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Old 12-21-2007, 03:25 AM   #1
sponaugle
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Default A look at the 2008 STI AVCS Exhaust Retard..

With the arrival of the new STI, the US market gets the Dual AVCS the JDM and European market has had for several years. Since we recently acquired one of these new STIs, I thought I would take a look at the changes.

The original AVCS on the US market STI allows advance of the intake cam up to ~25 degrees. ( The cam could be advanced more, but the stock ECU only does about 25 degrees of advance).

I made a diagram below that illustrates the relative timing of the intake and exhaust valves on the 04-07 AVCS cams:



Note that this diagram only shows 20 degrees of advance, only because that is the specification in the service manual. As you can see, with no intake advance, the exhaust valve closes at the same time the intake valve opens. In this case, the amount of intake valve advance is equal to the overlap.

Below is a diagram of a sample ECU run, showing the intake cam advance:



The cam advance starts out near 20 (once full throttle is applied), and rolls down to 0 by 5600 rpm. If you disable the AVCS, the lack of overlap reduces the low end torque and spool. If you leave the AVCS at 20 degrees up top, you will have reduced high end torque and power.

At maximum advance the intake valve opens as early as 15 degrees BTDC.

If we now switch to the 08 US STI, we see the effect of the exhaust cam retard:



From this diagram you can see that the intake cam advance has been extended a bit from 15 degrees BTDC to 25 degrees BTDC. In the case of no intake advance, the intake open point is the same as the previous years.

When at maximum advance, you could potentially have 10 degrees more of overlap. However the movement of the exhaust cams changes that. When the exhausts cam is in the rest state (not retarded), it closes at 28 degrees ATDC! With the exhaust cam AVCS unplugged, even zero AVCS intake advance would results in 23 degrees of overlap.

As soon as I saw a datalog from my STI I understood what was going on. Sure enough the exhaust cam Ďrestsí at -20 degrees of retard by the nature of the ECU calling for -20 degrees, which places the exhaust closeing at 8 degrees ATDC, much closer to the 04 5 degrees ATDC.

Letís take a look at a datalog from the 08 STI:



As you can see, the exhaust cam is used in the torque peak region only, and the rest of the pull stays at 20 degrees of retard. The intake Advance curve is similar to the 04s, falling to 0 at 5600 rpm.

However, if you take into account the combined angles of the cams you can figure out the overlap.



In the 08, the additional overlap from the exhaust cam movement gives a huge jump in overlap as the torque and boost peak.

There are lots of variables at play here, but there could be large potential upsides with early exhaust valve opening. Needless to say, stay tuned.

Cheers,

Jeff Sponaugle
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Old 12-21-2007, 04:11 AM   #2
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as always a good read
/subscribed!
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Old 12-21-2007, 04:57 AM   #3
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Interesting stuff, thanks Sponaugle. Now see what you can do with it!
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Old 12-21-2007, 06:01 AM   #4
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Quote:
Originally Posted by sponaugle View Post
.... there could be large potential upsides with early exhaust valve opening. Needless to say, stay tuned.

Cheers,

Jeff Sponaugle
You can say THAT again. Im jealous. Purely jealous.
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Old 12-21-2007, 06:14 AM   #5
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subcribed great info from the man
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Old 12-21-2007, 09:09 AM   #6
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may i attempt to CN what you said

old sti
intake open 15 btdc to 5 adc (range of 20?)
exhaust open 55 bbdc
exhaust close 5 atdc
exhaust duration 240

new sti
intake open intake open 25 btdc to 5 adc (range of 30?)
exhaust open 72 bbdc-32 bbdc
exhaust close 12 btdc-28 adtc
exhaust 240 duration

Last edited by Jaxx; 12-21-2007 at 11:07 AM.
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Old 12-21-2007, 09:33 AM   #7
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Quote:
Originally Posted by sponaugle View Post



From this diagram you can see that the intake cam advance has been extended a bit from 15 degrees BTDC to 25 degrees BTDC. In the case of no intake advance, the intake open point is the same as the previous years.
I wonder if the 07 STIs followed the same change in baseline intake cam advance before AVCS as the 08. I have heard severl people say that the 07 cylinder heads appear to have all the same parts as the previous years, but we all know that a tuned 07 can make more power.

