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Author Topic: Lean out upon cam advance?  (Read 18278 times)
jpurban
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« on: June 02, 2018, 07:22:59 PM »

Background...  I've noticed a temporary (about a second or so) lean out condition immediately after camshaft advance (about 30 degrees).  I have 2 settings on my camshaft advance 0 degrees and 30 degrees.   The spike in lean out varies from 20% leaner when cold to about 10% leaner when warm.  Generally, I don't notice this when driving with a warm engine.

Question...  Is there a good reason for this, is it by design (e.g. cutting spark during the changeover)?  If not, have any of you corrected the lean out? 

I've tried to search for some answers on this, but have found anything.  There wasn't anything obvious to me in the FR.  So, I'm asking for a little help.
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vwaudiguy
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« Reply #1 on: June 02, 2018, 09:28:57 PM »

Can you feel it while driving?
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mister t
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« Reply #2 on: June 07, 2018, 10:12:21 PM »

Lol, yeah, that's what happens when you change your cam timing.... It shouldn't come as a surprise.

Cam timing makes a massive difference in how your engine flows air.

Most tuners who deal with forced induction don't really know what to do with cam timing. However, having tuned 2 N/A 4.2 V8 S4's I've learned that cam timing can make some HUGE differences to the torque curve when done correctly.
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jpurban
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« Reply #3 on: June 13, 2018, 08:00:59 PM »

Mister T, I experience the lean out described above at STOCK cam timing -- I didn't make any changes.

Porsche 996 Turbo Variocam Plus doesn't have an EGR.  They use cam overlap at low RPM (cam advances no later than 1,500 rpm, even when cold) as a de facto EGR.

My thinking continues to evolve on this lean out, but ultimately I think the lean out is a direct result of the major difference in KFURL at low RPMs between the 2 cam settings 0 degrees and 30 degree advance.  The root cause of the problem is aggravated by insufficient enrichment on cam change (KFNWSL/H), likely caused by my upgrade from the OEM Siemens injectors to EV14s.  Since I don't have an FR specific to my car and the available FRs don't have any info on the KFNWSL/H tables and have been flying blind trying to adjust these tables.  The values vary from 0.92 to 1.00, but I don't know if these are lambda targets, multipliers or divisors of a target lambda. 

Just today, I tried a different approach...  I pushed the cam change above normal "street driving" range.  The ME7.8.4 seems to be a little more sophisticated regarding cam change than the Audi ME7.  I can specify the changeover point by 1) rpm, 2) load and, get this... 3) gear.  OEM settings are the same across gears, btw.  I basically logged to determine my street load profile by rpm and gear, then updated the table to "push" the cam change much higher, from 22% load (essentially idle) to 75% load.  Just think about what the OEM did for the sake of emissions... They switched from "torque" setting to "power" setting immediately off idle.  What a terrible emissions vs performance trade off!

Since the retarded position (0 degrees) is the "torque" setting, my part throttle, off boost behavior improved significantly, as you might expect...  Of course, this benefit comes at a cost of worse emissions due to the loss of the EGR effect (not that I care about the environment -- already stripped out my secondary air).  Still have some lean out, likely related to the the injector conversion and imperfect WFRL/BAKL/VAKL values, but it is much lower than when combined with the cam change (1.05 lambda peak vs 1.20 with a 1,500 rpm, cold engine cam changeover).

Now, I'm wondering if I should take it higher still... I've read many have pushed their cam change all the way up to peak boost or further (for faster boost spool up and improved boost PID behavior), which would probably be around 3,000 to 3,500 rpm and 220% load for my billet K16s.  That seems like a long way from the stock settings.  At my current settings, 75% at 3,000 RPM, the cam change happens before I build any boost.  So, I'll probably try higher load values for changeover, but I'll do it in multiple steps.

Care to offer any advice?  This is the first car I've tuned with variable cam/valve timing... For that matter, I'd be interested in how you optimize the variable valve lift too.  I can't find shit on variable valve lift regarding the ME7 control tables/scheme.
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zweistein
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« Reply #4 on: June 15, 2018, 07:42:13 AM »

interesting topic you are in:  I just looked into KFNWNGNG , and I see where you changed the load values to go to "torque" profile.  I suppose at idle rpm you do keep the 105% load value.

for the Valve lift control I see in VS:  MIVSNG  . It seems that valve lift is controlled by load and above certain load values valve lift increases.

