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Old 03-25-2016, 03:54 PM   #726
mrsaturn7085
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Quote:
Originally Posted by dr20t View Post
Re: the group n rom and the grb spec c roms above, my 2 cents on this is that it relates to compressor and turbine wheel efficiency as well as again, pre turbo egbp. Stock twin scroll setup tends to choke up top with the p25 twin scroll housing, and egbp would be high. Therefore makes no sense to run big overlap.
All good points! I was mainly posting those as help to get the AVCS timing back on-track for the guy a few posts above, as the pictured maps were all over the place. Doing a rough conversion of g/rev -> MAP is a good starting point, but certainly requires fine tuning after that point. The GpN map was chosen as it was clearly made in-house @ STi, and was set more aggressively than the factory tune. Comparing the two may be a good way to see where gains can be made (especially with VF-series turbos).
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Old 03-25-2016, 06:09 PM   #727
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Absolutely! And I hope you didn't think I was diminishing from the quality of your post. All great info so thank you.
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Old 03-25-2016, 07:28 PM   #728
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Not at all, this is a great thread with many people far more knowledgeable than I about AVCS tuning - honestly one of my least favorite details on modern engines. Great for performance (especially if you're the engineer designing the motor), but a variable VE on the motor doesn't exactly make it easily tuned since you're working backwards from what the engineer gave you to work with.
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Old 03-28-2016, 10:48 PM   #729
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Feel free to disagree but I'd like to hear more how exactly overlap on a FI engine will help improve spool? I see this statement tossed around and I haven't heard anyone explain how that actually works. If you can back it up by explaining the effect of it on the incoming charge, that would be great.

Not sure how you're linking compressor surge to AVCS efficiency. Comp surge is a build up of air against throttle plate when it closes, jamming the turbine and shocking it. That is a function of a poorly matched blow off valve.

Some things to think about with overlap:

- you have both intake and exhaust valves open, piston being near TDC. Even if you create 20 degrees of overlap, near TDC, most of the rotation is not moving the piston vertically at the time and with higher intake pressure, you end up blowing some part of your charge out the exhaust since that is the only place it can go.

- unless you can actually ignite the mix in the exhaust to raise EGTs, that cold charge will lower your EGTs and efficiency and you're dumping air/fuel you need for energy.

- since our AVCS has static duration, if you advance intake to create overlap, it means you close it sooner, before BDC, which is not optimal at high load. It has been proven that closing few degrees after BDC is best since air has mass and momentum and you can trap more by letting it rush in and "crash" against the piston that is starting to rise. And with our short intake duration, it means you cannot advance it and hope for good results.

Spool is really a function of gas velocity and how efficient the turbo is. The hard part is that it is actually really hard to quantify spool. Boost gauge reading is NOT a measure of spool which sadly is often used to quote "better" spool.



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Originally Posted by dr20t View Post
If you're saying this as an absolute statement, then respectfully, I disagree with what you.

If preturbo ebp is higher than inlet map absolute, then i would agree (assuming relatively stock setup).

However in all other instances, overlap will help spool. I've seen compressor surge on a gt2871 on a 2.0 litre with proper avcs tuning. Obviously don't want surge, but its indicative of the spool characteristics of a good avcs tune.
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Old 03-29-2016, 05:45 AM   #730
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Quote:
Originally Posted by Ziggyrama View Post
Feel free to disagree but I'd like to hear more how exactly overlap on a FI engine will help improve spool? I see this statement tossed around and I haven't heard anyone explain how that actually works. If you can back it up by explaining the effect of it on the incoming charge, that would be great.

Not sure how you're linking compressor surge to AVCS efficiency. Comp surge is a build up of air against throttle plate when it closes, jamming the turbine and shocking it. That is a function of a poorly matched blow off valve.

Some things to think about with overlap:

- you have both intake and exhaust valves open, piston being near TDC. Even if you create 20 degrees of overlap, near TDC, most of the rotation is not moving the piston vertically at the time and with higher intake pressure, you end up blowing some part of your charge out the exhaust since that is the only place it can go.

