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question about in/hg at idle and AFR at idle

 
Old 08-28-2014, 12:00 PM
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Default question about in/hg at idle and AFR at idle

Just had a random question about idle quality. I've got a swapped Buick Regal, basic setup is stock long block LS1, 224/230 on a 114 LSA cam, vic jr with elbow, siemans 80# injectors. Its a turbo car running on Microsquirt, it uses wideband for the O2 input vice narrowbands. Ive got it running pretty good. But i was curious on some of you guys' opinions about vaccuum vs AFR. At about a 13.3-13.5 idle AFR (which should be the target for a batch fire setup) i'm pulling about 8-9 in/hg, when i richen it up to about a 12.5-12.7 AFR at idle it pulls about 14 in/hg of vacuum. Would you guys rather have a leaner idle with less vacuum or richer idle with more vacuum?
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Old 08-28-2014, 02:19 PM
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Depends on what you are trying to do, if the car runs fine at the leaner idle that is what I would shoot for since it will use less fuel. You are not looking for max power at idle so the lower vacuum leaner set-up will give you better economy. I am making the assumption it still runs well.
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Old 08-28-2014, 03:54 PM
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it runs just fine either way, great actually. I might try and meet in the middle at about a 13.0, but i dont really sit at idle all that much so it mine be fine where its at. I was just curious on input from others.
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Old 08-29-2014, 10:24 AM
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If the brake booster is seeing enough vacuum at idle, then I would shoot for the leaner path. Just my preference though.
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Old 08-29-2014, 11:40 AM
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In the case of just stating a opinion without really knowing what is best. That being said back in the carburetor days you could almost always set idle mixture for emissions by going for best idle, fastest, smoothest then lean it out a little from there. That would give the best balance of low hydrocarbon and lowest c.o. leaning out further would increase missfires. More fuel would be a waste and increase c.o. best idle would be highest vacuum also. Just my thoughts.
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Old 09-02-2014, 12:12 AM
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Actually at idle, fuel consumption is minimal. WBO2 may not be all that accurate either at idle. As a rule of thumb, you want AFR and Timing set at max vacuum at A GIVEN RPM at idle. So if you bump up timing and richen it up and your idle climbs 200 rpm, then you are no longer comparing apples to apples. When you knock the airflow out to get it back down, you may end up with less vacuum than you had. Make sense?

The one exception in which you want to error lean is when you are firing up a new motor that needs the rings to seat. Rich idle in this situation can wash out the cylinder.
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