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LGT AVCS Tuning Discussion


Infamous1
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I honestly don't know. I would imagine that if they stay the same it will be reasonably close. The flow in and out of the cylinder heads will stay pretty close, it is just what happens in the cylinder that changes the most.

 

If you are on E85 (as in, not knock limited) I would quickly try three different maps, one w/ 7 degrees less AVCS, one w/ 7 degrees more, and one the same. Do it all in close succession (to eliminate variables) and you should be able to see if your original ACVS was the best, or if you have some work to do.

 

Road tuning is free, but doing this on a dyno (load bearing) is the easiest.

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I've been playing with it all day and have had the best results with slightly tweaked stock AVCS settings. The higher CR does change VE a bit. A deeper vacuum is created in the cylinder so it should be able to suck more air in faster and then spit more exhaust out faster with less mixing of new/spent air.

 

But because I'm at 5500 feet, the higher CR makes the engine react more like it were at sea level... which is why I think my minimally tweaked stock table is best.

(Updated 8/22/17)

2005 Outback FMT

Running on Electrons

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But because I'm at 5500 feet, the higher CR makes the engine react more like it were at sea level... which is why I think my minimally tweaked stock table is best.

 

That's a cool idea. Have you done the math to figure out how much your altitude affects your CR?

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I havn't done the actual math, but my guesstimate puts it really close. Sea level is 14.7psi and I'm at 12.2psi. My guess is that it makes me closer to 14psi relative.

Not saying your wrong or right, but if this were true how would it react at sea level then? :confused:

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lots of detonation on gasoline?

 

I probably wouldn't have changed the CR if I was at sea level. You'd probably have to back way off the timing and boost to prevent knock. You'd get a serious power bump off boost though.

(Updated 8/22/17)

2005 Outback FMT

Running on Electrons

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In understanding the design principles of variable valve timing, let's see what OEM engineers have to say. This is a document on the Toyota VVT-i system similar to AVCS.

 

http://www.legacygt.com/forums/attachment.php?attachmentid=72197&stc=1&d=1250541521

 

You can see that at idle overlap is essentially eliminated (as opposed to a cammed pushrod V8). Overlap leads to a rough idle and more HC and CO emissions as the blow back exhaust remains in the chamber during combustion.

 

While cruising on the highway, overlap is either pretty low or effectively eliminated. This is again to keep HC and CO emissions minimized. Remember, if it can't pass emissions it can't be built.

 

Let's move on to certain types of medium load driving. This can result in a high exhaust temperature, which generates more NOx and overheats catalytic converters (meaning costly repairs). The recirculation of exhaust gas cools the combustion temperatures (EGR). EGR solenoids are duty controlled and the EGR valves would be particularly active on some engines (90s 6 cylinder engines especially) during this conditions . In this case Subaru uses the AVCS to accomplish that effect.

 

Finally, we have high load conditions. Maximum volumetric efficiency and torque is achieved through greater overlap in the lower rpm band and less up top. On engines without variable valve timing (oldschool V8's etc) this can be achieved with aftermarket cams, but with a decrease in fuel efficiency, emissions compliance, and idle quality.

 

Here are these design principles applied to a Subaru AVCS intake cam advance map:

 

http://www.legacygt.com/forums/attachment.php?attachmentid=72198&stc=1&d=1250541521

2005_LGT_AVCS_notes_2.thumb.jpg.24ed19b1ce656dde6c0f7d99bac41897.jpg

2005_LGT_AVCS_notes.jpg.081081e3cc5a5ca5381c05e526ae72b6.jpg

Edited by Boostin
On the search for a new DD...
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In the spool-up area I found I had to run a little less timing with more AVCS.

Yes this would be true.

 

I also found the same to be true on the 2.0l dual AVCS JDM Subarus as well in the upper rpms when retarding the exhaust advance.

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Yes this would be true.

 

I also found the same to be true on the 2.0l dual AVCS JDM Subarus as well in the upper rpms when retarding the exhaust advance.

 

Just restating:

 

Anytime VE is increased, timing would generally decrease. The lower rpm intake AVCS usually requires less timing, upper rpm exhaust retard requires less timing.

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  • 1 year later...

Bump... any new news?

 

I've read somewhere (nasioc avcs thread and it was clark that hinted to it) that if make it so both cams aren't the same there is some power to be gained but i'm also assumimg this only works if you have an Un-EL header. Has anyone experimented with this

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Bump... any new news?

