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AFR Strategy change and results (Subaru)

Understanding AFR

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Hey boys.

I have an 09 STi with stock heads, a built bottom end with a slight turbo upgrade 20G TD05 and all the supporting mod's. I had been trying for quite a while to get a decent tune which mirrored the strategy that other have had with the setup, but had been having very limited success. The engine was very knock limited, even for an EJ. The Cobb 20G was providing some decent airflow but I just couldn't get the engine to take any timing. My area of Canada has awful fuel with no E85 options, so I though I was being conservative with targeting 245Kpa (Absolute) and tapering past 6000 RPM, with a target Lambda of about .77 tapering to .76. However, the tune did not want any timing. I was making decent torque as it came on to boost but my timing curve was almost flat, peaking at around 11 degrees. Power was OK, but it peaked before 6000 and fell off hard, clearly.

I was getting a bit frustrated so I decided to switch strategies completely. I looked at a few Off The Shelf calibrations that Cobb had created for areas with bad fuel and compared them to what I was trying to do. It was clear they were sacrificing AFR for timing. Some areas were even in the high .60's. Now clearly Off The Shelf calibrations have a high degree of safety built in, but it proved to be a good comparison of strategies when dealing with good fuel vs bad. I had to accept the reality of what I was putting in my tank and decided to change things up. For my new strategy I'm targeting .78 once it comes on boost tapering to .74, I also decided to drop the boost to 236Kpa. The result was pretty dramatic when it came to knock resiliency, I'm able to ramp the timing much more aggressively peaking at 20 Degrees. The result was a clear pickup in top end power even with the reduction in air flow. There has been a slight drop off in mid range torque but I think I may have some timing headroom there, I may even try and target a bit more boost under 5500, say 10Kpa or so.

I just wanted to share my story in case anyone else is struggling, sometimes switching things up is the way to go. Any advice or comments are appreciated.



Hi Jordie,

Thanks for sharing the info. I think many people will find it helpful.

I find Subaru's in particular are quite sensitive to this and it is sometimes hard to explain to people how adding some extra fuel can help reduce knock, or even lowering boost levels and introduction more timing can net the same power result, or sometimes more.

A lot of people ask "What AFR should I target" not realizing there is not 1 answer and it can vary on many factors, fuel quality being one of the biggest.

How I deal with all cars I tune is, tune to an AFR you think the engine wants, setup your boost level and ignition map. Then start moving the fueling around, target richer, see how the power reacts, see if it takes more timing, can you now add more boost etc. Its about learning how to read an engine and give it what it wants and not what you think it needs. Then a balancing act of maximum power while still having a good safety margin and economy in mind.

They best thing about your post is now you have learnt that yourself, you will start to apply it with cars you tune in the future and getting better results.

Those two posts should be compulsory reading!

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