I have found in tuning a Stage 2 07 STI that the AVCS map from the 04-06 STIs actually causes less power which I am surmising is due to my above question- if the intake cam is already mechanically advanced more (from 15 deg to 25 deg), then you would need LESS AVCS advance to make optimal power.

Anyone else have input?

BTW, to the OP, great comparison of the cam timing. I love a good, thorough analysis of things rather than the usual hearsay and speculation!
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Old 12-22-2007, 07:24 AM   #8
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Is this why the stock Timing maps were all over the place....

Realistically, With a BSP legal car...and the info your armed with now....what kind of power are you expecting to make on a pretty much a stage 2...

Yes...I too am getting excited...
Great Work...

Bill
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Old 12-22-2007, 10:33 AM   #9
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subscribed
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Old 12-22-2007, 03:26 PM   #10
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My plans were to keep mine as an AS car, but with new info coming out I don't know if I can contain myself!
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Old 12-23-2007, 05:01 PM   #11
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Quote:
Originally Posted by Scooby South View Post
Is this why the stock Timing maps were all over the place....

Realistically, With a BSP legal car...and the info your armed with now....what kind of power are you expecting to make on a pretty much a stage 2...

Yes...I too am getting excited...
Great Work...

Bill
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Old 12-23-2007, 09:18 PM   #12
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The 2008 is Dual AVCS arent they?
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Old 12-23-2007, 09:48 PM   #13
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I want an 08 ECU and Heads now along with the fuel pump. Nice work Jeff.
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Old 12-23-2007, 10:02 PM   #14
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Quote:
Originally Posted by tux121 View Post
The 2008 is Dual AVCS arent they?
Yes, Intake and Exhaust cams.
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Old 12-24-2007, 05:58 PM   #15
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Quote:
Originally Posted by sponaugle View Post
...

However, if you take into account the combined angles of the cams you can figure out the overlap.

In the 08, the additional overlap from the exhaust cam movement gives a huge jump in overlap as the torque and boost peak.

There are lots of variables at play here, but there could be large potential upsides with early exhaust valve opening. Needless to say, stay tuned.

Cheers,

Jeff Sponaugle
Liked your pun at the end.

One would expect that the larger valve overlap approaching peak load for the 08 would increase the proportion of exhaust gas trapped in the intake charge. What would that imply?

Trapped exhaust gas in the charge obviously raises the charge temperature and, as a diluent, slows combustion speed, which are both usually undesirable for avoiding knock. However, exhaust gas has an interesting chemical composition. According to "Design and Simulation of Four-Stroke Engines" by Gordon P. Blair on p. 425, the theoretic volumetric composition of exhaust gas from ideal combustion using super-unleaded fuel when lambda is 0.85 is:
6.7% CO
9.1% CO2
13.0% H2O
71.1% N2

Maybe increased valve overlap could be called internal water injection?

Last edited by Jon [in CT]; 12-24-2007 at 06:10 PM.
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Old 12-24-2007, 06:29 PM   #16
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Why? Why? Cosworth CamShafts on the way
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Old 12-25-2007, 04:57 AM   #17
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Quote:
Originally Posted by Jon [in CT] View Post
Maybe increased valve overlap could be called internal water injection?
Just speculating here, but... since you're "injecting" hot H20, in gaseous form, I'd be surprised if it still provided anywhere near the same advantage as convention water injection. That said, I'm really looking forward to seeing happens with all sorts of combinations of timing/rpm/load/boost/etc.
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Old 12-26-2007, 08:53 AM   #18
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Quote:
Originally Posted by Jon [in CT] View Post
One would expect that the larger valve overlap approaching peak load for the 08 would increase the proportion of exhaust gas trapped in the intake charge. What would that imply?