 

I've read somewhere (nasioc avcs thread and it was clark that hinted to it) that if make it so both cams aren't the same there is some power to be gained but i'm also assumimg this only works if you have an Un-EL header. Has anyone experimented with this

 

AFAIK there is no way to do that via tuning.

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Bump... any new news?

 

I've read somewhere (nasioc avcs thread and it was clark that hinted to it) that if make it so both cams aren't the same there is some power to be gained but i'm also assumimg this only works if you have an Un-EL header. Has anyone experimented with this

 

What do you mean by this? You have to have a dual AVCS head to adjust the cams independently.

On the search for a new DD...
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I read that someone fixed the hole on the stock AVCS map, does anyone have that for a stock ish stage 2?

 

It would be a simple matter to download your current tune from your ECU, fill in the 'hole,' and upload it.

 

The question is, would anything be gained?

 

The 'hole' you refer to only exists at low loads and cruising RPM, so it's not going to give you any extra power. I think it's safe to assume that Subaru did it for emissions and/or mileage, and I don't see a reason to change it.

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It would be a simple matter to download your current tune from your ECU, fill in the 'hole,' and upload it.

 

The question is, would anything be gained?

 

The 'hole' you refer to only exists at low loads and cruising RPM, so it's not going to give you any extra power. I think it's safe to assume that Subaru did it for emissions and/or mileage, and I don't see a reason to change it.

I remove the 'tree-hugger' spike in the cruise area because it makes the part-throttle drive smoother. I also like CL/OL delay at zero. Personal preference that I pay with in a small increase in fuel consumption. I ramp the AVCS up and down differently during spool for the balance I prefer between spool, torque and smoothness.

Obligatory '[URL="http://legacygt.com/forums/showthread.php/2008-gh8-238668.html?t=238668"]build thread[/URL]' Increased capacity to 2.7 liters, still turbo, but no longer need spark plugs.
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As fahr_side said, removal of that emissions spike is a first and easy step, and will reward the driver with a significantly better running car in the low RPMs. That alone will positively influence around-town MPGs. Vacuum at idle went from 9.x to 10.x psi.

 

Doing nothing other than lowering the spike to surrounding value, I made the largest single impact on my 11 WRX's everyday driveability, re AVCS. AVCS was left untouched while getting boost, timing, and AFRs where I wanted them.

 

FWIW, a comparison between an ADM 07 WRX Tuned by STI ROM's AVCS map and the OTS Cobb Stg2 one revealed Cobb's map surprisingly similar, unlike previous LGT maps I'm more familiar with, and once that "spike" is normalized they're even more similar. Taking "tuning" the AVCS too far off what is apparently a finite 'path,' however, can ruin a good thing fast.

 

Again, there are very few tuning changes I've done that have made a more positive difference in how I perceive the car from the driver's seat than a careful pruning of that one area. Changes at higher RPMs and loads are recognized more by analyzing logs than by how the car drives. This is something everyone can do and notice immediately.

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As fahr_side said, removal of that emissions spike is a first and easy step, and will reward the driver with a significantly better running car in the low RPMs. That alone will positively influence around-town MPGs. Vacuum at idle went from 9.x to 10.x psi.

 

Doing nothing other than lowering the spike to surrounding value, I made the largest single impact on my 11 WRX's everyday driveability, re AVCS. AVCS was left untouched while getting boost, timing, and AFRs where I wanted them.

 

FWIW, a comparison between an ADM 07 WRX Tuned by STI ROM's AVCS map and the OTS Cobb Stg2 one revealed Cobb's map surprisingly similar, unlike previous LGT maps I'm more familiar with, and once that "spike" is normalized they're even more similar. Taking "tuning" the AVCS too far off what is apparently a finite 'path,' however, can ruin a good thing fast.

 

Again, there are very few tuning changes I've done that have made a more positive difference in how I perceive the car from the driver's seat than a careful pruning of that one area. Changes at higher RPMs and loads are recognized more by analyzing logs than by how the car drives. This is something everyone can do and notice immediately.

which spike... aka load vs rpm, it is blank, it should be blank...

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which spike... aka load vs rpm, it is blank, it should be blank...

 

Here is a screenshot that may help visualize the "spike." The table shows the values and the 3D graph provides a visual representation. It is the "stock" Stg2 11 WRX AVCS table, and represents a similar map in other cars.

 

That area works best with higher advance in general, but spiking, literally, in only those few cells causes driveability issues, along with wasting fuel. If a person is uncomfortable doing anything else, cutting that spike down to near the other cells will significantly improve the way the car drives. It will be smoother, have no rpm related studder or bumps, and mileage will improve.

 

That said you'll be a little less environmentally green.

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