View Single Post
Old 08-28-2006, 08:45 PM   #1
Scooby Specialist
Member#: 56468
Join Date: Mar 2004
Chapter/Region: NWIC
Location: Seattle, WA
04 Improved STI
Dirty White

Default Let's talk AVCS tuning.....anyone...?

OK. I'm the first to admit the theory behind AVCS in vacuum versus boost starts to make my head hurt.

What AVCS does seems to be fairly basic. The intake valve opening early and closing early (more advanced) results in cleaner air (less residual exhaust, scavenging) and some improvement in the charge velocity at mid-RPM. Both result in improved mid-range torque and the increase in gross air induction in the volume and velocity of exhaust, which results in quicker turbo spooling.

Tuning it however is not easily accomplished. The benefits of AVCS seem to be most prominent in the mid RPM range and "on boost". Here the induction is blowing the the last reminents of exhaust out through the still open exhaust valve. In general, in this region (say the window from 10-20 psi, and 3500-5000 RPM) more advance is better. Agree? Disagree? How can you tell?

Ultimately a dyno can tell you about torque output in those regions. Perhaps you could tune to maximum torque? Are the same settings that benefit torque also going to benefit spooling of the turbo?

Now lets turn to a more mysterious area of AVCS tuning: the vacuum load sites. Here there is a theoretical issue in that on the intake side pressure is very low and on the exhaust side pressure is likely higher. If the intake cam is opened too early (advanced by AVCS), then there is the potential for "reversion" of exhaust back into the intake. When to advance, and by how much to advance, in the vacuum sites, will be a trick. Again, what will be the tuner's guide? Torque would be best, but perhaps we could use AFR to some extent.

Some argue that little to no reversion occurs with more overlap (as is the case with AVCS advance) except at very low RPM, like idle. Other's blame the sluggish "big cam" performance at low-mid RPM exactly on the increase in overlap and exhaust gas dilution.

Another question of interest to me is how and in what ways might a larger turbo affect AVCS tuning?

Another might be: if overlap is king at high RPM, why do people routinely taper AVCS advance as high RPM is reached? Does this have to to more is velocity of air through the motor, and AVCS has done it's thing in teh mid range and is unneeded at high RPM. Perhaps the engine needs all the induction effieciency it can get at high RPM, and AVCS is somewhat of an inefficiency (lowering mid-range VE) despite it's torque gains.

Not many have tuned AVCS, but perhaps we can at least get things going here. Theory, empirical approaches, data.......anything people can post would be useful.
* Registered users of the site do not see these ads.
bboy is offline   Reply With Quote