AVCS Timing And Overlap (2005-06 vs 2007-09)

[covertrussian]

While playing with AVCS valve timing in the fuel economy thread I came across some interesting information that explains a lot of AVCS timing and Timing differences between the 05-06's and 07-09's.

 

First lets get similarities and differences out of the way, this is data from FSM's

Intake Cams: 05-09's use the same intake cams: 26.55mm Cam Lobe height, 240* duration.

Exhaust Cams: They all have the same cam lobe height (46.75mm), but have different durations. 05-06's have 230* duration, while 07-09 have 240* duration exhaust cams. NOTE: I think the FSM typo is setting the 05 Duration down.

Valves: 05-09's all have same valve lengths

Compression Ratio: 05-06 = 8.2:1, 07-09 = 8.4:1

 

Valve Timing:

2005 NOTE: I believe there is a typo in the 2005 FSM, it says Exhaust Cam closes at 5* BDTC, while every other FSM says 5* ADTC

http://legacygt.com/forums/attachment.php?attachmentid=239734&stc=1&d=1477486376

 

 

2007-2009:

http://legacygt.com/forums/attachment.php?attachmentid=239738&stc=1&d=1477486376

 

Now I had a hard time visualizing that and calculating the overlaps with that data, so I graphed it on the degree wheel:

 

2005-2006:

NOTE: I believe there is a typo in the 2005 FSM, it says Exhaust Cam closes at 5* BDTC, while every other FSM says 5* ADTC

http://legacygt.com/forums/attachment.php?attachmentid=239743&stc=1&d=1477488132

According to this, 20* of AVCS is the max we can run on these motors. I believe we can run more though, more on this later.

 

2007-2009:

http://legacygt.com/forums/attachment.php?attachmentid=239744&stc=1&d=1477488132

According to this, 40* of AVCS is the max we can run on these motors.

 

The two have very different intake cam timings, 07-09's have the ability to advance their cams by 20* for one. The next part is actually the biggest difference, since 07-09's have 10* more duration on the exhaust cams, their exhaust valves are closed right as intake valve opens with 0* AVCS. While for 05-06's the exhaust valve closes 10* before the intake opens at 0* AVCS.

 

Overlap

Just get to the point already, what does all of this mean when it comes down to overlap?

 

For 05-06's since we now know that 0* AVCS is actually 10* after exhaust valve is closed, our 0* overlap is actually at 10* of AVCS advance. You can easily calculate your overlap by subtracting 10* from the current value.

 

05-06 Stock AVCS Map:

http://legacygt.com/forums/attachment.php?attachmentid=239736&stc=1&d=1477486376

 

05-06 Actual Overlap:

http://legacygt.com/forums/attachment.php?attachmentid=239737&stc=1&d=1477486376

 

For 07-09's, since the exhaust valve is closed right as intake valve opens at 0* AVCS, that means what you see in the AVCS table is what your actual overlap is.

http://legacygt.com/forums/attachment.php?attachmentid=239740&stc=1&d=1477486376

 

 

Idea: Swapping parts

If the the valve clearance allows it, 05-06 guys should be able to use 07-09's exhaust cams to gain 10* of duration (more power up top) and gain more overlap (would only benefit big turbo guys).

 

I was gonna suggest swapping AVCS cam gears, but It looks 05+ all have the same cam gears, which means they all can do +40* of advance. That means the reason 05-06's FSM says 20* of advance is because that's all they do in the tune. Please be careful when going above 20* on 05's, since valve clearance can be different then 07-09's.

[NSFW]

Nice. I had no idea that the 07-09 exhaust cams had more duration.

[covertrussian]

Yeah that was a pretty big surprise to me too. I was also very surprised to find out that we, 05-06's, have 10* of play before any overlap is introduced.

[Underdog]

Thought you might be interested in the specs from my Tomei Poncams (#143062). I'm running them on lightly ported B25 heads with stock valves and springs.

[covertrussian]

Alright if I'm reading that right, at 0 AVCS you have 10* of overlap, and at full intake advance (AVCS 20* on 05-06) you would have 25* of overlap. That's quite a bit for the stock header. I wouldn't run these cams unless your running a good aftermarket header too.

[Flinkly]

he's got twinscroll EL headers and up from JDM STI. twinscroll gives him ridiculous time between valve openings (exhaust, ovi), and much more equal pulses.

[covertrussian]

Is Tomei selling those cams mainly for TS applications though? It's one of those mods that could make you loose power on factory header/turbos.

[covertrussian]

This has been bugging me lately, 2005 Legacy overlap figures were just not adding up, so I started downloading every FSM I can find and comparing the exhaust valve closing time.

 

Every turbo 2.5L car's (even 2006 Legacy's) FSM says the exhaust valve closed at 5* ATDC. Odd ball is 2005 Legacy with 5* BTDC.

 

Searching for cam part numbers and every single avcs EJ255/7, till 2009, is said to use the following cams, which are the latest revision:

Left: 13052AA352

Right: 13034AA352

 

I thought maybe the latest version is ground differently so started looking up part numbers, original part number was: 13034AA350. This cam is said to be used by EJ255/7 up to 2006 and ll those cars say ATDC for exhaust valve closing, thus it can't be different cam grinds

 

Which brings me to I think there is a typo in the 2005 LGT manual (says BTDC instead of ATDC).

 

Why is this important? Well it drastically changes how you read the AVCS map. It changes how you tune things too, because now 10* of AVCS means 10* of overlap, which means more reversion and or fuel blowing out.

[covertrussian]

To know actual overlap figures we would need to find duration at 1mm lift for stock cams, does anyone have that handy?

 

Update: Just found this:

The duration at 1.27mm is 204/208...

 

With those numbers my quick math shows this:

Exhaust Valve Closing = 11* BTDC

 

AVCS 0 Intake Valve Opening = 23* ATDC

AVCS 0 Intake Valve Closing = 47* ABDC

 

So with 1.27mm lift we are not seeing overlap until AVCS is at 34*, doesn't sound right though

[covertrussian]

I was curious to see how much overlap 2002-2005 EJ205 WRX's had, since they don't have AVCS.

 

Manuals pointed out these figures:

http://legacygt.com/forums/attachment.php?attachmentid=256184&stc=1&d=1506952411

 

Plotted against the Degree wheel, we can see that they have 16* of overlap static.

http://legacygt.com/forums/attachment.php?attachmentid=256185&stc=1&d=1506952411

 

This makes them far more sensitive to exhaust back pressure, which means they would benefit the most from an equal length header.