Fuel Air Ratio

[R34 -_-]

lol oh great, now i know why iv got crap all over my back bumper.. I used to have a big bore but i had it changed to a twin tip that is angled up slightly so it just fires all the black stuff up into my bumper and due to drag etc small specs go all over the whole back bumper

[mad082]

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.

a lean tune with advanced timing will cause those issues as well, not just retarded timing. if you advance the timing to a point where it detonates, then EGT's will increase. 1 or 2 degrees advanced timing can have a worse effect than 1 or 2 degrees retarded timing

[zebra]

yeah thats true...basicly anything lean is bad

[SKiT_R31]

No its not, lean is good. Lean is better for your engine, better for your power and better for your economy/efficiency.

What is bad for your engine is excessive heat and/or detonation/too quick combustion.

I hate tuners that tune from A/F mixes without monitoring EGT's. Your running an N/A RB, your not going to melt valves or pistons running too lean, esp on a stock motor, all its going to do is cause flat spots in acceleration/lose power if too lean. My RB30DE used to run at flat 14.0:1 until 4500rpm where it richened up, I had amazing economy, no issues with detonation, and i had cool EGT's on NGK 5 range spark plugs.

On an N/A Engine you should definitely be able tune it no richer than 12.5:1 at WOT in peak rpm without any issues, with it a lot leaner at light throttle/lower rpms. Everyone has issues with economy on their N/A's, this should definitely help.

I would never tune a car richer than 12:1 (including my DE+T), if my EGT's are too high, over fuelling is just a bandaid solution - and a bad one at that (too rich can actually cause the same issues as too lean). The issue should be fixed elsewhere. (high octane fuel,water+alc/meth injection, colder spark plugs, less compression, less boost, change injector size, fix spray pattern, injector timing, ignition timing, cyl wall temp inc oil temp and water temp, under piston oil squirters, the list goes on)

In summary, if everything is functioning correctly and setup properly - i.e. you want clean flow tested (and matched) injectors, identical spark plugs in an appropriate heat range (7's are my preference for daily) preferably all indexed, equal cylinder pressures, you can lean your car out into the ideal range (12.5 - 13.2).

You need to monitor EGT's and AFR's, then just push it as close to the 13.2 as possible for the best power under acceleration, richening at higher rpm with full throttle as required (once again watching your EGT's) with some leeway of course, then tweak your ignition timing to suit. SAFC's are ideal for this reason, as you have part throttle and full throttle rev based mapping, you can squeeze a fair bit more efficiency out of your engine, just make sure its done with the O2 sensor unplugged and with your coolant temp sensor functioning correctly and your car at operating temp.

[zebra]

I should have clarified a little better

Im talking about long term longivety on an engine, Ie constant high RPM WOT use, Richer mixtures keep valves cool etc, which is why 99% of race cars dont use overrun fuel cut, as those few seconds of raw fuel cooling the exhaust valves can make the difference between finishing a race or burning chunks out of the exhaust valves.

I would also sacrifice 5hp and run a bit richer to help keep everything cool.

Street cars and such can get away with running lean as they dont sit on WOT for lap after lap.

E.g

On the highway I ran anything up to 17:1 in the skyline with 9:1 comp with out dramas,

and ran the car at 14:1 up to around 3-4psi

than I tried dropped it to 12.5:1 coming onto boost and the car accelerated much harder due to the extra timing i could run.

than was around 11.2:1 above 7psi.

And not all of us can afford a $2k EGT logging setup, as much as I would like one.

[SKiT_R31]

If your tuning on a dyno, you only need a $300-$500 gauge and sensor

[mad082]

actually you will find that lean mixtures actually do create higher EGTs than rich mixtures. when you have a rich mixture you will end up with a higher carbon monoxide levels, where as leaner will create more carbon dioxide, and creating carbon dioxode has a greater reaction than creating carbon monoxide, thus creating more heat.

also once you go leaner than 14.7:1 AFR you will start losing power and not gaining power.

also if AFRs were safe being lean, ALL of the car manufacturers wouldn't be tuning their ecu's to run stock levels around 11:1 at full throttle and high rpm.

sure you could run easily run a car at 14:1 afr and have low egt's but you may find that your timing is more retarded than a car running a richer AFR and your power may not be any higher.

and finally, a safc is a poor tuning tool compared to other units (such as a nistune which isn't much more money) because when you tune with the safc you are simply bending the afm signal. when you alter the afm signal, not only are you altering the AFR but you are also altering the timing (because you are simply altering the load mapping the ecu is seeing. you aren't actually altering the injector timing like on a stand alone ecu), so you may start getting detonation before you get to the ideal AFR. this will then cause your EGT's to skyrocket well before you reach the ideal AFR, and thus you won't make the same power you could with a comprehensive tune.

[zebra]

Dont forget that retarding the timing raises EGTs so running lean and having to pull timing will also see EGTs go through the roof.

[Galois]

actually you will find that lean mixtures actually do create higher EGTs than rich mixtures. when you have a rich mixture you will end up with a higher carbon monoxide levels, where as leaner will create more carbon dioxide, and creating carbon dioxode has a greater reaction than creating carbon monoxide, thus creating more heat.

also with more oxygen available the explosion is faster, allowing more expansive force when the cylinder is high in the compression chamber which generates more torque/power

also once you go leaner than 14.7:1 AFR you will start losing power and not gaining power.

why is this? is it to do with timing to reduce detonation or something to do with the chemistry?

