Effects of running lean, and how do you fix it...

**Robert Viehweger** · 02-17-2012

Someone asked on the forums, Figured others might ask. If anyone wants to be a bit technical about it, please post!

**HolyCrapItsFast** · 02-20-2012

And massive knock as a result of a lean condition can destroy your rod bearings also. Ask me how I know this

We should really understand the relationship between fuel, timing and boost before we can make an assumption as to what constitutes lean. There are some ways of determining a lean condition and ways of fixing or compensating for them. Tuning methods include a combination of fuel, timing and boost adjustments and or changes to hardware such as injectors or fuel pump.

#1 Fuel
If your AFR's are not appropriately defined then increasing fuel is the most obvious thing to do to correct it. But there could be many reasons a lean condition exists and finding the reason why is important. The tune could be off or there could be a hardware issue some where. Your fuel pump may be inappropriate for the job or the injectors could be to small or your fuel pressure is not correct. You will usually see this by observing your IDC's in combination with your AFR's at WOT. In this case you will observe your IDC to approach or exceed 100% and your AFR's lean out in the same area. A log will reveal allot so everyone should be logging from time to time just to see if things are normal. Another point is to make sure you are using the appropriate fuel. You should be using the highest octane fuel available to you at all times.

#2 Timing
If your AFR's are defined properly in the tune and you are seeing corresponding AFR's on the wideband then fuel may not be the issue. For instance if you are at peak torque and your AFR's are say 12.2:1. This is appropriate fuel IMO. Let's say you are seeing knock as a result then you would need to reduce your timing in that area to compensate for the knock or you can increase fuel. However if you reduce timing to much you will exceed optimal exhaust temperatures and you could be doing more harm than good as Rek has pointed out. At that point you should be monitoring your exhaust gas temps and you may need to increase fuel to control exhaust temps. This strategy works well when you choose to make power with boost, which brings me to the next parameter...

#3 Boost
Again as listed above, if your AFR's are appropriate and you are seeing knock you can compensate for the conditions by reducing boost. This will allow you to increase timing and run slightly leaner fuel if you plan to make much of your power with timing.

So to sum it up, an optimal AFR for making power will put you right around the low 12's air to fuel ratio. Anything richer than that will not net you power but is meant to compensate for knock or temperature and any power you gain from that point is coming from timing and/or boost. For E85 users some of the strategies discussed here change somewhat because of the fuels resistance to knock, but that is a subject for another thread

If you have RomRaider or AccessTuner you can have a look at some fuel tables to get an idea. If you get anything from this, don't just set your AFR's to 12:1 and leave it. Again focus on the concept above and understand why we richen the fuel to 11.3:1 or less even.

**HolyCrapItsFast** · 02-20-2012

Thank you Rek. Coming from you it means allot.

**Zach@TTLM** · 02-20-2012

great job holy...this is a big problem with more than just the subie world...This should be a sticky I agree

**JAC Wagon** · 02-20-2012

Id make it a stickey, but Im not a global mod or the mod for this section. I agree, this is a great thread. Good info!

**HolyCrapItsFast** · 02-21-2012

I will leave it up to John... He runs the show over here and I wouldn't want to upset the balance. I believe he has a table of contents thread that we can link it to.