After a lot of questions and not a lot of answer on this forum. I figured I'd make a AVCS FAQ.
What is does the name AVCS imply?
It stands for Active Valve Control System.
What does it do?
Its the system Subaru developed to vary cam timing. In other words its a complicated system that can change how
valves open and close to change the fueling and exhaust of each cylinder.
Which USDM WRX/STI models have this?
The difference being that the 08 and up STIs have Dual AVCS (according to Cars 101 Subaru Research Site
What is different in dual AVCS vs single AVCS?
In the very basic sense. Single is Intake side only on the Intake side and Dual is Intake and Exhaust Side.
How does this affect my engine?
Lets try and understand how this relates to the cam. The cam is a rod with a series of bumps or lobes. When the
cam spins it opens and closes the valves. Normally the rate at which the valves open and close is a fixed rate.
The timing belt controls how fast the cam spins. Which would control how fast the valves open and close. ACVS
AVCS allows the cam to advance and retard the timing, on the fly. It does this through a series of oil pressure
switches and solenoids. Instead of the cam being on a fixed axis, you can not make the valves open sooner or later in
the combustion cylce. Why would you want to do that?
Altering how you can change the air and fuel mixture determines how you can make your power. In a turbocharged car,
that means you can take advantage of your exhaust gasses. Which in turn can help you increase your spool characteristics,
if you can vary the valves to get the pressure to the turbo faster.
What is the major technical difference in Dual vs Single?
Well, basically the overlap of the cams is the major difference.
Single AVCS allows for 40 degrees of advance on the Intake cam, combined with the fixed exhaust cam. Which gives a total
variable overlap of 0 to 40 degrees.
Dual AVCS allows a more restricted 30 degrees of Intake cam advance, to combine with 40 degrees of exhaust cam retard.
Combined the two allows for a total variable overlap of -17 degrees (no overlap) to 53 degree of overlap. Not only do you
get more overlap, but for a given overlap you also have the ability to move the overall cam timing.
What does that mean in real life?
In short, dual AVCS allows for slightly faster spool and slightly more torque down low.
Would this be a worth while mod? Converting my single to dual?
NO! Its not worth it. The only individual that would discuss this conversion in depth was Ron from rawperformance and this is
what he said.
"You can do dual AVCS, but you will need to totally swap out your cylinder heads and cams, the entire engine wiring harness,
the front bulkhead harness, the harness from the main connection to the ecu, and you need the dual AVCS ecu. You will also
run into some issues with the speedometer funtions as the 08 cars get the signal from the ABS sensors, and the earlier cars
get it from a speedo sender in the transmission itself. For all the effort you are going to put into performing the swap,
the power gained will really not be worth it. You would probably be better off running a set of aftermarket camshafts in
your existing heads. Price would be similar and gains would be more with the aftermarket cams
Thats pretty much all I gathered so far. I hope it helps anyone that was wondering these questions. If anyone else knows
more about this subject and wants to help me correct/update this. That would be welcomed.
My Sources are:
Modifying the Subaru STI AVCS | Driving Sports
Raw Performance Home
Cars 101 Subaru Impreza
NewToTheGame - a.k.a. Josh helped me on my search for information
and some forum searching on other forums.