As also cam time is load dipendent  both are influencing each other. 

In 1 month I can try this on a car with logging, time permitting I will report back.

Thank you for your findings
Andreas
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jpurban
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« Reply #5 on: June 15, 2018, 02:36:09 PM »

Andreas,

Thanks for the response. I'm glad to have someone else reviewing this.

I did some research on the Variocam Plus system and learn some interesting things that are especially relevant. First, the system varies the intake cam timing and intake valve lift in a binary fashion. Intake cam has either 0 degree overlap, used only for idle (really), and a 30 degree overlap, used everywhere above about 1,200 rpm.  The intake lift also has 2 settings. It is either 3mm (idle and low rpm) or 10mm (high rpm).

This information really changes the cam discussion. It isn't worthwhile to maintain zero overlap WITHOUT also altering the valve height to 10mm at low rpm.  Here's why... The overlap improves intake flow at low valve height because air can be pulled from both the exhaust side (recirc) and the intake side, minimizing pumping losses.

Delaying the cam changeover while still in low valve lift actually REDUCES your ability to produce "torque" and impedes boost spool due to reduced mass flow (from pumping losses arising from high cylinder vacuum).

I verified this myself in my logs.  At higher rpm, my boost immediately increases upon cam changeover WHEN in low valve lift.

So, altering cam changeover on the Variocam Plus system isn't as straightforward as what has been done by others on their Audis with different VVT strategies.

At this point, I'm not sure how to move forward. Because I'm not sure of my valve clearance, I don't know if I can safely run a retarded cam (zero overlap) while also running the valve in high lift. If the Porsche engineers really milked the design (resulting in limited valve clearance), you can't do it without valve-piston interference.

So, for now, I'm left with a choice between lean out on cam change (stock settings) or lower power from a higher cam changeover at low valve lift. I guess I'm back to looking for a way to address the lean out with fueling.  I'll post some logs of the issue so you can see the undesirable lambda coupled with a drop in load and requested torque, which has to be the reason for the fueling shortage.

Any thoughts?
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prj
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« Reply #6 on: June 15, 2018, 02:42:46 PM »

I'll tell you how to move forward.
Don't do anything at all. Literally.

If you have serious backpressure issues on top you can try to move it, but the way the engine is designed I think it will hurt performance everywhere.

The reason the cam even moves is to make idle acceptable and not sound like a bag of sh...
As for leanout, well, the lambda will show a bit leaner during spool due to scavenge effect.
It doesn't mean the mixture is that lean, it's kinda tricky there.

And without proper logging it's difficult to tell. I could check my logs from when I was tuning to see?
I had some 800+ cc injectors... and full ram logging.
« Last Edit: June 15, 2018, 02:46:16 PM by prj » Logged

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jpurban
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« Reply #7 on: June 15, 2018, 03:37:43 PM »

PRJ,

Thanks for joining in...  I can really benefit from your experience.  I'll take any advice you have to offer.  Hell, I'm so frustrated with this niggling issue that I'd pay for help.

I've attached a short log that shows the lean out following the cam change on a cold engine (45C).  This is a good example of my issue after adaptation has had a chance to "work" (the lambda spikes above 1.2 immediately after a fresh flash).  Unlike the stock 996 Turbo, I've enabled transient adaptation (CUKA) and this reflects the best it can do.

Please know that I'm logging with Durametric and they don't use any of the Bosch terms (like PSSOL for Boost Setpoint).  So, I'm left to my own devices to figure out what variable they are really logging.  I have no idea what "calculated mass flow" represents, as an example.  BTW, I called Durametric and they wouldn't tell me...  as they're clearly part of the glass bead society.  They said, "Call your tuner".

As you review, note that my requested torque and ignition angle drop on cam changeover, which probably has something to do with the lean out.  My problem...  I don't know how to appropriately address it.  I found cam changeover enrichment tables (KFNWSL/H), but don't know what to do with them without an FR that explains how they work.  I'm not even sure they are enabled at this point (since I've tried various values and seen no detectable change).

