What would the ideal A/F curve look like?
04-07-2004, 07:34 AM
#1
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What would the ideal A/F curve look like?
12.5:1 is supposedly best for max power, but after seing some of the DBW dyno plots, which run ~14.5:1 until about 5000RPM, I'm curious if it would be better to run it a little leaner during the midrange.
There are three main regions of the curve:
1. Low Boost (amount of time until boost starts building)
2. Boost transition (area from little boost to full boost)
3. Full Boost
So, let's say that 2000 to 3000 rpm is the Low Boost area, Boost Transition is 3000 to 3500, and full boost is 3500 - 7500 RPM
I'm thinking that keeping it leaner during Low Boost and Boost Transition will have higher EGTs, which is more energy for the turbo, and maybe result in quicker spoolup.
So maybe 14:1 during Low Boost, 13.5:1 during Boost Transition, and slowly dropping to ~12:1 during Full Boost.
Stolen from Ray's PP:
<img src="http://pictureposter.audiworld.com/4477/128s3dbwvsnondbw.jpg">
There are three main regions of the curve:
1. Low Boost (amount of time until boost starts building)
2. Boost transition (area from little boost to full boost)
3. Full Boost
So, let's say that 2000 to 3000 rpm is the Low Boost area, Boost Transition is 3000 to 3500, and full boost is 3500 - 7500 RPM
I'm thinking that keeping it leaner during Low Boost and Boost Transition will have higher EGTs, which is more energy for the turbo, and maybe result in quicker spoolup.
So maybe 14:1 during Low Boost, 13.5:1 during Boost Transition, and slowly dropping to ~12:1 during Full Boost.
Stolen from Ray's PP:
<img src="http://pictureposter.audiworld.com/4477/128s3dbwvsnondbw.jpg">
04-07-2004, 10:57 AM
#5
Re: What would the ideal A/F curve look like?
What you said is my general take on it also. On big turbo just make sure you got fuel for the transition since it happens so fast, just dont wait till the last 100rpm ;-). Yup lean then rich, it does help with spoolup and so does a few degree's of timing:-)
Since you have an egt gauge watch that but I still think WOT in high RPM's I'd want it to be in the 11's. But yes we are lucky these engines are so damn durable and tough. Even on my lean 400whp I couldnt hear any detonation and timing still held. But I just redid my fuel system to suport my power so Im not advocating running the a/f I have on that pull but good to know the engine is leaniant(chucklechuckle) and built like a tank.
Since you have an egt gauge watch that but I still think WOT in high RPM's I'd want it to be in the 11's. But yes we are lucky these engines are so damn durable and tough. Even on my lean 400whp I couldnt hear any detonation and timing still held. But I just redid my fuel system to suport my power so Im not advocating running the a/f I have on that pull but good to know the engine is leaniant(chucklechuckle) and built like a tank.
04-07-2004, 11:52 AM
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14's before and while boost is building
12's to 11's once you hit full boost and start moving alot of air. If you run the car too lean while at full boost you will have to lower the adv timing.
Before my manifold cracked I was able to hold 11's and dipping into the 10's from the time I hit full boost to 7K. This was done without the pressure line to the adjustable fpr which was set at 40 lbs.
Before my manifold cracked I was able to hold 11's and dipping into the 10's from the time I hit full boost to 7K. This was done without the pressure line to the adjustable fpr which was set at 40 lbs.
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04-07-2004, 01:00 PM
#9
Mike's looked really good IMO.
But yea, in a perfect world, what you're describing would be great. I'd only say in addition to what you did, that under WOT/full boost, I'd take the afr down to 11's:1, depending on the max boost level.
04-07-2004, 01:51 PM
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Re: What would the ideal A/F curve look like?
Check this out John, "nmeyti" is a tuner at TXS, he is known to be pretty good in Subie land.
12:1 is too lean, you typically run race gas (104 or better) that lean.
http://www.wrxhackers.com/modules.php?name=Forums&file=viewtopic&t=2 174
12:1 is too lean, you typically run race gas (104 or better) that lean.
http://www.wrxhackers.com/modules.php?name=Forums&file=viewtopic&t=2 174
