[Supras] Spark retard and top end fuel redux

[Wade Tobin]

That's some interesting reading but perhaps you're going into this a bit deeper than necessary.:)  There's dozens and dozens of 7m's running around w/ Lexus AFMs, 550s, piggybacks and no problems.  Been using a Lexus AFM/550s since 02 on the same engine which came from a junkyard (140k miles).  The car has made 420rwhp on the dyno 50 passes on the track both 1/8 mile and 1/4 mile.  About 20 of those passes were over 17psi.  Best time was a 12.4 @ 117.
   
  Don't know if you got my last email Bernie, but running the stock CT26 elbow, DP and both cats is an area I'd far more concerned with than Lexus AFM tech.  Surely, a choked up exhaust will cause higher EGTs and detonation, Lexus AFM or not.  Also, a big intercooler is also a good idea.  The stock MK3 exhaust and intercooler = garbage.

  Your absolutely correct about the ECU not retarding the timing w/ the Lexus.  MSD sells a boost timing retarder box for your needs.  I don't run one.
   
  -wt
"berniek at technicaldevelop.com" <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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