[zebra]

A bit of both really, at 14.7:1 all the petrol is burnt using all the avaliable oxygen, any leaner and there is oxygen there that is doing nothing apart from wasting space.

NB - that is it in a nutshell

[SKiT_R31]

Of course full management is the best way to go, but SAFC's are quick and easy. Of course your not altering injector timing, ugh.

Also once you go leaner than 14.7:1 temps start cooling down again, they don’t just keep getting hotter and hotter. Heat is a good thing; you want it, more expansion, better burn, more power etc. Stock tunes are rich for emissions and reliability, so you can run hotter spark plugs for low load/rpm emissions testing (they then richen the mixture to stop plugs from melting etc) also as mentioned before, they don’t know how your going to drive the car, prolonged heavy load and high rpm will stress components, not only internally, it could damage your CAT and O2 sensors etc, these are the reasons manufacturers tune things so rich. Also, they don’t know what other variables could change. Older Japanese cars are tuned stupidly rich for these plus other reasons, different manufacturers/newer/older cars are not all like this.

My base timing was 20degree's on a factory turbo ecu (because they have more ignition timing up top + I ran turbo injectors) I also had an AFC neo with a Q45 Y piecing twin throttle body’s. I now run MAP through a Microtech LTX12s, altering fuel mixes/ignition timing via vacuum and revs with air temp adjustment (using 550cc injectors and enjoying sequential injection to alter the injector timing for more efficiency.

The idea of getting power out of an engine is to have the peak pressure of the explosion as close to TDC as possible, with the burn lasting the full stroke providing it is complete by the time the combustion stroke has finished and the exhaust stroke starts. Regardless of anything else that’s your goal, you adjust your ignition timing and fuel mixtures to make this the most efficient as possible, if your EGT's spike, you have either prolonged the burn too much, OR your combustion is too intense. Likewise with detonation, the engine is combusting too quickly, you need to slow it down, either by igniting further away from the peak pressure (when the piston is further down the combustion stroke), by dulling with too much fuel or some other method (less compression/boost).

Edited May 19, 2010 by SKiT_R31

[Galois]

but any heated gas is going to expand, so more gas means more expansion for the same amount of heat released? the increased oxygen conc would increase detonation tho.

[MBS206]

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.

I got to this post and stopped reading, so if someone said this below, good on em!

The leaner an air fuel mixture, the hotter it burns. The hotter it gets the more the gas expands, the more the gas expands the higher the pressure. Pressure is a force over a certain area, the area is staying constant, hence force increases. Torque on the crank is equal to the force on the piston (with some funky angular changes) multiplied by the distance from where the conrod joins the crank, and the middle of the crank.

There is a point though where there isn't enough fuel being put in to burn properly, or burn enough to make enough alternate gases.

Remember, Pressure, volume and temperature are all related.

Hence, leaning it out, more pressure, more force, more torque. Power = Torque * RPM... Hence, more power.

As mentioned though, leaning out increases temperatures to a point that certain vital components are not adequately cooled, and hence, melt.

[mad082]

Dont forget that retarding the timing raises EGTs so running lean and having to pull timing will also see EGTs go through the roof.

depends on how much you are retarding the timing by. only backing it off by 1 or 2 degrees won't alter the EGT's by much. if you take off 10 or 15 degrees than you will.

[Duchie]

Hmmm very intriguing but complicated for the untrained i.e. ME. So would it be a good idea to se the Apexi FC Neo to change the ratios so its leaner for lower rpms and richer for higher rpms? And can you also change your timing with the Apexi FC Neo? How do you match the timing to the mixtures so that your temperatures dont go / ?

[Super Drager]

Hmmm very intriguing but complicated for the untrained i.e. ME. So would it be a good idea to se the Apexi FC Neo to change the ratios so its leaner for lower rpms and richer for higher rpms? And can you also change your timing with the Apexi FC Neo? How do you match the timing to the mixtures so that your temperatures dont go / ?

SAFC is a fuel only computer, you can't adjust timing with it.

[zebra]

SAFC is a fuel only computer, you can't adjust timing with it.

This is true, but when you bend the AFM signal to lean it off, it often adds timing as the engine thinks its under less load.

Something to watch for with all signal benders, SAFC, Fuel Cut defenders etc.

[mad082]

yeah zebra is right. the timing gets altered as a result of trying to alter the AFR. this means that if you don't know what you are doing you could end up with a nice AFR but have the car pinging its arse off

[SoFreshSoClean]

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.

Sorry but that is just completely wrong.

A high flow cat is one that is physically larger to compensate for the lack in flow of a smaller unit, it will do the job just as effectively as a standard unit (assuming it is a good quality item).

The staining occurs from running a rich tune.

[Galois]

i haven't read any emissions reports comparing the two, but from a design perspective you either have thin piping and need less time in the cat or wide piping and more time in the pipe. the exhaust has to contact the piping for the catalyst to work, and such contact is promoted with smaller piping and high turbulence (by direction changes and heat differentials).

so if you're using the same catalyst, and same residence time, wider piping will lead further from complete combustion. the residence time will have to be much higher to compensate for a small change in pipe diameter, as is the issue with all surface area to volume based systems