The Magic 12:1 A/f Ratio

[R31Heaven]

I have read and there seems to be a belief that 12:1 is the A/F ratio that you should use when tunning an RB engine, My problem is My car was tuned using 12.9 and it runs as rich as and is fowling plugs I had it tuned again with 13:1 mixtures it is slightly better but my plugs are still black and sooty. I must admit I dont drive the car on long trips or hard, only short ones which may be causing the problem. Would I be better to change plugs to hotter plugs because of my driving conditions/style or just have it tuned close to stoich 14.7? I am runnig an SAFC. Or can it be somthing else causing the plug fowling?

Edited November 8, 2005 by R31Heaven

[Warpspeed]

I have read and there seems to be a belief that 12:1 is the A/F ratio that you should use when tunning an RB engine, My problem is My car was tuned using 12.9 and it runs as rich as and is fowling plugs I had it tuned again with 13:1 mixtures it is slightly better but my plugs are still black and sooty. I must admit I dont drive the car on long trips or hard, only short ones which may be causing the problem. Would I be better to change plugs to hotter plugs because of my driving conditions/style or just have it tuned close to stoich 14.7? I am runnig an SAFC. Or can it be somthing else causing the plug fowling?

<{POST_SNAPBACK}>

Unleaded fuel always burns black and sooty, that is the way it is. You can only get a nice light grey exhaust pipe colour with leaded fuel or avgas. Unleaded cars always have dark black exhaust pipes, even when running lean.

Spark plug heat range has a lot to do with it too. Too cold a plug may foul, even in a well tuned engine that does not burn any oil.

12:1 air fuel range adds just enough fuel to burn all the available oxygen and will give maximum power. Any more fuel than that cannot burn because there is not enough air in there to do the job, so power will not increase below 12:1

14:1 air fuel ratio ensures that all the fuel is burnt completely, no unburned fuel will be present in the exhaust. Leaning it off beyond 14:1 will not improve economy.

[Sydneykid]

I have read and there seems to be a belief that 12:1 is the A/F ratio that you should use when tunning an RB engine, My problem is My car was tuned using 12.9 and it runs as rich as and is fowling plugs I had it tuned again with 13:1 mixtures it is slightly better but my plugs are still black and sooty. I must admit I dont drive the car on long trips or hard, only short ones which may be causing the problem. Would I be better to change plugs to hotter plugs because of my driving conditions/style or just have it tuned close to stoich 14.7? I am runnig an SAFC. Or can it be somthing else causing the plug fowling?

<{POST_SNAPBACK}>

Few suggestions;

1. If it isn't warming up then it is always on cold run enrichment. So either you don't drive far enough for it to warm up or the water temp sensor is not working. I would check that.

2. Low speed, constant throttle and idle is at stoick because it runs in closed loop.

Have you ever replaced the lambda sensor? Or checked that is is in fact running closed loop A/F ratios when it should be?

3. 12.9 to 1 is not a very safe A/F ratio, neither of the above 2 problems are fixed by further leaning out. But you are already in dangerous territory.

4. Are you sure about the accuracy of the A/F ratio meter you are using? Standard is 10.5 to one and I have never heard of a standard Skyline fouling plugs.

5. Since you have not reported any missing under load, I don't believe coil packs are an issue.

6. What heat range plugs are you running? The NGK "7" heat range should be fine.

Hope that was of some help

cheers

[R31Heaven]

So War speed what you are saying is 12:1 is for max power or performance and 14:1 is for optimal economy?

Sydneykid, I have a new O2 sensor and temp sensor so have ruled that out I am runinig NGK BCP 6ES-11 plugs. so may go to a 5 I have a SAFC and get no knocking readings on 13:1 even under load. I am running a RB25 Turbo on an RB20 NICS motor with only 5 psi boost (stock) and a 600 x 300 x 80 FMIC.

With the acuracy the setting were with a probe in the exhaust while on a dyno? Even the Dyno guy wanted to set my tunnig to 14:1, so this has realy got me thinking.

[Beer Baron]

14.7:1 is only for light load cruise conditions, under load and full throttle you shold be MUCH richer than that. under 12:1 even.

[BHDave]

What sort of ecu are you running? Does it still have closed loop mixture control?

I suspect your tuner isn't correctly setting up your low load mixtures and that is why you are fouling plugs. You should be aiming for 14.7:1 for cruise and up to about 11.5-12 (depending on boost and timing run) under load.

If he is seriously talking about running 14:1 under load then cut and run as fast as you can and get yourself another tuner before you pop an engine.

[ebola]

Seconds on the finding another tuner.

My understanding is that leaner mixtures burn hotter - as in hot enough to burn a piston, or through the head.

Under cruise load, the extra heat is not an issue, but when you start making (using?) a decent amount of power, bad things happen. Which is why :

You should be aiming for 14.7:1 for cruise and up to about 11.5-12 (depending on boost and timing run) under load.

there needs to a be a little room for error

Driving style notwithstanding, lean mixtures at full throttle.... bad.

hope it helps (and hope its right)

[gurglr]

From what I've read, 12.6 is max power and 14.7 is stoich for unleaded.

Running at 12:1 whould be using fuel to cool the intake charge wouldn't it?

[xRHETTx]

Air fuel ratio meter (even expensive wide band ones) are not very accurate, can even be out as much as a whole ratio.

So tuners usually stay on the safe side of 12.5:1

[BHDave]

From what I've read, 12.6 is max power and 14.7 is stoich for unleaded.

Running at 12:1 whould be using fuel to cool the intake charge wouldn't it?

<{POST_SNAPBACK}>

I lost 3rwkw between 6500 and 7000 by dropping my af ratio from 12.5 to 11.9.

I'm happy with that little bit of extra buffer.

[Angry]

mine now sits on a 12:1 ratio ...pickd up nearly 100km's to a tank at the same time........ my car was stupidly rich before.... AFR's would bottom out on the graph under 10:1 under WOT. Previously i had issues with fouling plugs......must be a redtop ecu thing.... coz they are just too rich.

12:1 is were u should be at John.. leaning it out is probably going to ask for pinging. had mine "quick tuned" at ICE on the dyno day within the space of 3 runs it pickd up 30rwkw mid range... .. was basically just to bring the AFRs up till i take it back with this bigger turbo now on it.

[rob82]

14:1 air fuel ratio ensures that all the fuel is burnt completely, no unburned fuel will be present in the exhaust. Leaning it off beyond 14:1 will not improve economy.

<{POST_SNAPBACK}>

[/quote

Leaning it off will improve economy - this is why older cars as in pre 2002 run lean cruise mixtures. The problem with a leaning AFR is that you introduce more NOx which mixes with photochemical smog to produce O3 which depleates the ozone. Damn hippies f*#%ing our fuel economy.

AFR up to about 17:1 can be seen under lean cruise. However nissan ECU's do not use the lean cruise function probably because of the reason above.

[Cubes]

Which makes me wonder....

Solder a resistor inline with the o2 sensor to achieve a leaner cruise and idle... providing it likes to idle at those leaner mixtures.

meh ozone smozone... We are heading in to an ice age anyway.

Edited November 12, 2005 by Cubes