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Old 03-28-2005, 07:39 PM   #1
kenlee
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Default ideal afr's vs. rpm

I've recently logged a few runs and it looks like I'm running a bit rich in the 4000-6750rpm area. It looks like I'm running around 10.7.1 from 4000-4500,then it richens to 10.3.1 from 4500-5750, and 10.5.1 up to redline.
I'm logging in 4th gear and running 97-98% tps. My question is will I be o.k running close to 11.1.1 in 4th with the possibility of running in 5th occasionally? I typically run a 94-95 octane mix.

Also what are ideal afr's for the lower rpms, say 2000-4000 rpm vs. load? I'm running high 14's while cruising and would like to compare my logs with what is ideal.

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Old 03-28-2005, 07:51 PM   #2
dosage0
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Just my take.. and I'm not a pro tuner.

I dont compare AFRs to RPMs. AFRs correspond to PSI with me. The more pressure you ram in, the more fuel you need to control mistimed fires. You can either make your car richer and run higher timing, or leaner with less timing. Its a personal preference, but I like to keep my AFRs:

up to 7 lbs of boost you can run 14.5:1
Big drop 7+ to 10.5 on 91 or 10.8 on 94-95 in the midrange.
As boost begins to taper off I lean out to 10.8 on 91 and 11:1 on 94-95.

Just what I run... you can also try leaning it out until you knock, then richen it a bit. That way you'd find your own cars max.

Rob
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Old 03-28-2005, 09:01 PM   #3
antimullet
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Yup tune afrs versus load not rpm or your setting yourself up for knock/det or overall lean conditions.
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Old 03-28-2005, 09:48 PM   #4
kenlee
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Thanks for the input. I realize fuel should be tuned to boost levels and said timing. It just gets alittle confusing when dealing with the utec fuel table looking at rpm and load. I'm looking at it like this. From 4000-5500 rpm the car runs max boost around 22psi. past 5500 it starts to taper off to 20 psi and holds 20 till redline. For my fuel mixture 94-95 shouldn't 11.1.1 be safe? The only worry I can think of is the occasional pull in 5th gear which would run abit leaner.

Thanks for the afr/boost for given boost levels. I'll try to log what I'm running against the values you gave.
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Old 03-29-2005, 09:50 AM   #5
ride5000
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ken, also consider the intended use.

i run 11.5 or so on the street. i rarely blast over 100mph, and when i do, it is a very short spurt... (i like my licence and i don't want to die!!) as a result the egt is never really a concern.

now, if i was going to a track to road race where i was repeatedly hammering the throttle hard, i would want to richen it up, maybe even to the high tens. i might give up 10-20hp, but i would gain reliability. what works for daily drivers or 1/4 miles does not for extended high speed runs.

jm2c
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Old 03-29-2005, 07:14 PM   #6
antimullet
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I wouldn't run stoich on anything but vacuum ie no/low load/cruise. Any kind of boost should have some more fuel into it.

Ask around but if your on pump and wanna keep that car in one piece IE: no Det then I'd go overly rich first to make sure your semi safe then work from there. But use caution on overly rich settings as that can raise egts also.

11:1 is a pretty good thing to shoot for just make sure you aren't logging any det events and adjust fuel/timing accordingly. Might wanna start around 10.9:1 on fuel at max boost and work lean from there just to stop the det.

What is your timing setup to look like IE: load versus timing?
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Old 03-29-2005, 07:17 PM   #7
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Quote:
Originally Posted by antimullet
Yup tune afrs versus load not rpm or your setting yourself up for knock/det or overall lean conditions.
Everything's still relative. Bosch says that most engines make the most power from 12.5-14 to 1 a/f ratio. That makes me think that at hard throttle, ya never really wanna go leaner then 14 to 1 unless you wanna lose power. The only reason we make the a/f ratio richer then 12.5 to 1 is to cool and slow down combustion to avoid det and melting parts. If there was no problem with the heat created by a lot of boost, high rpms, or more weight,(in general heat created by more load), then noone would ever run their engine richer then 12.5 to 1. If you can get away with running from 14-12.5 to 1, then do it. Ie-at lower pressures, air flows, and rpms(like -10psi to 5psi below 5000rpm), ya really don't need to make the a/f ratio really rich to fight off the heat, so you could run something like 14-12.5 to 1. As the rpms, boost, and airflow rise though, so does the heat making it necessary in many cases to run richer a/f ratios. How much richer is upto you.

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Old 03-29-2005, 07:21 PM   #8
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Quote:
Originally Posted by ride5000
ken, also consider the intended use.

i run 11.5 or so on the street. i rarely blast over 100mph, and when i do, it is a very short spurt... (i like my licence and i don't want to die!!) as a result the egt is never really a concern.

now, if i was going to a track to road race where i was repeatedly hammering the throttle hard, i would want to richen it up, maybe even to the high tens. i might give up 10-20hp, but i would gain reliability. what works for daily drivers or 1/4 miles does not for extended high speed runs.

jm2c
ken
What he said
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Old 03-29-2005, 10:22 PM   #9
kenlee
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Good stuff guys! I'll shoot for 11.1.1 and if everything seems o.k.(no knock) I might decide to lean it out slightly more given the fuel mixture I'm running.

My timing runs 15-18 from around 3k-5k and bumps up to 25.5 by redline.
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Old 03-29-2005, 11:10 PM   #10
Jon [in CT]
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Quote:
Originally Posted by hippy
Bosch says that most engines make the most power from 12.5-14 to 1 a/f ratio.
I'm pretty sure that no Bosch literature has ever made that claim. Suppose you're running gasoline with 10% ethanol (winter gas), whose stoichiometric A/F ratio is about 14.0:1?

What Bosch does say is: "Spark ignition engines with manifold injection achieve their peak power output at a air deficiency of 5...15% (lambda=0.95...0.85), ..."
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Old 03-29-2005, 11:34 PM   #11
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You're correct. Bosch speaks of things like that in terms of lambda. Since they say max power is made at a lambda of .95-.85, it would come out as a reading of 14-12.5 to 1 on wbo2 sensor kits we use which are calculated for gasoline. This is true no matter what fuel you use. Ya might not actually have an a/f ratio of 14-12.5 to 1 for max power depending on the fuel, but that's the reading you're shooting for with our wbo2 kits. At least that's the way I thought it worked.

peace

Last edited by hippy; 03-30-2005 at 12:09 AM.
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Old 03-30-2005, 01:16 AM   #12
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Since you are tuning a UTEC, there is a nice post or two over at wrxhackers that can show you a load map style layout of AFRs. Do a search over there for "target AFR."
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