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Old 09-08-2006, 04:14 AM   #60
Scooby Specialist
Member#: 54052
Join Date: Feb 2004
Location: Auckland, NZ
2005 Spec C Type RA
WRC Blue


I just got my car back today, after it was found my so called AVCS aftermarket cams were actually non-AVCS cams. Anyway, the cams have been fixed and are now moving as expected.

I haven't had a chance to do detailed analysis, but what I found was quite interesting. The car has been tuned with the cams at a fully retarded position of ~130 crankshaft degrees (as reported by the camshaft position sensor).

I chucked in 110 degrees (~20 degrees advance) across the entire map (I have Autronic EM) and this is what happened:

1. Car was running much leaner - hitting peak boost the car was running 11.5:1 AFR with cams at 130 degrees. With the cams at 110 degrees, car was running 13.5:1 AFR - didn't let it rev past 4000RPM for fear of blowing the engine.

2. Turbo spooled up faster - previously hitting 15PSI @ 3600RPM in 3rd, now hitting 15PSI @ 3200RPM in 3rd. How lean the car was running probably aided the spool up time.

What I believe this shows is with 110 degrees advance, at the RPM range I looked at (2000-4000RPM), the engine was flowing significantly more air, hence this level of advance is much more optimum for that range.

An idea I now have for tuning AVCS without a dyno is:

1. Set AVCS map to 0 advance. Tune fuel map rich - e.g. 10.5:1 for high load sites.
2. Set AVCS map to 5 advance. Do a few power runs, log AFRs
3. Set AVCS map to 10 advance. Do a few power runs, log AFRs
4. Set AVCS map to 20 advance. Do a few power runs, log AFRs
5. Set AVCS map to 30 advance. Do a few power runs, log AFRs

Once you are done, compare the AFRs at various RPMs. The leaner the AFRs become relative to your first run the more optimal (since the fuel map is constant for each run, the AFR reading shows how much more/less air is going into the engine). Doing the base run rich is to avoid running too lean during your testing. Running it at 10.5:1 allows for you to detect if the engine runs richer (most wideband AFRs lose resolution below 10:1).

The above technique I believe should work with a MAP-based engine management system (like Autronic or Motec), not sure on a MAF-based system as load is expressed in air mass. On a MAF-based system it might be as simple as looking at the MAF readings for each different level of advance but it's kinda chicken in egg.
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