- unless you can actually ignite the mix in the exhaust to raise EGTs, that cold charge will lower your EGTs and efficiency and you're dumping air/fuel you need for energy.

- since our AVCS has static duration, if you advance intake to create overlap, it means you close it sooner, before BDC, which is not optimal at high load. It has been proven that closing few degrees after BDC is best since air has mass and momentum and you can trap more by letting it rush in and "crash" against the piston that is starting to rise. And with our short intake duration, it means you cannot advance it and hope for good results.

Spool is really a function of gas velocity and how efficient the turbo is. The hard part is that it is actually really hard to quantify spool. Boost gauge reading is NOT a measure of spool which sadly is often used to quote "better" spool.
All very valid points and thank you for the detailed response.

In my post above, you wil notice I said that if you meant what you said as an absolute statement then I disagree. In some instances I believe you are correct in that overlap can harm spool. But that cannot be taken as an absolute.

My statement about compressor surge was mainly to point out that I am able to spool a medium frame turbo quite quickly with my 2.0 litre. Of course compressor surge is not a good thing at any time. But it was an indicator for me that I was able to produce more boost than my motor could ingest at one point. This has been fixed (by tuning to improve the engine's volumetric efficiency and its ability to ingest this amount of boost - or more correctly, remove the level of restriction causing this high level of boost- as boost is a measurement of inlet restriction).

Brace yourself for this, but I currently run a peak of 108 degrees dynamic overlap (in cam degrees) in my dual avcs ej207. That's not to say that everyone should do this. And here's why.

My view is it comes down to a few questions:

1. What exactly drives turbine spool? (In your above statement you say its gas velocity which is one driver but not the only one)

2. What do established fluid dynamic principles tell us about the flow between low and high pressure areas?

3. How can we utilise variable cam phasing to improve the engine's ability to breathe at all rev and load ranges?

Mick
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Old 03-30-2016, 12:23 AM   #731
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Quote:
Originally Posted by Ziggyrama View Post
Comp surge is a build up of air against throttle plate when it closes, jamming the turbine and shocking it. That is a function of a poorly matched blow off valve.
<pedantry>

Compressor surge can also occur with a wide open throttle, if an overly large turbo tries to feed the engine far for air than it can ingest. Similar problem, but the engine itself is the restriction, not the throttle plate. Since the throttle is open, the engine's exhaust is still driving the turbine, and damage may follow.

</pedantry>
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Old 03-30-2016, 12:25 AM   #732
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And I just realized that dr20 already covered that.

<homer> D'oh. </homer>
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Old 03-30-2016, 04:46 AM   #733
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Concerning ebp dr20t is absolutly right.

I just wanted to make an autox map for my Sti (stock turbo) and thought running 90+ wgdc in lower rpm will make spool in lower gears faster...

But when I saw the datalogs I was surprised... it took longer to spool the turbo and I achieved lower boost.
The only thing I changed was wgdc.
Then I switched over to my normal map and suddenly spool was as fast as always and more boost.

Long story short, the amount of overlap (significantly more than stock) was messed up by the higher ebp. Exhaust gas temperatures during spool with higher wgdc were higher than with the normal map, so the effect of some blow through was gone.

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Old 03-31-2016, 12:59 AM   #734
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I really don't see why increased WGDC would cause what you're talking about. More WGDC -> slower spool just makes no sense. I'm not saying it didn't happen, but I'm pretty sure the cause wasn't WGDC. It doesn't cause increased backpressure either, except indirectly if it leads to MORE boost (not less).

Did you see a spool difference for ramping up from vacuum to 10psi? The reason I ask is that when boost is below the wastegate spring pressure, WGDC is completely irrelevant. So if you were seeing a difference in the sub-10psi area, then WGDC was definitely not causing what you were seeing.

Did ambient temperatures change between those two sets of data logs? There's gotta be another explanation.

