct for a gas motor. (and anyways, perfect stoichiometric is very hard to accomplish) but when it runs a little lean it will run hot which will reduce NOX gases (oxides of nitrogen)(only a very lean condition will cause detonation and cause internal engine failure) which reduce emissions. and while running a little lean helps make sure most of the hydocarbons get burned in the combustion, reducing emissions. and yes XXP is correct about the octane rating, it has nothing to do with running rich or lean.
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Unread 05-15-2005   #6 (permalink)
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I know the ECU is constantly modifying a/f mixture around stoichiometric, and wouldn't argue that running a bit lean is better for emissions....but....and I'm just taking a poke at this, I bet it gets pretty rich at WOT. You gain power by richening the mixture to a point...13.5, 12.7, somewhere in there. My only point about a stoichiometric mixture is that's where you completely burn a gasoline/air mixture.
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Unread 05-15-2005   #7 (permalink)
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so the ECU is monitoring the a/f mix... why is a tune necessary for a CAI? Someone had mentioned that the ECU has top and bottom limits on how much air flow it will recognize, and thus the amount of fuel it will deliver. sounds kinda odd as Ford must know that one of the first products of aftermarket is CAI. Hard to imagine them not taking that into consideration.

But I digress.... I'd really like to know exactly what the tune is doing. Is it something as simple as removing the upper/lower limits? is it altering the a/f mixture that Ford supplied? altering the ignition timing? so many things that it could be doing to gain the extra hp I wonder if the CAI is actually doing anything.

there's additional parameters I'd like to mess with, too. namely the slow throttle response.
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Unread 05-15-2005   #8 (permalink)
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The more air you flow, the more fuel you can add which means the more power you can make. And, the cooler the air the better. I have some ideas on what the tune does, but nothing firm, so I'd prefer not to speculate, but it's more than just mixture. It handles fuel and timing curves.
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Unread 05-24-2010   #9 (permalink)
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Default Rich/lean

Well both of you are bouncing around the fundimentals of combustion. Stoichiomety will only be preserved at closed throttle. The factory MAF sensor that identifies the amount of air interring the engine only operates in a limited perameter. A true AFR will of course only be acheived with a Wide band sensor. Factory sensors on all Ford oem parts are narrow band and limited. On the factory tune the ECM will make adjustments to the fuel mixture based on senory data recieved from the MAF sensor and part/closed/WOT. The up/down stream o2 sensors identify the out-come of the combustion process. If the exaust does not fall under operating guide lines it will signal back to the ECM which turns on that nasty Check engine light. Through numerous tunes and research I have found that max power is leaned around 12.4 and should lean out after 4000 revs. Most likely you should see 14.7(8,9 (15)) lean out to 12.2 to 12.8 at Red line. Anything beyond that will be dangerous. You most likey not acheive this unless you operated on a custom tune and have a after market CAI. Preditor is a great tuner for the slighty Moded mustang. Gives the ability to adjust the WOT fuel trim and lean/rich level. Hope this helps
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Unread 05-24-2010   #10 (permalink)
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The reason you need to "tune" when you change components is to compensate for the changes. For a CAI, the only time you need to change anything is if the MAF sensor is moved or the MAF housing is increased or decreased in size. The MAF sensor measures the amount of air passing by, but it only samples the air, and the PCM uses that reading to determine the amount of air going into the engine based on the area of the MAF housing. If you increase the MAF housing size, more air flows, but the computer would "see" less air, so it would "lean" the fuel mixture. The tune needs to be modified to compensate for the different sized housing so that you can get back to the proper AF ratio. If the MAF sensor is even moved, and sits in a different angle than before, it will be off a little, and the tune needs to compensate for it.

If you tune for performance, you are adjusting the two parameters that affect power: AF ratio or spark timing. If you go a little leaner, you make more power (within reason). However, as you go lean, you make more heat. Heat can cause pre-detonation of the AF mixture which can damage the engine. Also, the leaner the mixture, the easier it burns, so both factors tend to lead to pre-detonation. Advancing your spark timing leads to more horsepower at higher RPMs (typically less torque at lower RPMs too), but just like leaning out your mixture, if you advance the timing too far, you run the risk of detonating your AF mixture too early.

The reason you would run higher octane fuel is because the higher the octane rating, the harder the fuel is to burn. This allows for leaner fuel mixtures or more advanced spark timing. You have to adjust the tune for more spark or leaner AF ratio if you want to take advantage of higher octane fuel, and once you do that, you can't start running lower octane fuel again unless you want to risk causing damage to your engine. Newer cars (like the 2011 mustang) can detect when you have higher octane fuel and automatically give you more timing, giving you more power. However, even if you don't have a tune for higher octane, there is nothing wrong with running higher octane fuel with a lower octane tune. It's the other way around that can be bad for your motor because of the risk of pre-detonation.

When running boost, the AF ratio is very important. At WOT, if you are running 14:1 Air to fuel, with even a small amount of boost (or high compression in a naturally aspirated engine), you can start to melt aluminum in a very short amount of time. The higher compression combined with lean fuel creates some extreme heat, and not only is detonation pretty much a guarantee, the pistons, block, and heads can be damaged just from the heat alone. Boosted engines run in the 10.5-11.5 range at WOT, depending on fuel, spark timing, and intake air temperature.

It all comes down to safety of the engine, with intake air temp, AF ratio, fuel octane rating, and spark advance being the deciding factors on which parameters you can push and which ones you can't.

The engines are fairly safe from pre-detonation with the knock sensors constantly "listening" for a ping. If pre-detonation is detected, up to 25 degrees of timing can be pulled from the tune automatically. This will drop power drastically, but allow the engine to run under abnormal conditions without any threat of damage. However, a major change to the engine without compensating for it in the tune will lead to all sorts of problems.
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Unread 05-24-2010   #11 (permalink)
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Wow all that to answer a question that was asked 5 years ago...
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Unread 05-24-2010   #12 (permalink)
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Quote:
Originally Posted by TheKurgan View Post
Wow all that to answer a question that was asked 5 years ago...
LOL I didn't see that.. the thread was at the top of the list.. haha
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