Stoichiometric AFR for N55 with COBB Stage 1

iuhutch · 07-13-2013, 09:25 PM

I believe the air/fuel ratio for the n55 in general should be the same regardless of tune/AM parts but curious as to what this ratio should be in general?

Garrett states that 14.7:1 is an ideal AFR for compressor engines.

For Example:
15.0:1 = Lean
14.7:1 = Stoichiometric
13.0:1 = Rich

However, when looking at my COBB datasets, I am seeing an average idle AFR at around 13.82 and on a 20 minute drive I am seeing an average of 25.36 with a few high values in the 100's? Do I have a leak somewhere? What causes the intermediate spikes in the AFR table?

Any thoughts, suggestions or research on this topic would be great. I have a 2011 335xi with BMS intake and COBB Stage 1 Aggressive map. Charpepipe is on its way but not yet installed...

Thanks guys!

iuhutch · 07-14-2013, 12:10 PM

Any knowledge on this?

boostd92 · 07-14-2013, 12:20 PM

At idle, your AFR's should be above 13, but it depends on the design of the engine, there is no strict rule. When you're cruising, it's normal for them to be way lean, that's how manufacturers get good gas mileage.

AFR's at WOT is the only thing you need to be concerned with... on port fuel injection cars, safe AFR's on pump gas would be in the 11.5 range at WOT, with our direct injection cars, they can get quite a bit leaner.

iuhutch · 07-14-2013, 12:41 PM

Quote:

Originally Posted by boostd92

At idle, your AFR's should be above 13, but it depends on the design of the engine, there is no strict rule. When you're cruising, it's normal for them to be way lean, that's how manufacturers get good gas mileage.

AFR's at WOT is the only thing you need to be concerned with... on port fuel injection cars, safe AFR's on pump gas would be in the 11.5 range at WOT, with our direct injection cars, they can get quite a bit leaner.

Thanks for the response. These numbers are what I am seeing at both idle and WOT. Makes me feel better that these engines run leaner on average, makes sense with the fuel conservation but I always thought running lean is bad on the engine in the long term...

N8N · 07-14-2013, 12:44 PM

Running lean is bad because it can cause detonation. If you aren't experiencing detonation, there isn't really a problem.

iuhutch · 07-14-2013, 12:55 PM

Car is running smooth as silk. Just saw my average AFR's at different times during a drive and was surprised by the anomalies... 13.8 at idle and right around 11.2 at WOT matches up to what boostd mentioned. Still open to more analysis on this and I can copy my data if needed as well

nkkenme · 07-14-2013, 08:53 PM

On average from the mazdaspeed platform that i came from which is also turbocharged and direct injected and tuned the hell out of with cobbs, at idle youll be anywhere from 14.7-15.1ish WOT will be what its set to but should be around 11.4-12.5 and under cruise youll see anywhere out to 25. Hope it helps and if you have any other questions about the ap let me know.

JETmn · 07-16-2013, 02:25 PM

14.7 is the perfect mixture for a complete burn in perfect conditions. The above is correct and you need to be much richer under boost. Idle can flirt with 14.7 and cruise can be in 13's to low 14 on most platforms.

iuhutch · 07-16-2013, 06:42 PM

Quote:

Originally Posted by JETmn

14.7 is the perfect mixture for a complete burn in perfect conditions. The above is correct and you need to be much richer under boost. Idle can flirt with 14.7 and cruise can be in 13's to low 14 on most platforms.

Got it, what about average runs in the 25 range with a few data spikes in the 100's!?

I see you are from Minneapolis, what part? know of any local events/forums/people?

iuhutch · 07-16-2013, 08:35 PM

Ok, so all the data in the org post was from excel where I was logging the data and not paying attention to the live sets. I just started the car up and watched the AFR. Dead on 14.7 at cold start up idle; also 14.7 at standard 2rpm's cruising. Getting on it I saw 13s:1 and and when I lifted the throttle is when I saw it spike to 25. My tune is dead on (who would argue with COBB?