(I think the wastegate spring pressure is around 10psi but my memory is fuzzy. Adjust accordingly if I'm wrong.)
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Old 03-31-2016, 02:41 AM   #735
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Initial und max wgdc were set to 97-98 until 4000rpm, because I wanted extra good spool and more boost (mainly in second gear) for autox in lower gears...
Normally I can get about 24psi in 3rd gear, then I could only achieve 22psi...
I was so surprised and my autox was the next day, that I changed to my normal wgdc settings 1 hour later and everything was back to normal.
Don't forget that suddenly exhaust gas temps were higher.
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Old 03-31-2016, 07:20 AM   #736
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My view on the above situation is this:

High wgdc holds the wastegate closed tighter on spool.

If you saw a negative impact on spool with higher wgdc and the same avcs settings, it would indicate a leaking wastegate to me.

Reason is when wgdc is higher and therefore wastegate closed tighter, it should show what your true ebp is pre turbo, as the wastegate is completely shut.

If having the wastegate shut caused a reduction in spool, then it tells me you have too much overlap for the level of ebp you're seeing pre turbo, thereby potentially causing reversion.

Tl:dr - your wastegate might be leaking / loose, and by running more overlap you have tuned around this. When you "fixed" the wastegate leak by running higher wgdc, you increased the back pressure. The increased ebp may have caused reversion.

The other way to look at it is that by increasing wgdc, you kept your wg closed longer than previous, which also resulted in increased ebp.

Either way, I believe its ebp related, or more accurately, by increasing your wgdc, you exposed a potential contributor you had to lower ebp, which was allowing you to run more overlap previously.

I could be wrong, but if all you literally changed was wgdc, then they are my only logical explanations.

Mick
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Old 03-31-2016, 11:33 AM   #737
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Quote:
Originally Posted by dr20t View Post
All very valid points and thank you for the detailed response.
...
My statement about compressor surge was mainly to point out that I am able to spool a medium frame turbo quite quickly with my 2.0 litre. Of course compressor surge is not a good thing at any time. But it was an indicator for me that I was able to produce more boost than my motor could ingest at one point. This has been fixed (by tuning to improve the engine's volumetric efficiency and its ability to ingest this amount of boost - or more correctly, remove the level of restriction causing this high level of boost- as boost is a measurement of inlet restriction).
...
Mick
This is the point I was getting at. Sounds like you're looking at this the right way and you're aware of the trap that many fall into.

From my perspective, evaluating spool is very hard because we don't have good way of doing it. We can look at boost but that can be very misleading. You could use EGTs as that "should" help but it in itself will not measure it. Flow may be a good metric since that tells you how much air you're actually moving. In fact, what I am thinking is, perhaps measuring how much time it takes to achieve certain flow is what we should be looking at? Isn't that what we perceive as spool? Unless you have a way of measuring the actual turbine behavior in the compressor itself. And I am guess most, if not all of us don't.
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Old 03-31-2016, 11:50 AM   #738
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My view on the above situation is this:

High wgdc holds the wastegate closed tighter on spool.

If you saw a negative impact on spool with higher wgdc and the same avcs settings, it would indicate a leaking wastegate to me.

Reason is when wgdc is higher and therefore wastegate closed tighter, it should show what your true ebp is pre turbo, as the wastegate is completely shut.

If having the wastegate shut caused a reduction in spool, then it tells me you have too much overlap for the level of ebp you're seeing pre turbo, thereby potentially causing reversion.

Tl:dr - your wastegate might be leaking / loose, and by running more overlap you have tuned around this. When you "fixed" the wastegate leak by running higher wgdc, you increased the back pressure. The increased ebp may have caused reversion.

The other way to look at it is that by increasing wgdc, you kept your wg closed longer than previous, which also resulted in increased ebp.

Either way, I believe its ebp related, or more accurately, by increasing your wgdc, you exposed a potential contributor you had to lower ebp, which was allowing you to run more overlap previously.

I could be wrong, but if all you literally changed was wgdc, then they are my only logical explanations.