BTW, this lean out isn't a scavenging event.  RPM is too low for that to happen.  I doubt backpressure is a concern since I have high flow headers and cats and an exhaust much less restrictive than stock.  The K16 turbine is perfectly sized for my application, but the compressor side got a larger, billet wheel upgrade.  Anyway, this lean out results in stuttering and poor throttle response, albeit only briefly and noticeable only when cold, but I still would like to address it.

Any and all advice is welcomed.
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zweistein
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« Reply #8 on: June 16, 2018, 12:44:05 AM »

if lean out is only a concern at warm up,  in the 997S tables there is KFBAKL, enrichment based on engine temp. Should exist also on 996tt.

p.s. wenn du Deutscher bist, schick mir doch dein mobile per PM.

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Andreas
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prj
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« Reply #9 on: June 16, 2018, 01:11:05 AM »

KFBAKL is mostly throttle movements IIRC, if your throttle is static, it's not gonna do that much.
Durametric sucks for logging Sad And they won't know which value it's really logging because it's logging MVB's not RAM.

Also from that log it doesn't look like cam overlap itself is causing the spike, but rather load increase. Try tweaking the wall wilm to be a bit bigger.... and possibly KFBAKL.
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jpurban
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« Reply #10 on: June 18, 2018, 03:42:41 PM »

Alright...   I've taken another run at my WFRL.  This time I've examined the derivatives of WFRL because the FR says the curve should be smooth.

I tried this out this morning and it helped some, but the throttle response was definitely smoother.  I'll need to take it further...  (removing the dip described below).

Anyway, just wanted to ask if anyone else has reviewed the derivatives of their OEM WFRL curves.  My problem ocurrs between 30% and 60% load, which is also where the WFRL derivative has a "dip" in it.  Do the AUDI OEM WFRL curves have the same dip?

My thinking is that the 1st derivative slope should be steep to start and then gradually flatten out.... clearly, my OEM curves don't do that.  And the second derivative is really telling... Note the OEM curve is very inconsistent from point to point.

So, why did the OEM have these weird undulations in the 1st derivative?  Does the cam overlap and valve lift affect wall film?  I'm not sure...
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mister t
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« Reply #11 on: June 19, 2018, 12:37:54 AM »

Looks like you're talking about getting a tiny lean spike when you lift off the throttle on relatively light loads correct?

Like prj said, leave it alone. A slight lean spike is to be expected on throttle lift as the fuel film on the manifold walls is momentarily depleted.

I can promise you that it's not causing your engine any harm. Hell, you should see how lean my S4 with headers goes when I lift, it's literally off the scale lol.

The one suggestion I would make is to park the exhaust cam at full lobe separation unless you go over about 60% load. The EGR function is only for fuel economy and emissions.

I found that keeping the lobe angles separate at part throttle made for much better transient response as it always takes the cams time to go from overlap (EGR) to separation (torque). It may only be a tenth of a second, but it was noticeable enough for me to do it on my 3.0 AVK Audi. 
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prj
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« Reply #12 on: June 19, 2018, 06:24:38 AM »

The one suggestion I would make is to park the exhaust cam at full lobe separation unless you go over about 60% load. The EGR function is only for fuel economy and emissions.

I found that keeping the lobe angles separate at part throttle made for much better transient response as it always takes the cams time to go from overlap (EGR) to separation (torque). It may only be a tenth of a second, but it was noticeable enough for me to do it on my 3.0 AVK Audi. 

YO SUPERMEGANATUNER, nothing what you've said is relevant whatsoever to this topic. In this topic we're talking about cars with 2 point NVS on intake cam + variable lift. I understand you've never even seen one, but at least pay attention to what's written in the thread.

@jpurban
I attached a dyno pull from a 996TT I was tuning.
I think I went kinda nuts on KFBAKL and wallfilm to sort the entire issue with the Dekas that were on it. So much so that I have a rich spot where you have a lean spot.
But I don't let it bother me too much because actual tracks very well and elsewhere mashing the throttle and lifting off the throttle my lambda correction was reasonable.

You need WinlogView to view the file, that you can download from the 14point7 website (or use whatever else you want).
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mister t
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« Reply #13 on: June 19, 2018, 11:12:05 AM »

ohhhh, SUPERMEGATUNER, I like the sounds of that lol. Actually, my post was right on point if you cared to look at the title of the thread "leaning out on cam advance" AND my point about parking the exhaust cam for transient response is also an excellent point that no one else has ever raised to my knowledge.

