Avcs Faq - Subaru WRX Forum
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#1 Old 01-26-2010, 04:29 PM
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Avcs Faq

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?

2006-2010 WRX
2004-2010 STIs
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
fixes this.

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.

Kirt
378awhp/373wtq Custom Dom 3 & CBRD tuned 2006 STI.
"When in doubt...flat out" - Colin McRae
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#2 Old 01-28-2010, 04:17 AM
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Aftermarket cams are a tradeoff; you sacrifice driveability (smooth, low-rpm idle) and low-end torque for more power uptop. I was under the impression that the point of having AVCS (and dual AVCS) was to reap some of the benefits of cams without any of the drawbacks. If this is the case, then aftermarket cams wouldn't really be a "replacement" for AVCS/dual AVCS, right?
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#3 Old 01-28-2010, 08:47 AM
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Nice writeup turmic. I tried to rep you again, but I still have to spread more around - which is tough to do considering there have been so few posts lately. What happened to everyone?

John
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#4 Old 01-28-2010, 09:05 AM
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Nice writeup turmic. I tried to rep you again, but I still have to spread more around - which is tough to do considering there have been so few posts lately. What happened to everyone?
Eh, the rep system doesn't work right. I use manual intervention when the need arises.

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#5 Old 01-28-2010, 09:40 PM
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LOL mosc, Thanks btw

WReXd: Ron's statement was aftermarket cams to a single AVCS system, would be more worth it, than the swap. If you don't have AVCS...you're just changing your fixed rate to the higher profile all the time. Some avcs has to be better than none at all. But, i wouldn't know how difficult that would be. If you have any insight...please let me know.

Kirt
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#6 Old 02-06-2010, 11:01 AM
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I had the same thought in mind, and did a little digging myself. Since my build is going to incorporate a new ECU, I emailed Phil at Element Tuning a while ago if this is something the Hydra 2.6 could do. His response was that they did make a plug-and-play ecu for the 07 STI with the dual avcs option but it would be a special order, which was fine by me. But my impression was that I would need to run additional wires for the sensors/control, not replace the entire engine wiring harness. Might need to revisit this again if that's the case.
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#7 Old 02-08-2010, 05:03 PM
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Quote:
Originally Posted by Amuzzing1 View Post
I had the same thought in mind, and did a little digging myself. Since my build is going to incorporate a new ECU, I emailed Phil at Element Tuning a while ago if this is something the Hydra 2.6 could do. His response was that they did make a plug-and-play ecu for the 07 STI with the dual avcs option but it would be a special order, which was fine by me. But my impression was that I would need to run additional wires for the sensors/control, not replace the entire engine wiring harness. Might need to revisit this again if that's the case.
It's what ron told me. I'd be very interested in your build and how Phil at Element tackles it. It'd be a wealth of knowledge on how it can be done. So, please let us know if you go forward on doing the dual AVCS upgrade. Plus, let us know how difficult and expensive it is. If you could...thanks.

Kirt
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#8 Old 02-09-2010, 07:04 AM
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Originally Posted by turmic View Post
It's what ron told me. I'd be very interested in your build and how Phil at Element tackles it. It'd be a wealth of knowledge on how it can be done. So, please let us know if you go forward on doing the dual AVCS upgrade. Plus, let us know how difficult and expensive it is. If you could...thanks.
Yup, Iíll keep you posted. What Ron told you is correct in the sense that itís not worth the money if all you are doing is trying to add dual avcs on your engine. My dilemma is that Iím most likely going with new heads and an aftermarket ecu, so the only difference for me would be to get the 08+ heads instead and wire up the electronic parts (+ oil lines, etc). Fit wise, I was told that they would bolt up perfectly with the í07 block.