).

Thanks for the comments, I hope this helps others.

JETmn · 07-17-2013, 03:56 PM

I am not familiar with the specifics of the N55 powerplant, but it most turbo cars 13's under full boost is dangerously lean. Most platforms are looking for 12-12.5:1. I haven't hit up any local gatherings, but a few buddies from the Mitsu crowd have also migrated to the 3 series. I work on the west side and live on the north side. What color is your car?

N8N · 07-17-2013, 05:14 PM

I'm assuming the spikes to 100+ are at 0% throttle. Most modern FI systems will cut off all fuel at 0% throttle and some RPM above idle speed.

iuhutch · 07-18-2013, 01:53 PM

Quote:

Originally Posted by JETmn

I am not familiar with the specifics of the N55 powerplant, but it most turbo cars 13's under full boost is dangerously lean. Most platforms are looking for 12-12.5:1. I haven't hit up any local gatherings, but a few buddies from the Mitsu crowd have also migrated to the 3 series. I work on the west side and live on the north side. What color is your car?

White/Red. I found the forum and think I bought the car you were looking at! LOL, did you go in one day a few minutes late and someone was signing the paper work?...that was me

JETmn · 07-18-2013, 05:18 PM

Quote:

Originally Posted by iuhutch

White/Red. I found the forum and think I bought the car you were looking at! LOL, did you go in one day a few minutes late and someone was signing the paper work?...that was me

Nope, I bought the one I wanted. Wasn't even really looking to buy, the wife just wanted a ride in a 335xi to see if she approved. I ended up buying in the next day. Space grey M sport sedan from Minnetonka BMW. Are you in the Plymouth area much? I see a red one with a white stripe, thought it was a 135i, but not sure as it is always going the other way.

iuhutch · 07-18-2013, 06:47 PM

Quote:

Originally Posted by JETmn

Nope, I bought the one I wanted. Wasn't even really looking to buy, the wife just wanted a ride in a 335xi to see if she approved. I ended up buying in the next day. Space grey M sport sedan from Minnetonka BMW. Are you in the Plymouth area much? I see a red one with a white stripe, thought it was a 135i, but not sure as it is always going the other way.

I am; however it is white with a red interior, sorry I didnt specify. I agree on the test riding to be sold on the car. I was orginally wanting an m3 but was hesitant due to the awd needed here in MN....my buddy said I should test drive a 335 and I instantly fell in love!!!

07-13-2013, 09:25 PM	#1
iuhutch Captain 118 Rep 656 Posts Drives: 18%E2%80%99 F80 ZCP Join Date: May 2013 Location: local iTrader: (0)	Stoichiometric AFR for N55 with COBB Stage 1 I believe the air/fuel ratio for the n55 in general should be the same regardless of tune/AM parts but curious as to what this ratio should be in general? Garrett states that 14.7:1 is an ideal AFR for compressor engines. For Example: 15.0:1 = Lean 14.7:1 = Stoichiometric 13.0:1 = Rich However, when looking at my COBB datasets, I am seeing an average idle AFR at around 13.82 and on a 20 minute drive I am seeing an average of 25.36 with a few high values in the 100's? Do I have a leak somewhere? What causes the intermediate spikes in the AFR table? Any thoughts, suggestions or research on this topic would be great. I have a 2011 335xi with BMS intake and COBB Stage 1 Aggressive map. Charpepipe is on its way but not yet installed... Thanks guys!
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07-14-2013, 12:10 PM	#2
iuhutch Captain 118 Rep 656 Posts Drives: 18%E2%80%99 F80 ZCP Join Date: May 2013 Location: local iTrader: (0)	Any knowledge on this?
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07-14-2013, 12:20 PM	#3
boostd92 Captain 63 Rep 762 Posts Drives: '11 E92 M-Sport Join Date: Jun 2009 Location: Chicago, IL iTrader: (1) Garage List 2011 BMW 335i M-Spo ... [7.26]	At idle, your AFR's should be above 13, but it depends on the design of the engine, there is no strict rule. When you're cruising, it's normal for them to be way lean, that's how manufacturers get good gas mileage. AFR's at WOT is the only thing you need to be concerned with... on port fuel injection cars, safe AFR's on pump gas would be in the 11.5 range at WOT, with our direct injection cars, they can get quite a bit leaner.
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07-14-2013, 12:44 PM	#5
N8N Lieutenant Colonel 179 Rep 1,843 Posts Drives: 2009 E92 335i 6MT Join Date: Sep 2012 Location: Berwyn Heights, MD iTrader: (2)	Running lean is bad because it can cause detonation. If you aren't experiencing detonation, there isn't really a problem.
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07-14-2013, 12:55 PM	#6
iuhutch Captain 118 Rep 656 Posts Drives: 18%E2%80%99 F80 ZCP Join Date: May 2013 Location: local iTrader: (0)	Car is running smooth as silk. Just saw my average AFR's at different times during a drive and was surprised by the anomalies... 13.8 at idle and right around 11.2 at WOT matches up to what boostd mentioned. Still open to more analysis on this and I can copy my data if needed as well
	Appreciate 0 Quote