Mick
Some more food for thought. We need to consider gas velocity and its effect on the turbine. Gas that moves has momentum which means it will exert more force on the turbine. A wastegate that is completely closed can have initially detrimental effect on gas velocity since you're essentially slamming the flow against the turbine. This can be a good news/bad news situation:

- by feeding all the exhaust in, you should get faster spool, at least that is a common assumption

- exhaust gas that collides with something will cause a significant slowdown in velocity initially, which will affect efficiency of the overall system. Impact of anything produces a pulse that travels back up, which has further negative impact on the system upstream.

So, the question is, if you crack the wastegate open just enough to maintain the flow, does that effect maintain more gas velocity which translates to better overall system performance? I bet if you were to plot it, there is a sweet spot, somewhere in between 0 and 100.
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Old 03-31-2016, 03:47 PM   #739
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Originally Posted by Ziggyrama View Post
So, the question is, if you crack the wastegate open just enough to maintain the flow, does that effect maintain more gas velocity which translates to better overall system performance? I bet if you were to plot it, there is a sweet spot, somewhere in between 0 and 100.
i think you're going down a dead end here.

maintain what flow? the flow from the head to the wastegate? i'm not interested in that flow. i'm interested in pressure ratio across the turbine. period! that's what induces a mass flow rate across the turbine and that is what gets it moving.

this idea that an open wastegate (literally an exhaust leak) will spin up a turbo faster makes no sense.
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Old 03-31-2016, 07:10 PM   #740
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i think you're going down a dead end here.

maintain what flow? the flow from the head to the wastegate? i'm not interested in that flow. i'm interested in pressure ratio across the turbine. period! that's what induces a mass flow rate across the turbine and that is what gets it moving.

this idea that an open wastegate (literally an exhaust leak) will spin up a turbo faster makes no sense.
I think you misuderstood what I said. I said potential good news/bad news.

Good news:

If you send all you got at the turbine, you will get maximum spool up. No argument there.

Potential bad news:

Once you go WOT, and you send a rush of gas at an obstacle, what does that do upstream? This is the point I was speculating on. Surely, it must have an effect. I don't know if it is 5%, 15% or 0.005%. Perhaps this is relatively insignificant.

To use an analogy, consider a revolving door and a rush of people running at it, in close formation. As they hit the door, what happens? Disruption. That effect propagates up through the rank upstream. Now, introduce a small door on the side that allows some people to jump off the line. It provides congestion relief at the expense of getting the revolving door moving. The question is, which is better? My guess is, keeping the wastegate closed is still better, just putting it out there.

Engine is a pump. You need to move gas through it , you know this. It requires flow. Whatever you push in, you have to move out the other side. If you disrupt exhaust, it will have a net effect on the system. Turbo presents a restriction and the act of spinning up the turbo requires significantly more energy then to keep to spinning. If things do not move well in the manifold/header and uppipe, that will effect the efficiency of the pump. Granted, the piston does push the gas out but to your point, pressure differential will make things smoother. Consider the gains alone from going to equal length headers on our engine.

I am curious to hear your thoughts on overlap vs. spool idea. I am not seeing the connection there. Thoughts?

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Old 03-31-2016, 09:18 PM   #741
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Ziggyrama I think we will keep going in circles until and unless we agree on a few things (some of which are well beyond the scope of the discussion in this thread but important to touch on that may affect our view on VE and therefore avcs tuning):

1. Yes we agree that Boost is a measurement of inlet restriction. The greater the restriction, the greater the pressurization of inlet airflow.

2. We do not neccessarily flow a higher volume of air volume with more boost, we flow more oxygen per particle of air as it is more denseley compressed. The means to achieving this is the compressor wheel pushing air against a static restriction to build "boost pressure" above ambient.

3. Compressor surge that i referred to, is caused at the point that the engine can no longer "process" the volume of intake charge being sent to it - this can be because the air between compressor and inlet valve has been boosted to a point that cannot be ingested, or because of the volume of air being moved by the compressor wheel is "built up", causing a corresponding imbalance of pressure seeking to flow through / across the inlet and exhaust valves.