@WReXd
A few other builders told the same thingÖ that aftermarket cams were a better bang for your buck but you are right, swapping out the cam doesnít give you all the benefits of having avcs on the exhaust. Again, in my case Iíll be 95% of the way there, so Iím trying to figure out what the return would be for that extra 5% . Need to do some more digging, still not convinced one way or another.
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#9 Old 05-16-2011, 05:53 PM
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I may be showing my age here... but VTEC VVT VVL AVCS or whatever you want to call the system, has it's uses (theoretically) But I've Allways found they actually have Less down low than a good ol 2 Valve engine with a nice old fasioned Rally cam (peaking at around 7500rpm) But this can be put down to multi valve engines sucking ass in low revs in general!

Now imagine if you could get decent low end with 27/65/65/27 angles like the 1600 Kent crossflow & even the lotus1600 Twin cam 8V has... and that Sound! exuse me, I have a mess to clean up now...
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#10 Old 05-16-2011, 07:25 PM
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Quote:
Originally Posted by 4kita View Post
Now imagine if you could get decent low end with 27/65/65/27 angles like the 1600 Kent crossflow & even the lotus1600 Twin cam 8V has... and that Sound! exuse me, I have a mess to clean up now...
^ What the? ^

Kirt: you come up with the extra 5% yet?

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#11 Old 05-16-2011, 10:49 PM
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Brad: Nope, he never responded.

4kita: What are you talking about? The original V-TEC was limited to just only helping on the high end. IVTEC is much more like every other electronic Variable Valve Timing set-up.(Read: Just about everyone uses a electronic VVT these days. Why? Better gas mileage and performance out of the engine.) It can help the down low portion of your engine just as much as the high end. Especially Dual AVCS where you can control the exhaust side timing with the intake side. That's why 08+ STIs can gain TQ over a compariable older gen STI. Better and quicker spool on the low side.

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#12 Old 05-23-2011, 10:59 AM
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Hi guys

I've got a few questions about AVCS that I hope somebody can answer.

I recently bought a new, built longblock for my 05 STi. I got a great deal on the cylinder heads and they included BC 272 non-AVCS cams.

I got the new block installed and ready to fire. Tried starting, the car would fire, run very poorly for a few seconds and die. CEL came on, so we started there. P0345 (Camshaft Position Sensor A Circuit Malfunction (Bank 2))

So I unplugged the sensor and checked the plug for continuity. Check. Swapped the bank 2 CPS with the bank 1 CPS, reset the CEL and tried firing again. Same result, same code. It appears the sensor is not the problem.

So I unplugged the CPS on bank 2, started the car and after several seconds of cranking, the car fired up and ran fine. Car drove just fine, although since its on break in, it hasnt seen more than 3k rpm, and little boost.

My questions are:
1. Do I have to retune the ECU so that it knows the cams being used are non-avcs?
2. Why does only bank 2 show an error, but not bank 1?
3. Could the crankshaft position sensor have anything to do with this?
4. What would you do?

Thank you in advance for any help. Im really stuck here and not real familiar with AVCS. You can respond here or email me, sutton.tomt@gmail.com
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#13 Old 05-23-2011, 08:16 PM
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First off why did you get the non AVCS cams? Are the cams the kind that just need the screws pulled out of the ends to make them AVCS capable. I couldn't tell you which screws. Wagonracer or someone else would have to clarify how to do that or which cams would be capable of that. The second thing is this should've been a new post in problem diagnosis section. I understand why you put it here. But, problem diagnosis would get you a lot responces for more help.

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#14 Old 05-25-2011, 08:42 PM
 
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I can't be much help with this one, having basically no personal experience with AVCS engines aside from some fairly simple troubleshooting with issues that came through the shop. I will take this up with the chief tech at work, and see if he can assist. I'll post what I learn tomorrow
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#15 Old 05-26-2011, 01:35 PM
 
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Here's the scoop, yes you'll need it tuned esp. to run without CELs. The ECU is looking for input that's not there, since the cam pulleys are non AVCS. The oil switching valves are also serving no purpose which is "confusing" the brain.

Subaru always defaults to bank 2, don't know why, just that's how the ECU works.

Those cams cannot be changed over to AVCS according to our tech.
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