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Old 10-05-2018, 01:46 PM   #801
Scooby Specialist
Member#: 375462
Join Date: Dec 2013
Chapter/Region: NWIC
Location: Portland, OR
2006 Impreza WRX STI
WR Blue


Originally Posted by NSFW View Post
What do you think the factory ECU uses?

Has anyone actually measured the mechanism with a protractor? What did you / they find?

I have a partially disassembled exhaust AVCS mechanism, but no protractor.

If I understand how the AVCS solenoid and mechanism work, oil pressure fluctuations wouldn't matter much unless it drops WAY down. If oil pressure goes up or down (and the solenoid doesn't vary the split) the oil pressure goes up or down the same amount on both sides of the lobes, so there shouldn't be much movement. Unless the oil pressure drops so low that it can't overcome the return spring on the exhaust side or the... opposition (for lack of a better word) from the valvetrain on the intake side.

My LGT runs 30 degrees of intake advance in cruise, with BC 272 cams, and it's very stable. Unless you really want max advance/retard below 1500 RPM I'd be surprised if you had trouble getting it. And if you do want max at such low RPM, I gotta ask, why?
Since you have one in-hand - post your picture of the mechanism with the inner lobes installed and pressed against one stop (take a straight-on view photo for me).

I have measured this before in the past, and I believe this thread actually has the info... but I'm just too busy to dig at the moment.

Oil pressure fluctuations DO matter, and oil temperature matters even more. Achieving 45 deg advance at idle, for example... well you can get 50 easily, because that slams up against the physical stop. Anything about about 30 deg @ idle or a little above and you'll have a decent amount of jitter. As this affects VE strongly, this is a very bad thing for stability. Using my 2006 EJ257 motor as an example as I've logged more data on that than anything else - the 35-49 degree range is the no-go zone.

You hit the nail on the head though - 30 degrees at cruise is fine. It's the 35-49 degree range you'll likely have issues with. Again, 50 is easy - the way the PWM works on these is by having a center value that holds advance at a stable position. Move the PWM % up 1 point and the advance will climb slowly, 2 points and it will climb extremely fast, and 3 points and you'll slam into a stop. I've had to program the control from scratch for my MoTeC M800 and it was eye-opening to realize the PWM control range used is like 7% out of the full 100% range.

EDIT: Read my comments here - https://forums.nasioc.com/forums/sho....php?t=2809286

These were posted closer to when I was performing the PWM programming of my motor and now I remember what it was! The EJ207 motors have a center PWM value that targets about 25 degrees advance allowing for full tuning within the 0-50 degree advance range (the factory used 0-45 deg on single AVCS EJ207 motors). The 04-06 EJ257 motors had a center PWM value that targets about 10-15 degrees advance. What happens with this, is that the 0-30 (maybe 35) degree range is stable, but the 35-49 degree range is unstable due to the necessary PWM values to achieve this advance being too far from the center value (causing hunting).

EDIT 2: Took a look at an older AVCS mechanism picture I found online - cam advance max = 25 degrees equating to 50 degrees @ crank. The 'tooth' with the 2 side bumps at the bottom is the physical stop 'tooth'. If I had to guess, those raised stops are probably manufactured long and machined to accurate size during the manufacturing process. Factory ECU tables are in crank degrees.

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Last edited by mrsaturn7085; 10-05-2018 at 02:10 PM.
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Old 10-05-2018, 10:30 PM   #802
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Member#: 140444
Join Date: Feb 2007
Chapter/Region: NWIC
Location: Near Seattle, WA
05 Stage Free LGT


As far as I can tell, the "center" PWM value doesn't target any particular amount of advance, but rather it targets zero change of advance. Small deviations from that center will cause the amount of advance to increase or decrease, and the ECU returns to the "center" value when the desired advance is achieved. (Or desired retard, for the exhaust cams.)

I will confess to not logging it enough to be absolutely certain of that, but I made a standalone exhaust AVCS controller for my single-AVCS car based on that assumption, and it works. I got the idea from looking at an exhaust AVCS log from an 08 STI. The "center" values moves around over time but it doesn't seem to correlate with anything obvious like RPM.

The factory tune for my wife's 2009 LGT has an island of 40 degrees of intake advance.

The exhaust AVCS pulley that I mentioned is a paperweight on my desk at work, but I'll get a picture of it on Monday.
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