Fuel Air Ratio

[Mooney]

what should be a FAR for an rb25de from r32?

[mad082]

the air/fuel ratio for a non turbo running stock ecu can be anywhere from about 12:1 all the way down to 10:1. generally though it will be around 11.5:1 in stock form at high rpm, but does vary from car to car. when tuned though you generally try to get them up around 13:1 for a NA. bith richer for a turbo.

[SKiT_R31]

As above for the stock air ratio.

The optimum A/F ratio for N/A applications is 12.5:1 for best power when rich, and 13.2:1 for best power lean, so somewhere around there (If your intention is to tune it with an SAFC or something for better power or economy). WOT enrichment is generally to protect the engine/engine components like spark plugs. If you were to run a cooler plug, good fuel with appropriate ignition timing, you really shouldn’t need as much enrichment, will get more power and better economy. Sometimes when maintaining WOT at high RPM on leaner fuel mixtures, components such as pistons can continually absorb extra heat (while the head and block are cooled sufficiently), that’s when you use fuel to cool them. Generally you'd use somewhere at the lean end of the best power for part throttle, and somewhere around the rich end for at WOT, it really depends how high strung your engine is (compression/ignition timing/revs) and how hard it’s driven.

[Mooney]

thanks, i was wondering because i got the car tuned and the mechanic said it was 12:1 and that was way too lean and said it should be 14:1, but i think hes full of shit because he reakoned r32's didnt come standard with 25's

[mad082]

14:1 is leaner than 12:1. it is measure in parts of air to parts of fuel. so 14:1 is 14 parts air to 1 part fuel. 12:1 has 2 parts less air for the same amount of fuel.

between 14 and 15:1 is the sort of range it should be when on cruise when the ecu is in closed loop mode and using the stock narrow band o2 sensor. once you give it the beans it will go to open loop and should drop down to around 11 or 12:1 on a stock ecu. if you are at 12:1 then you aren't doing too bad and the power gain by going leaner wouldn't be that much.

[Galois]

can someone explain why there's a power gain from leaning out?

is it just because the combustion reaction is more complete (for the same amount of fuel pumped in) when you force more air into the engine, ie simple equilibrium and kinetics?

cheers

[mad082]

i will have a bit of a go at explaining it, but this is from any knowledgable background, just how i understand it.

you are pretty much right about the more complete combustion. too much fuel will take longer to burn, so you don't get as much fuel burnt when the compression is the highest (this is similar to how advancing the timing can improve power). obviously you will get to a point where you will stop making power by leaning it out even more, but in the case of cars, they generally run richer than they could because other factors come into play, such as keeping cylinder temps down to prolong engine life (hence when people will say that the tune is "safe"). a certain amount of fuel requires a certain amount of oxygen to combust fully. if you don't have enough oxygen then you will end up with some unburnt fuel, and you will also end up with the last bit of fuel not combusting as quickly as it could if there was a little bit more oxygen. there is also the fact that if there is too much fuel it will almost put out the flame. if you tip a bucket of fuel over a match you will get a small flame but then the flame will go out.

as i said, that is only my understanding of it. someone who has a better understanding of the physics/chemistry behind it could probably give a better description.

[R34 -_-]

I'm guessing you need a piggy back to tune a/f ratio? my car smells really rich and would like to have it tuned so its running mint, but justifying the expense of a aftermarket ecu etc on an NA car is hard to do lol

[SKiT_R31]

A piggy back like an SAFC will give you some power and economy improvements and fix up all your fuel mixes to the right amount, but full aftermarket management will net you more power due to being able to alter the ignition timing maps.

But with an SAFc you can somtimes get some extra benefits from the ecu seeing less airflow, therefore pushing you onto slightly more aggressive maps (especially if you have a couple modifications, so your car is breathing mroe than standard).

[R34 -_-]

How much would one of those cost? If its not cheap I'll probably forget about it and put the money towards a much needed turbo conversion

[Galois]

thanks mad082

ok, so leaning out the afr increases the rate of reaction, so bigger bang over shorter time, or more importantly, bigger bang at top dead center, which means there is more gas volume produced in the small tdc space which produces more power.

does that affect the engine temperature that much? the vast majority of the fuel is going to combust anyway right? which means all that heat is released during the combustion anyways, so why is it hotter if its leaner? cos if there is a significant difference between the amount of complete combustion between lean and a normal run that's an efficiency loss...

[jrm]

From what I understand, the extra fuel ends up 'cooling' the combustion (like smothering the combustion). The reduction in heat in the combustion then results in less likely to cause detonation, etc. That's how I've always understood it anyway - feel free to correct

[Galois]

hmm, interesting, so that fuel is effectively wasted, or only reacted to produce carbon monoxide instead of dioxide, until it reaches the cat? and fuel that isn't reacted combusts in the exhaust? or leaves the exhaust?

sorry for all the questions, just interested in how the combustion works exactly.

also, i read somewhere else that adding a bit of water to the airflow keeps the temps down, so would it be more efficient to do that and lean the engine out instead of wasting petrol energy with incomplete combustion?

thanks for the info

[zebra]

A lean tune with retarded ign timing can in severe cases melt pistons and exhaust valves,

As said above factory cars run rich too a) keep them cool and B) it helps keep the cat converter hot so it lowers tail pipe emmisions, Its kinda funny to think that to keep emmisions down cars need to use more fuel....bit of a catch 20 there.

Water injection works to keep temps down rather well, as it absorbs the heat and turns to a gas.

As a side note a lot of overheating problems can be cured by adding ign timing and more fuel...even a .5:1 afr increase can stop exhaust manifolds glowing.

[R34 -_-]

If i installed a high flow cat, would that stop the carbon staining my bumper?

[zebra]

will more than likley make it worse

[R34 -_-]

All i have is a straight through exhaust and a resonator, i would have thought a cat converter would reduce carbon not make it worse?

[zebra]

Well yeah will be better than no cat

[Galois]

A lean tune with retarded ign timing can in severe cases melt pistons and exhaust valves,

As said above factory cars run rich too a) keep them cool and B) it helps keep the cat converter hot so it lowers tail pipe emmisions, Its kinda funny to think that to keep emmisions down cars need to use more fuel....bit of a catch 20 there.

Water injection works to keep temps down rather well, as it absorbs the heat and turns to a gas.

As a side note a lot of overheating problems can be cured by adding ign timing and more fuel...even a .5:1 afr increase can stop exhaust manifolds glowing.

whoa, i had no idea it worked like that.

makes sense about the cat tho, using the extra fuel to lower emmissions. fascinating, what like of thought did the designers go down to come up with that design conclusion, clearly in a time of cheap fuel lol.

thanks for the info, i get it now. i'll have to look up some numbers but im guessing the engine never actually reaches stoichiometric combustion ratios (sorry to go all chemical lol)

ok, some explanation for the above questions:

no cat means soot and crap that would have been combusted in the cat instead is fired out of your tail pipe.

a high flow cat is effectively a shitty cat, not doing its job properly, because to do it properly the cat needs a large surface area of exposure for the exhaust air, but large surface area means tiny thin piping, which impedes exhaust flow. hence us performance minded people go for larger arrangements for higher/free-er exhaust flow, which means more sooty crap coming out the back, ie: more staining.

with no cat at all, none of the carbon left over from the engine is combusted in the exhaust system and it goes straight out of the exhaust pipe.

[bloodsword27]

afc neo's are going for round $250