[Supras] Spark retard and top end fuel redux

[Jim Jobe]

It doesn't stop retarding ignition timing.  It simply changes the scaling
of the ignition timing table.  The factory ignition timing map is going
to be fairly conservative to begin with, and combined with a decent
92-94 octane fuel you should be fine.  I in fact ran 15 degrees base
timing when I was on the stock ECU (92T).  There was a set of dyno
pulls we did long ago on a 90T with a 57 trim turbo, 550's, lexus
afm, and 18 degrees base.  I think fuel was 93 octane with some
octane booster added.  *laugh*  How crazy we were/little we knew
back then.

On Tue, Sep 11, 2007 at 01:15:52AM -0400, berniek at technicaldevelop.com wrote:
> The popular 550cc-Lexus AFM modification does not appear to take spark 
> retard at higher boost into account.  Since the Lex infers boost from 
> airflow, it will read about 25% low in airflow, the very principle of 
> using it with 550cc injectors.  So 8 PSI boost to the stock AFM is 
> interpreted as 6.4 PSI by the Lex (gauge pressure basis; may need to 
> rethink this).  15 PSI boost read by the stock AFM if fuel cut did not 
> get in the way would be interpreted as 12 PSI by the Lex.  However, 
> since the ECU is set for the stock AFM, the difference between 15 and 8 
> PSI boost (fuel cut not withstanding) is a difference of 7 PSI.  
> Indications are than there should be between 0.7 degrees and 1.3 degrees 
> of spark retard per psi of boost on pump gas.  So if the stock ECU will 
> track up to say 8 PSI, that leaves about 7 degrees of spark retard to 
> account for from 6.4 PSI to 12 PSI with the Lex (equivalent to 8-15 PSI 
> range with the stock AFM).  So it appears I will need to retard the 
> spark an additional 7 degrees at 12 PSI boost, and perhaps 10 degrees at 
> 15 PSI boost with the Lex.
> 
> Anyone dealt with this?  I'm just beginning to get into the dizzying 
> world of electronic add-ons and would rather avoid them.  This will just 
> be a daily driver, nothing more.  Will the knock sensors remap the ECU?  
> That would seem logical with differences in gasolines.  From an 
> electronic standpoint, it is not hard to build a simple circuit to 
> retard the spark signal sent to the ignitor from the ECU with a linear 
> retard of 7-10 degrees starting at about 6 PSI of boost using the output 
> of the boost sensor which presently only drives the dashboard gauge 
> (actually works up to just about 15 PSI).  I'll be happy to share the 
> construction details for anyone who knows some electronics and can 
> handle a soldering iron.  But this would not be necessary if the ECU 
> learns from the knock sensors. 
> 
> Thanks. 
> 
>     BernieK
> 
> 
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