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Old 01-15-2005, 10:16 AM   #1
crazyhorse
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Default Great AVCS explanation!

I know there have been threads in the past but this is a great resource. I found it in the Drive magazine issued to our dealership and then located it online for you all to check out. I'm sure there are still plenty of people that have no idea how AVCS works.

AVCS explained: http://www.drivesubaru.com/Win05_WhatsInside.htm
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Old 01-15-2005, 11:05 AM   #2
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That is a pretty good explanation.

For cars with variable intake cams as well the later the intake cam timing the more the overlap, and the more it allows port-flow to continue since intake reversion will occur later in the cycle.

Then there's VTEC, which actually just runs two different sets of cams. Weird stuff.

Adrian~

edit: Waaait a minute. Intake variable timing? I thought it was exhaust?

They also confused advance with retard. While the cam's graph appears to move forward with respect to time, the valve will open and close LATER and thus I would view them as "retarded" and not advanced since they will actuate at a later angle.

Last edited by SaabTuner; 01-15-2005 at 11:11 AM.
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Old 01-15-2005, 11:46 AM   #3
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Nice find. I added it to the 2.5 FAQ thread.
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Old 01-15-2005, 01:00 PM   #4
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Thanx crazyhorse. . . Found that very useful.
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Old 01-15-2005, 01:04 PM   #5
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Always glad to help.. No one here to buy cars from me so I figured I might as well make myself useful
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Old 01-15-2005, 02:58 PM   #6
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Great read.

Don't know if I'm the only one, but I actually had the AVCS cam sprocket fail on my STi. Made a horrible clicking sound under certain conditions.

Here was my log of the event and repair... AVCS Cam Sprocket Failure
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Old 01-15-2005, 05:43 PM   #7
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Quote:
edit: Waaait a minute. Intake variable timing? I thought it was exhaust?
This is currently only available in JDM models, unfortunately and believe me makes much difference especially in spool ups!
JIM
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Old 01-15-2005, 06:08 PM   #8
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Quote:
Originally Posted by SaabTuner
edit: Waaait a minute. Intake variable timing? I thought it was exhaust?

They also confused advance with retard. While the cam's graph appears to move forward with respect to time, the valve will open and close LATER and thus I would view them as "retarded" and not advanced since they will actuate at a later angle.
it is advance, and the graphs show it clearly because it's not the exhaust valve that changes.
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Old 01-15-2005, 07:53 PM   #9
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thanks for the info

but i still don't understand why advance more timing will cause denotation and how higher octane gas will solve the problem?

Last edited by kool168; 01-15-2005 at 08:01 PM.
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Old 01-15-2005, 09:00 PM   #10
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Quote:
Originally Posted by SaabTuner
Then there's VTEC, which actually just runs two different sets of cams. Weird stuff.
Incorrect. Read the new Sport Compact Car... It explains it fairly well.
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Old 01-15-2005, 09:12 PM   #11
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Quote:
Originally Posted by kool168
thanks for the info

but i still don't understand why advance more timing will cause denotation and how higher octane gas will solve the problem?
AVCS affects the valve timing, which is completely independent from ignition timing, which is what you're referring to. Ignition timing is a topic for another thread.
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Old 01-15-2005, 11:40 PM   #12
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Quote:
Originally Posted by awilson529
Incorrect. Read the new Sport Compact Car... It explains it fairly well.
VTEC doesn't run two sets of actual camshafts, but that's silly. I'm sure it's not literally what he meant.

For a pair of valves, the camshafts have a separate 3rd lobe between the two lobes that are normally there, and when VTEC activates, it does something funky I never quite understood to start making both valves follow the more agressive center lobe.

So while AVCS can change overlap and cam timing, VTEC can actually change lift and duration. Although I think some newer honda engines actually do both. They have variable cam timing like AVCS as well as the 2nd set of cam lobes that can be activated.

-Mike
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Old 01-16-2005, 05:33 AM   #13
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Quote:
Originally Posted by grippgoat
VTEC doesn't run two sets of actual camshafts, but that's silly. I'm sure it's not literally what he meant.

