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Old 04-07-2005, 11:25 AM   #1
jblaine
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Default Big vacuum target AFRs

I realize this isn't a sexy area to talk about but I am curious what kind of AFR targets make sense (and why) for high vacuum situations. I recall datalogging my stock setup targeting ~20:1.

My thinking (hypothesizing) is that maybe a little fuel (14:1?) would be good to run at high RPM + high vacuum in our cases since this is usually indicative of the end of a WOT run and either:
  • a shift up, in which case it might be better left alone
  • engine braking (entering a turn) - could use the time to do a little cooling while off load
  • braking just before a downshift - could use the time to do a little cooling while off load

Perhaps I'm naive, but it seems like this is an area we're kind of free to play with a bit.

I'm not really able to find any good information on it.
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Old 04-07-2005, 12:56 PM   #2
hippy
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What engine management? If it's with the utec, you have no control over the fueling at part or off throttle. Well, there might be a way if you can make the utec think that it's always at hard throttle(by adjusting the utec tps calibration so the utec always thought the tps was above 25%....). I dunno how good of an idea that is though. You probably saw the really lean reading while off throttle. If there was fuel going into the engine at this point, the rpms wouldn't drop as quick. It would be like having the throttle at 1%. At and above 1% while at part throttle(assuming you're using a utec), the ecu will always shoot for 14.7 to 1.

If you're not using the utec(like if you have a hydra or something), there are many things to think about. First would be that the stock ecu shoots for 14.7 to 1 because that's the a/f ratio at which catalytic converters work the best. If you have no cats in your exhaust, there's no real reason to ever try for 14.7 to 1. From what I hear, 14 to 1 or richer(like downto 12.5 to 1 depending on the situation) makes the most power, and the best fuel economy is at a point which is a bit leaner then 16 to 1. I wouldn't know what you can do about off throttle fueling since I dunno much about standalone ems systems.........

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Old 04-07-2005, 03:51 PM   #3
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Quote:
Originally Posted by hippy
What engine management?

peace


pssst. Be quiet. He's stabbing at his HYDRA.
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Old 04-07-2005, 04:02 PM   #4
jblaine
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Old 04-07-2005, 06:19 PM   #5
ride5000
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jeff, the stock ecu will target a fairly rich afr (12s at least iirc, maybe richer) on throttle close while >5k5, then cut injectors entirely from there back down to idle. that 5k5 cutoff coincides with the open loop crossover. to my mind, there is probably a reason for that.

ken
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Old 04-07-2005, 08:55 PM   #6
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On the Hydra you can leav it a bit richer (ie, not shut off injectors) but beware of big fireballs. It does help cool off the cylinders a bit on deceleration on the track but will decrease gas mileage and not necessary for street use so depends on use of the car I guess.
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Old 04-07-2005, 09:51 PM   #7
drees
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Quote:
Originally Posted by ride5000
jeff, the stock ecu will target a fairly rich afr (12s at least iirc, maybe richer) on throttle close while >5k5, then cut injectors entirely from there back down to idle. that 5k5 cutoff coincides with the open loop crossover. to my mind, there is probably a reason for that.
Probably to help avoid shift knock?
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Old 04-08-2005, 07:42 AM   #8
ride5000
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i think "shift knock" is more a matter of the utec-->ecu transition. there is no reason that the ecu maps can't be programmed to limit the max rate of change of ignition advance, which is the reason for "shift knock" imo.

i think it does have to do with cylinder cooling, as well as the fact that the wrx oem ecu stops listening to the knock sensor above 5k5 rpms (iirc). better to be safe than sorry.

ken
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Old 04-08-2005, 08:23 AM   #9
jblaine
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Doesn't hurt those fuel economy numbers on the marketing brochure either

Thanks for the replies.
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