Trapped exhaust gas in the charge obviously raises the charge temperature and, as a diluent, slows combustion speed, which are both usually undesirable for avoiding knock. However, exhaust gas has an interesting chemical composition. According to "Design and Simulation of Four-Stroke Engines" by Gordon P. Blair on p. 425, the theoretic volumetric composition of exhaust gas from ideal combustion using super-unleaded fuel when lambda is 0.85 is:
6.7% CO
9.1% CO2
13.0% H2O
71.1% N2
I think they are introducing a type of EGR effect for emmissions. Adjustable EGR if you will. I don't think water at those temperatures would provide any cooling of the intake charge...
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Old 03-22-2008, 10:41 PM   #19
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Excellent discussion!!

Bit of a bump on this one, as I discovered this thread when I was looking for info on the dual AVCS heads.

For the experts....I'm considering an 08 STi motor swap for my car, and I understand they are running the dual AVCS heads.

It's going to be a huge head f*ck trying to wire up the dual avcs and run them with my MoTeC...so does anyone know if single AVCS cams (AVCS intake cam, fixed timing exhaust cam) will fit in these heads, or are they completely different?

Thanks
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Old 06-17-2010, 03:45 PM   #20
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Bringing this back from the dead.

What is the range of angles for IVC ABDC?
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Old 02-05-2012, 10:54 AM   #21
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Quote:
Originally Posted by sponaugle View Post
When the exhausts cam is in the rest state (not retarded), it closes at 28 degrees ATDC!
Bringing this thread back to life again.

This does not sound right to me. When you retard exhaust valve, you delay it's opening. How can the exhaust cam be at rest in maximum retard? If retarding the exhaust moves up the valve opening, that's called advancing, which is what is applied to the intake valve timing.

I would expect that the exhaust valve would rest at 12 BTDC, which seems more realistic. Its hard to believe that Subaru would run 23 degrees of overlap before AVCS kicks in. Let us consider the stock map on '11 STI. In the cruise conditions, the map calls for large exhaust retard and moderate intake advance with the purpose of creating EGR effect to control emissions and improve gas economy. If the exhaust valve timing moves in relation to 28 ATDC, the stock settings would create no valve overlap. Does that make sense? Another example, would it make sense to run overlap in high RPMs when you certainly not want it? I think the diagram for 08+ STI is correct but the interpretation is off.
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Old 02-06-2012, 07:46 AM   #22
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This is one Sponaugle thread that I've missed. Subscribed now...
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Old 02-13-2013, 10:33 PM   #23
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Funny how time flies..

I was just sitting here looking at my exhaust AVCS map. Perrin, Bailey, and I switched my 08 over to a Borg 7670 turbo, and am in the middle of getting it tuned up.

I did a quick google to see if there were many new threads, and this is one of the first threads that came up.. So I click on it and think hmm.. who is this sponaugle guy.

On a more serious note, I hope to be able to shed some more interesting analysis on the tuning side of things as I have outfitted my STI with three important sensors: Exhaust side pressure, Intake side pressure, and combustion pressure (using a fiber optic Optrand sensor). Lots of data to work through, but I hope there are some nuggets to be found.

Stay tuned.

Cheers,

Jeff Sponaugle
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Old 02-14-2013, 07:17 AM   #24
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Your findings and insight are eagerly anticipated.

Frank
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Old 02-14-2013, 01:36 PM   #25
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Quote:
Originally Posted by sponaugle View Post
Funny how time flies..

I was just sitting here looking at my exhaust AVCS map. Perrin, Bailey, and I switched my 08 over to a Borg 7670 turbo, and am in the middle of getting it tuned up.

I did a quick google to see if there were many new threads, and this is one of the first threads that came up.. So I click on it and think hmm.. who is this sponaugle guy.

On a more serious note, I hope to be able to shed some more interesting analysis on the tuning side of things as I have outfitted my STI with three important sensors: Exhaust side pressure, Intake side pressure, and combustion pressure (using a fiber optic Optrand sensor). Lots of data to work through, but I hope there are some nuggets to be found.

Stay tuned.

Cheers,

Jeff Sponaugle
Jeff, please keep us informed. I am very much curious to see what you record and what kind of conclusions we can draw from your data. We'll have to keep in mind that this will be in context of the Borg turbo but the info will be very interesting nonetheless.
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