You'll also note that I also mentioned the wall wetting as being the root cause of the lean spike (and gave you credit for being the first one to point it out (although I did know it in advance of reading your post).

Further, parking the exhaust cam on an FSI engine also has the advantage of eliminating EGR which is the cause of most, if not all of the carbon buildup on the intake valves that plagues that engine series. Again, a point that I have yet to see anyone raise around here.
« Last Edit: June 19, 2018, 11:21:56 AM by mister t » Logged
jpurban
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« Reply #14 on: June 19, 2018, 10:29:29 PM »

PRJ and Mister T, thanks for the responses.  I'd like to run some more thoughts by you...

Mister T...  How would you recommend altering KFNW?  I've attached a screenshot of my OEM values.  You'll see I quickly drop to the low 20 RL as a changeover point.  My cam changeover seems to aggravate lean out by 10% or more when it occurs at low rpm in the range of 30 to 50 RL.  So, 60% seems like a good call on your part.  I get the feeling you know the crap I'm encountering.  Not undriveable, but not ideal either.

Also, have you altered NNWS or DRLNNWSG?  I ask because raising the changeover point from near idle to the middle of the cruising range could aggravate oscilliations in cam position -- Think uphill vs downhill in cruise control.  You'd cross the 60% RL threshold a lot, right?  May not be a concern, but you'd probably increase the number of cam changeover events, which could cause less-than-smooth transients in cruise as well as increase wear on changeover mechanisms.  So, it seems to make sense to increase protection against unnecessary cam changes...  just thinking out loud, if you will.

PRJ...  I completely agree with you -- Appropriate transient compensation is the ideal solution.  Altering cam changeover point isn't ideal with this engine design -- reducing overlap with the low valve height ultimately reduces the engine's ability to breathe (as rpm rises and scavenging benefits grow).  That's never good.  BUT...  I have tried to alter my ESUK tables so many times and only seem to make matters worse.  Frustrated is an understatement.

I'm willing to keep working on ESUK adjustments, but could benefit from your experience.  First question... Do you have to have a dyno to effectively update ESUK for new injectors?  Second...  Do you disable KFBAKL/KFVAKL (Set to 0) when trying to tune WFRL, as the FR instructs?  I guess simply want to know your process for ESUK tuning... Any key steps?

I've attached a screenshot of my relevant ESUK tables.  I mentioned the lack of continuity in my WFRL derivative.  Is this normal?  I see similar weirdness in my KFABAK table, where the short term portion drops to nothing a 60C.  You can definitely feel the "numbness" in throttle response, but I'm sure the OEM had good reason.  Only reason I can find is that LRA is enabled at 60C.  Since the OEM has ESUKA disabled in my file, could it be that the OEM expected LRA to address the transients above 60C?!?  Do you see the same weirdness in your Audi files?

Revisiting a solution to my cold start lean out that I first read in the EV14 thread...  Adjusting ESWL warm up tables.  I did just this, but found limited benefit.  Think I now know why... My warm up is super short -- My ATISLATM is only 250 injections at a 40C TMST.  That's about 30 seconds, barely enough to get out of the driveway.  My warm up ends and I end up with a lambda target of 1.00 while my motor is still quite cold.  Are your Audis similar?  (Edit:  Ithink ATISLATM is the threshold for switching from lamns to lamwl, but the Audi FR shows a less-than symbol to set lambwon.  A little confused, obviously.)

Perhaps this isn't a problem if you directly from short warm up to cat heating, which has it's own rich lambda requirements.  However, I've removed my secondary air, disabling KH as a result.  That may be the root cause of my lean out when cold...  That begs the question... How do we replace the KH rich lambda targets?  I'm guessing the answer is setting CWWL bit 0 or bit 1, which enables LASWLTM (inactive LR) or KFLASWLR (active LR), respectively.  My OEM CWWL = 0, and I have neither enabled.  Am I n the right path?  What settings do you use in the cars you tune that lack KH?  Please forgive me if I'm asking a silly question.  This area (ESWL) is especially new to me.
« Last Edit: June 19, 2018, 10:57:19 PM by jpurban » Logged
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