07-14-2013, 08:53 PM	#7
nkkenme Private 1 Rep 56 Posts Drives: Alpine White 2011 335i Sedan Join Date: Jul 2013 Location: Va Beach, VA iTrader: (1)	On average from the mazdaspeed platform that i came from which is also turbocharged and direct injected and tuned the hell out of with cobbs, at idle youll be anywhere from 14.7-15.1ish WOT will be what its set to but should be around 11.4-12.5 and under cruise youll see anywhere out to 25. Hope it helps and if you have any other questions about the ap let me know.
	Appreciate 0 Quote

07-16-2013, 02:25 PM	#8
JETmn Lieutenant Colonel 247 Rep 1,595 Posts Drives: 2011 335i xDrive Msport Join Date: Apr 2013 Location: Orlando iTrader: (0) Garage List 2011 335Xi [0.00]	14.7 is the perfect mixture for a complete burn in perfect conditions. The above is correct and you need to be much richer under boost. Idle can flirt with 14.7 and cruise can be in 13's to low 14 on most platforms.
	Appreciate 0 Quote

07-16-2013, 08:35 PM	#10
iuhutch Captain 118 Rep 656 Posts Drives: 18%E2%80%99 F80 ZCP Join Date: May 2013 Location: local iTrader: (0)	Ok, so all the data in the org post was from excel where I was logging the data and not paying attention to the live sets. I just started the car up and watched the AFR. Dead on 14.7 at cold start up idle; also 14.7 at standard 2rpm's cruising. Getting on it I saw 13s:1 and and when I lifted the throttle is when I saw it spike to 25. My tune is dead on (who would argue with COBB? ). Thanks for the comments, I hope this helps others.
	Appreciate 0 Quote

07-17-2013, 03:56 PM	#11
JETmn Lieutenant Colonel 247 Rep 1,595 Posts Drives: 2011 335i xDrive Msport Join Date: Apr 2013 Location: Orlando iTrader: (0) Garage List 2011 335Xi [0.00]	I am not familiar with the specifics of the N55 powerplant, but it most turbo cars 13's under full boost is dangerously lean. Most platforms are looking for 12-12.5:1. I haven't hit up any local gatherings, but a few buddies from the Mitsu crowd have also migrated to the 3 series. I work on the west side and live on the north side. What color is your car?
	Appreciate 0 Quote

07-17-2013, 05:14 PM	#12
N8N Lieutenant Colonel 179 Rep 1,843 Posts Drives: 2009 E92 335i 6MT Join Date: Sep 2012 Location: Berwyn Heights, MD iTrader: (2)	I'm assuming the spikes to 100+ are at 0% throttle. Most modern FI systems will cut off all fuel at 0% throttle and some RPM above idle speed.
	Appreciate 0 Quote