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Old 12-17-2007, 06:42 PM   #1
williaty
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Default How Does AFR Affect the "Feel" of the Car?

OK, so I'm slowly changing simple things about the car. One of the things I'm working with is the OL fueling map because the stock map holds the car at 14.7 even in OL over much of the map, then lets it crash down to a ~10 effective AFR at WOT+redline. So the first map I made basically kept the cells involved in cruising at 14.7 to keep the car in CL, then tapered to an requested AFR of 12 everywhere else. This, coupled with the removal of the CL/OL delay, made the car very much smoother to drive and instilled a sense of urgency in each press of the gas pedal. Mind you, I don't think a dyno would have showed much, if any, difference, it just felt like it was much more willing to respond and that it really wanted to just leap up and pull for redline from even very low revs. It made the car MUCH easier to drive because there were many situations in which I no longer had to downshift before accelerating in traffic.

After speaking at length to someone who has tuned a copy of my car on a dyno with a WBO2, I altered the fuel map based on the differences he noted between the AFR called for in the OL fueling map and what actually resulted, as well as some of his personal opinion on how the fueling map should he shaped. Basically, I leaned it out some more and tapered it slightly richer with increased load and RPM. The new map still feels very very smooth, but the sense of urgency when I put the pedal down is gone. Note that it isn't so lean that it's pulling timing, timing is the same as the first fuel map I created and it's not knocking significantly.

Unfortunately, at the same time I flashed the newest fuel map, the humidity went through the roof. So I can't take weather out of the equation. On the days I was running around on the first map I made, it was cold, very dry (<30% relative humidity), and high pressure. Now it's cold, very wet (70-90% relative humidity), and slightly lower pressure.

So my question is this. Is it more likely that the sense of urgency was lost due to the slightly leaner AFRs (going from >12 to 12-13, NA motor, remember) or the change in the weather?
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Old 12-17-2007, 09:30 PM   #2
Master2192
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AFR and weather effects can be different based on location. For instance, the local fuel I tune on does not like much timing advance even at rich AFRs. But will take much leaner AFR without knock and good EGTs than you would usually expect. Your local fuels are probably much different.

I haven't experimented enough with N/A models to know how they would feel, but going to a rich mixture quickly before full boost on a turbo car will slow spool and response. The car can still feel fast in this condition but the snappy response can make it feel a bit sluggish especially if you are on and off the throttle in something like a corner.

I would think you'd have lost torque going to a 12 afr immediately (especially with no change in timing), are you sure you weren't pushing the throttle further to make up for the torque loss and thus the car felt faster since you'd just end up at a higher load than usual?
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Old 12-17-2007, 09:55 PM   #3
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I've attached a graph of what different af ratios do for a f/i k20. I've done this test using a similar graphing program that you guys use to plot your graphs. I know the wrx and sti have a different beast (ej32) but im still learning, but know the jist of the engine and what not.

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Old 12-17-2007, 10:36 PM   #4
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Quote:
Originally Posted by Master2192 View Post
I would think you'd have lost torque going to a 12 afr immediately (especially with no change in timing), are you sure you weren't pushing the throttle further to make up for the torque loss and thus the car felt faster since you'd just end up at a higher load than usual?
It doesn't go directly to 12, it tapers across 3 cells. Yes, I'm sure I'm not using more throttle since it's always WOT
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Old 12-18-2007, 12:08 PM   #5
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Are you tuning with a wideband O2 or an EGT? I'm just curious how close your requested A/F ratios are to the actual A/F ratio.

I've never burned down a subaru motor. But I used to run a Formula 500 in the SCCA. It was based on a rotax motor. I had a needle in the carb lose it's snap-ring. It fell and lodged itself in the main jet partially blocking it. It melted the spark plug and part of the piston without ever knocking. In fact it ran really strong right up until it quit. Based on that experience, I don't think that I would rely on the presence of knock alone to indicate safe A/F ratios. High EGT's can cause aluminum to start leaving areas it shouldn't.

Also someone please correct me if I'm wrong, but doesn't the stock ECU start ignoring the knock sensor after a certain RPM (5500?)
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Old 12-18-2007, 01:49 PM   #6
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i just recently had my car tuned. before the tune, my afr was in the 9.5:1 range. the car still felt like it was pulling strong and no different than before the intake i installed.

i'm still tweaking my map but i'm now up to 10:1 and the way the car pulls feels totally different. the engine is much happier to wind out to redline, my idle has become smoother, and it pulls so much stronger throughout the rev range. i can't even imagine what it'll feel like once i get to 11:1.

all i've done to achieve this so far is to scale the maf for the intake and adjust the fuel table.
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Old 12-24-2007, 04:52 PM   #7
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best torque is around 12.5. that's all i'm gonna say.
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Old 12-27-2007, 01:44 PM   #8
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Quote:
Originally Posted by nhat View Post
i just recently had my car tuned. before the tune, my afr was in the 9.5:1 range. the car still felt like it was pulling strong and no different than before the intake i installed.

i'm still tweaking my map but i'm now up to 10:1 and the way the car pulls feels totally different. the engine is much happier to wind out to redline, my idle has become smoother, and it pulls so much stronger throughout the rev range. i can't even imagine what it'll feel like once i get to 11:1.
I had the opposite experience. My AFR was set to the same 9.5:1 and i changed it to 11:1 straight away, then to 12:1. Didnt notice any difference in power. I left it at 11.2.
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Old 12-27-2007, 03:39 PM   #9
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For me, running 14s and 13s at lower boost pressures 5-10psi certainly feels better than running low 12s and 11s at that pressure. It depends on your turbo too.
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Old 12-27-2007, 03:41 PM   #10
williaty
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Oh crap, that's what I'm doing wrong! I forgot the turbo!


(psst. look at the OP's profile)
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Old 12-27-2007, 03:47 PM   #11
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Damn it! Why do you NA guys bother??

jk jk..

I don't think it would matter as much to NA guys as the turbo guys. I'm a fan of the feel of a leaner mixture though.

Quote:
Originally Posted by williaty View Post
Oh crap, that's what I'm doing wrong! I forgot the turbo!


(psst. look at the OP's profile)
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Old 04-11-2008, 12:21 PM   #12
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While not intending to revive the dead, Williaty, what did you end up settling on for OL target AFR's? I've richened up lower RPM's/higher loads to 13.4 and transitioned to 12.5 or so in higher RPM's Loads. Curious what you found to work best for your car/mods/climate.
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Old 04-11-2008, 03:33 PM   #13
williaty
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12.5:1 at all RPM from 1.0g/rev up.
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