For a pair of valves, the camshafts have a separate 3rd lobe between the two lobes that are normally there, and when VTEC activates, it does something funky I never quite understood to start making both valves follow the more agressive center lobe.

So while AVCS can change overlap and cam timing, VTEC can actually change lift and duration. Although I think some newer honda engines actually do both. They have variable cam timing like AVCS as well as the 2nd set of cam lobes that can be activated.

-Mike
im stuck at the same place as u about v-tec. i know what it does but im not sure w/ all of the mechanical parts about it. i know that once u reach predetermined RPM a solinoid moves the cam to provide w/ a bigger lift and overlap. but im not sure about duration. i cant see how they can make the valve stay open longer.
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Old 01-16-2005, 06:18 AM   #14
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The only thing I'm not really sure about is the exact mechanics of how the VTEC rocker arm gets engaged.

As for how they keep the valve open longer.... It's a completely separate cam lobe. They can grind it any way they want. They just have to grind it with a longer duration.

-Mike
(Former NSX owner... Turbo > VTEC )
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Old 01-16-2005, 06:33 AM   #15
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If you retard the cam timing DURING valve actuation..then you could extend the duration
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Old 01-16-2005, 07:45 AM   #16
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Quote:
Originally Posted by JIM THEO
This is currently only available in JDM models, unfortunately and believe me makes much difference especially in spool ups!
JIM
I believe the new USDM Legacy has intake and exhaust.
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Old 01-16-2005, 08:49 AM   #17
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Great find - it was always a question if we had AVCS.

By advacing the intake sooner you can help the turbo spool up faster.
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Old 01-16-2005, 02:44 PM   #18
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Quote:
Originally Posted by Abe Froman
I believe the new USDM Legacy has intake and exhaust.
yeah thats true. but in japan the AVC actualy gives it more lift as well, not just overlap
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Old 01-16-2005, 04:27 PM   #19
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VTEC- you're dealing with changing the cam profile during low and high rpms. a follower connects three cam lobes together through use of oil pressure. since the high rpm lobe is longer, the cam profile of the other two lobes becomes longer, hence the term LIFT.

AVCS- on the other hand, is just adjusting the cam timing (advancing or retarding) by degrees of up to 35 degrees advance. this allows more air/fuel into the combustion chamber, but uses the ECU to determine the ideal position of the camshaft according to sensors.

AVCS in a way is much like VTEC, however you're just dealing with two different approaches to the system. one is using cam timing to adjust a/f ratios, while the other is mechanical and uses valve lift at higher rpm's for better fuel economy at high rpm's.

hope this helps.

VTEC explanation

and the AVCS one is above.

Last edited by clamdip; 01-16-2005 at 04:42 PM. Reason: added information
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Old 01-16-2005, 04:36 PM   #20
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Quote:
Originally Posted by grippgoat
The only thing I'm not really sure about is the exact mechanics of how the VTEC rocker arm gets engaged.
-Mike
(Former NSX owner... Turbo > VTEC )
The oil pressure engages a pin and "locks" the rocker arm together.
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Old 01-16-2005, 05:25 PM   #21
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I had this article a long time ago, I think it originated from Subaru of Australia, but then couldn't find it. Good explanation of how the system works.
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Old 01-17-2005, 11:15 AM   #22
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Incidently... The current 3.0L H6 has variable lift as well. The camshafts actually shift to a higher lobe, etc.
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Old 01-17-2005, 12:56 PM   #23
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Quote:
Originally Posted by crazyhorse
Incidently... The current 3.0L H6 has variable lift as well. The camshafts actually shift to a higher lobe, etc.
that is correct. but it only does it on the intake. i belive.
now we need that on both the intake and exhaust, for the ej series
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Old 01-17-2005, 02:58 PM   #24
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Quote:
Originally Posted by crazyhorse
Incidently... The current 3.0L H6 has variable lift as well. The camshafts actually shift to a higher lobe, etc.
you mean the legacy's have lift too? not the ones here in the states right?
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Old 01-17-2005, 03:34 PM   #25
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I dont know where all the misinformation is comming from but IIRC all the 2.5L blocks (sti, fxt, leg and so on) all come with AVS just on the intake side.
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