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Location:
sunny coast, qld
Registered:
October 2002
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optimum air fuel ratio
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Fri, 27 May 2005 03:29
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Hi guys,
just drove my car back from exhaust shop. First time driven legally on the road for a year or so.
Anyway, I have a wideband o2 sensor kit and was wondering what the air fuel should be on cruise and idle? I know it should be around 12-12.5:1 on WOT.
Thanks
Chris
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I supported Toymods
Location:
Australia
Registered:
November 2003
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Re: optimum air fuel ratio
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Fri, 27 May 2005 03:45
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As lean as you can get it without it pinging or overheating.
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I supported Toymods
Location:
Adelaide
Registered:
July 2004
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Re: optimum air fuel ratio
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Fri, 27 May 2005 04:03
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congrats on finally getting to drive it dude!!
feels great doesnt it 
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Location:
Brisbane
Registered:
May 2002
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Re: optimum air fuel ratio
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Fri, 27 May 2005 05:37
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awesome stuff mate! Great to hear it's going again!
Running it as lean as you can will give the best economy (some jap econboxes do this and run like 18:1 afr's or something like that!!), this causes very hot exhaust exhuast gasses tho and some people recon its bad for the exhaust valves? (meh no idea).
The EPA would prbly like you to aim for 14.7 and make as little carbon-monoxide as possible 
I think somewhere in the range or 15-16.5? as an uneducated guess?
Cheers
Wilbo
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: optimum air fuel ratio
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Fri, 27 May 2005 06:13
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Manufacturers and the EPA would prefer 14.7:1, cos thats where the cat works the best.
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Location:
sunny coast, qld
Registered:
October 2002
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Re: optimum air fuel ratio
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Fri, 27 May 2005 06:29
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Yeah, I knew that leaner ratios caused increased exhaust gas temps. The problem is I don't have an exhaust gas temp guage.
So shall I say 15-15.5:1 on idle and cruise then? If 18:1 is what is used in econoboxes, maybe a little richer should be used in my situation.
Thanks guys It is really quiet again. It must be the muffler I have. 4" dump pipe, 3" cat, no resonators and 3" muffler with a bend in it and it is a little louder than my parents vienta on idle
I was going extremely slow on the way home as my a/f was at 22:1 on very light throttle. However, it was idling at 12:1 so too rich.
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Location:
Brisbane
Registered:
February 2003
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Re: optimum air fuel ratio
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Fri, 27 May 2005 07:55
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12 - 12.5 is good and safe for making power. 14-15 would be good to aim for cruise.
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Location:
sunny coast, qld
Registered:
October 2002
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Re: optimum air fuel ratio
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Fri, 27 May 2005 08:37
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I was just having a look then and it didn't seem to run very smooth at anything leaner than 12:1? That is at 1.2ms on 550cc injectors at 750 and 1000rpm.
I think I will have to wait until next week to get the tune done.
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I supported Toymods
Location:
Australia
Registered:
November 2003
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Re: optimum air fuel ratio
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Fri, 27 May 2005 08:41
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The bigger injectors are the less happy low idle will be.
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Location:
sunny coast, qld
Registered:
October 2002
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Re: optimum air fuel ratio
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Fri, 27 May 2005 08:47
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but I was leaning it out so they would be like running smaller injectors for a longer time.
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I supported Toymods
Location:
Australia
Registered:
November 2003
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Re: optimum air fuel ratio
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Fri, 27 May 2005 08:53
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They lose resolution.
Especially with aftermarket ecu's.
Its easier to make a 220cc injector punch out 10cc of fuel for idle than it is a 550cc injector.
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Location:
sunny coast, qld
Registered:
October 2002
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Re: optimum air fuel ratio
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Fri, 27 May 2005 09:02
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Ok, so they physically can not open for that short a time. So it sounds like 1.2ms is their lower limit. Thanks
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I supported Toymods
Location:
Australia
Registered:
November 2003
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Re: optimum air fuel ratio
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Fri, 27 May 2005 09:06
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The probably phyiscally can, becuase the 2jz used 550cc injectors on the factory ecu.
But the Wolf3d probably doesnt have the resolution to do it.
At least that is the way I understand it.
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Location:
Montrose, VIC
Registered:
May 2002
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Re: optimum air fuel ratio
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Fri, 27 May 2005 14:10
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The 'perfect' ratio is different for every application - I suppose, pull the engine out, and with a few hours on an engine dyno, attempt to find the balance between BSHP and BSFC (brake specific fuel consumption). However, if you're a little more realistic, then I guess you can go off generalisations - i.e. 12-12.5 for power, 14.7 = stoic, and 16-17 for maximum economy. The emissions side of it is tricky though, with CO decreasing under leaner mixtures, and NOx increasing. Which is why car makers aim for 14.7 as much as possible, and fit a cat converter. 'Lean cruise' functions are illegal these days anyway.
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Location:
Canberra
Registered:
May 2002
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Re: optimum air fuel ratio
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Fri, 27 May 2005 14:23
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at idle, 12-12.5:1 is really not that bad with 550cc injectors...
Idle mixtures are generally richer than the ideal 14.7:1. Don't stress too much. Its hard to control bigger injectors at less than 1.5ms
Cheers
Phil
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Location:
South Australia
Registered:
July 2002
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Re: optimum air fuel ratio
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Fri, 27 May 2005 14:28
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Actually I think it's safer to say that manufacturers look towards getting close to stiochiometric ratios or slightly rich ratios near the ignition area - using direct injection, intake swirl design, combustion chamber shape etc - thus allowing the rest of the chamber to be filled with 'whatever' (recycled exhaust or fresh air) and hence run leaner than would normally be possible given emissions requirements.
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Location:
sunny coast, qld
Registered:
October 2002
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Re: optimum air fuel ratio
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Sat, 28 May 2005 01:19
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Thanks guys.
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