So I have a bit of a noob question about AFR for this car. I know the "ideal" AFR is 14.7, but I have seen around posts by various people saying "my tuner didnt feel comfortable with the AFR getting above 12" or some other number indicating a somewhat rich mixture. My question is this, why are some leery of raising the AFR above 14 with this car? isn't anything less encouraging carbon buildup and a whole host of other bad things with the car? does this risk detonation from a boost spike or whats the deal. Its not my intention to start a flame war or anything, but Im just curious about this aspect.

Andrew

 

14.7 = cruising.

when you see people talking about afr's in the 10's-11's (and maybe even some crazies in the 12's) they are referring to WOT.

 

ah, ok... question answered

 

I would like to extend this question somewhat.
At some point the ECU must go from open loop mode (direct table lookup, such as during WOT) to a closed look mode (using AFR's to adjust the injector timing, such as when cruising). When exactly does this transition occur/when is closed loop mode activated?

I would guess it is perhaps combination of some of the following: Rate of change of RPM, throttle position, throttle position rate of change, expected throttle position for the current RPM or injector load. Or is it a whole lot simpler?

 

The most documented speed density system out there is the system used on the turbo 4.3L GMC Syclone/Typhoon and some obscure GM engines from the late 80s. On those engines, returning to closed-loop driving appears to be a two stage process. The engine will first return from rich heavy load operation to normal A/F targets (leave a "power enrichment" state). Then it will test a number of conditions before entering closed loop.

To leave power enrichment, the TPS voltage must first fall below a particular value along with having a MAP reading and coolant temp reading within a certain range.

After leaving a power enrichment mode, the Syclone ECUs tries to make sure the engine is ready for emissions controlled driving. First the O2 sensor is tested to see whether it is up to temp or not. Then the ECU checks the coolant temperature and a series of timer circuits. Finally, it makes sure certain check engine codes are not flagged.

see http://www.aces.edu/~parmega/efi/turbo_p4_doc.pdf document page 56-59 (pdf page 64 - 67)

a full dump of the Syclone ECU is available here: http://www.nwstp.com/forum/P4_bbzb1580.asp

a description of the most important tables are available here: http://www.nwstp.com/forum/chiptable.asp

on MAF based systems, the main triggers are mostly throttle position and a "load" threshold, which is usually some kind of airflow measurement divided by rpm. That's how it works on STi's for example. There are multiple trigger tables and delay/timer values in the various Subaru maps I have.