4. Spool as we are referring to, is the ability of the turbine to pressurise the inlet in the most efficient manner. I realize that boost is a measure of inefficiency in a way, but we seem this inefficiency so that at the next phase of intake, compression, power, exhaust, that inneficiency has resulted in a higher oxygen content for the combustion process to make use of, thereby generating more torque / power.

So whilst you may see measuring spool as futile, the reality is spool = boost sooner, boost = more oxygen, which equals more power, sooner in the rev range, and therefore creates more torque.

I agree that we could theoretically reach a point where we spool a turbo to reach a certain boost level, but not utilize this boost to full capacity, and therefore create less torque. An example of this is a rich / low ignition advance spool, where we may reach 10 psi sooner but make less torque due to the rich mixture and low timing. This is not ideal and therefore not what we are chasing.

When you optimise this boost level by allowing the engine to breathe, ingest this intake, convert it to torque and expel the gases, in a more efficient manner, you are creating efficiency out of perceived inefficiency (boost).

This is precisely why optimised avcs tuning is important to us in this thread. It allows you to tune the effective VE of an engine for given load / conditions, and convert the negative pressure ratio which robs you of spool time (and creates lag) into an opportunity to make the engine breathe better.

Mick
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Old 03-31-2016, 10:05 PM   #742
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I wrote the above before seeing your latest post.


I think I know why you're not seeing the link. And its why I asked the first question a few posts back about what drives a turbine.

Exhaust pressure is what drives a turbine, along with heat (which assists in raising that pressure) and obviously we need gas velocity to do that.

With smaller (ie stock or similar sized) turbos and exhaust manifolds, the back pressure betwen exhaust valve and turbine wheel is high (smaller turbine wheel, smaller rear housing, smaller diameter piping etc). This is what drives quicker spool. Once its spooled, a stock turbo / header system creates excessive pre turbine back pressure, so your inlet to exhaust pressure ratio starts suffering in favour of the exhaust.

Example - at sea level, td04 turbo with stock wrx headers. Reaches 14psi map pretty quickly. Once there and above say 5000rpm, i can assure you your inlet to pressure ratio looks something like 1:2. (14psi + 14.7 psi ambient : 14.7psi ambient + 28psi ebp).

With this setup, creating valve overlap induces exhaust gas reversion, as the low pressure area is now in your inlet compared to exhaust, so fluid dynamics dictates the flow will go from the higher pressure area to low. This is one of those instances where I said overlap doesn't work.

With a larger turbo and header sustem, the inlet to exhaust pressure ratio is in favour of inlet. Therefore overlap assists in air flow in and out of the engine.

You are answering your own question here - by running more overlap when pressure ratio is in favour of the inlet, you're spooling the turbo and allowing the engine to breathe.

Much like an n/a vehicle - with the difference being we want the overlap down low as they're still is an exhaust restriction of sorts.
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Old 04-02-2016, 10:53 PM   #743
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1. Yes we agree that Boost is a measurement of inlet restriction. The greater the restriction, the greater the pressurization of inlet airflow.
And also a measure of how much work the compressor is doing, which (for a given turbo) is a measure of EGBP and turbine speed. It is a measure of many things, so you don't improve anyone's understanding by proclaiming that it is a measure of just one.

BTW, if measuring "spool" is hard, consider measuring the elapsed time between stomping WOT at X RPM and reaching Y RPM. Pick an X and a Y and be consistent. Maybe 2000 to 4000 with a stock turbo, or 3000 to 5000 with something bigger. Acceleration is the goal anyway, however it is achieved.

Or pick a starting RPM and see how high the RPM climbs after a fixed time period.
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Old 04-14-2016, 11:21 PM   #744
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I am curious to hear your thoughts on overlap vs. spool idea. I am not seeing the connection there. Thoughts?
For a better view what's happening inside the engine we will need a valve profile which shows the duration of overlap at low lifts and maximum intake advance. As long as I do not have it I will try to explain it as best as I can.
We don't have to see the valve overlap like huge openings especially for turbocharged engines. Most of it happens at less than 0.05" of lift.
The overlap affects the turbo spool together with the engine's Volumetric Efficiency which is raising due to earlier closing of the IV.
How it works?
The exhaust gases which are leaving the cylinder have a mass. Travelling at very high speed they have also an inertia.
Due to the inertia they tend to create a mini-vacuum behind them, that means without valve overlap their speed will be reduced which translates in less velocity for striking the turbine wheel With overlap and with positive pressure in the intake manifold the exhaust gases velocity will not be hindered so much and more of them will leave the cylinder (more flow->better spool)
Al this overlap thing depends of how good is your engine breathing. A freeflow exhaust, pnp'd head , a bigger turbo or any mod that will increase the flow through the engine and remove the restrictions will affect how efficient will be the overlap. If the engine is flowing very good with very low restrictions, actually you can go wrong with overlap by letting fresh air mixed with fuel to go straight to exhaust , losing power. Just think that this "valve overlap" for the stock engine is just enough to pull a part of residual gases inside the cylinder.

I hope I was explicit enough even I am not US native and my english is not so good.
You can watch the following video for a better view of the phenomena. The engine presented is not similar to the WRX one but there is not any difference between them regarding exhaust and intake valves opening and closing principle
http://www.youtube.com/watch?v=oJDe9YTMEPw

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Old 04-16-2016, 04:15 AM   #745
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Here is the datalog..(Stock turbo VF43)
First one with higher wgdc...
Second with lower wgdc...
Nothing else has been changed between these two runs

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Old 04-16-2016, 12:13 PM   #746
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Throttle went wide open about 200 RPM later in the first log, so boost climbs a couple hundred RPM later.

I don't have time to look closer, but did you comparison take that into account?
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Old 04-28-2016, 12:04 AM   #747
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Here is a data point, using EJ207 with 3-port/MBC combination.





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Old 04-28-2016, 12:27 PM   #748
Clark Turner
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I have to chuckle inside everytime someone trys to tune intake cams based on airflow and load.

C
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Old 05-01-2016, 02:40 PM   #749
NSFW
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Some of us chuckle every time you post so I guess it all evens out.
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Old 05-01-2016, 08:03 PM   #750
mrsaturn7085
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Quote:
Originally Posted by Clark Turner View Post
I have to chuckle inside everytime someone trys to tune intake cams based on airflow and load.

C
Care to elaborate? I assume this is due to cam timing changes being too slow to keep up with transient MAP, correct?

If this is the case, I've noticed that most baseline performance AVCS maps with load vs. RPM typically become 2D (RPM only) maps after hitting about 7.5 PSI (above atmospheric), which seems plausible as a part- to full-throttle tuning scheme in performance/racing engine. Group N base maps would be my example for this (see page 29 for GpN examples as provided by STI/Subaru). I feel like attempting to vary AVCS timing at higher load would require a 4D fuel/timing setup (cam advance vs. RPM vs. load).

EDIT: I realize what you are saying now; I was taking what you said as mapping AVCS timing using an axis of load (vs. RPM) to be incorrect, but you simply meant that selecting your AVCS advance values by looking at logged calculated load peaks (w/o fuel and timing adjustments) to be inadequate.

It's not a bad start (for a big ballpark or 'base map' before you make it to the dyno), but I think I understand what you mean - even if you pick the same target AFR and optimal ignition timing values that were determined to be the 'best' at one cam advance (through load-holding dyno testing - where you clearly didn't tune AVCS timing right the first time if you're changing it now), changing the cam advance changes the VE at that load point, which in turn changes the target AFR (somewhere between mean- to rich-best-torque), which in turn changes the optimal ignition timing.

Last edited by mrsaturn7085; 05-12-2016 at 02:44 PM.
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