AVCS tuning for power and economy

[KoolBRZ]

I modified the intake and exhaust AVCS tables because I have a theory that a change in exhaust back-pressure makes a change to the engines volumetric efficiency. I believe that if you run an after-market exhaust, your valve-timing might not be correctly compensating for the change in back-pressure. It depends on your tune of course.

Here are pics from my first try at compensating for the difference in back-pressure. The modded tables are the safe tables that are used when the IAM is at or above .95. The unmodded tables are the normal tables used when there is an IAM below .95. So if it becomes unsafe to run the modded tables, it will automatically use the unmodded normal tables. The tables labeled safe are the modded tables, first Intake, then Exhaust, and the tables labeled normal are the unmodded tables.

Athough I've posted this elsewhere, I believe it's better suited to the software tuning forum.

[KoolBRZ]

Originally Posted by steve99

http://www.romraider.com/forum/viewt...hp?f=15&t=6258

(From the Forum)
"AVCS is a way to adjust your VE at low RPM. It can help with around-town drive ability and spool up time, but it won't help you with WOT power, because people have shown time and time again that you make the most power with AVCS set to 0 after you pass peak torque. The easiest way I've seen to adjust AVCS is pretty straightforward. You set your entire AVCS table to 0, go make 3 pulls in some gear. Set your entire AVCS table to 5, go make 3 pulls, then 10 degrees, then 15, 20, 25, etc. Eventually you'll have 3 WOT logs for each AVCS advance, then you compute the average power (or VE, but see below for some caveats with doing that) for the 3 runs, and overplot them on each other. What you'll have at the end is a HP vs RPM graph with 6-7 curves on it, showing the HP for each AVCS advance value. Then you simply pick and choose. At 1000 RPM, which AVCS advance value made the most power, and use that one. At 2000 RPM, which AVCS advance value made the most power, etc. You'll probably see that at low RPM it's the high AVCS advance that are making the most power, but as you approach 5000 RPM it will trail off and the lower advance values will make more power, until above 5k RPM it's the 0 advance table that's making the most power."

Very good info in this link! Even if it is slanted towards FI.
Is this talking about Intake AVCS, or both Intake and Exhaust AVCS? I would do one thing further with our cars though, I would adjust and log Intake AVCS, and then adjust and log Exhaust AVCS, Since they each have different effects at different RPM's.

Here is some further info about what VVT does and is used for, from Wikipedia;

"Late intake valve closing The first variation of continuous variable valve timing involves holding the intake valve open slightly longer than a traditional engine. This results in the piston actually pushing air out of the cylinder and back into the intake manifold during the compression stroke. The air which is expelled fills the manifold with higher pressure, and on subsequent intake strokes the air which is taken in is at a higher pressure. Late intake valve closing has been shown to reduce pumping losses by 40% during partial load conditions, and to decrease nitric oxide (NOx) emissions by 24%. Peak engine torque showed only a 1% decline, and hydrocarbon emissions were unchanged.[2]

Early intake valve closing Another way to decrease the pumping losses associated with low engine speed, high vacuum conditions is by closing the intake valve earlier than normal. This involves closing the intake valve midway through the intake stroke. Air/fuel demands are so low at low-load conditions and the work required to fill the cylinder is relatively high, so Early intake valve closing greatly reduces pumping losses.[2] Studies have shown early intake valve closing reduces pumping losses by 40%, and increases fuel economy by 7%. It also reduced nitric oxide emissions by 24% at partial load conditions. A possible downside to early intake valve closing is that it significantly lowers the temperature of the combustion chamber, which can increase hydrocarbon emissions.[2]

Early intake valve opening Early intake valve opening is another variation that has significant potential to reduce emissions. In a traditional engine, a process called valve overlap is used to aid in controlling the cylinder temperature. By opening the intake valve early, some of the inert/combusted exhaust gas will back flow out of the cylinder, via the intake valve, where it cools momentarily in the intake manifold. This inert gas then fills the cylinder in the subsequent intake stroke, which aids in controlling the temperature of the cylinder and nitric oxide emissions. It also improves volumetric efficiency, because there is less exhaust gas to be expelled on the exhaust stroke.[2]

Early/late exhaust valve closing Early and late exhaust valve closing can also reduce emissions. Traditionally, the exhaust valve opens, and exhaust gas is pushed out of the cylinder and into the exhaust manifold by the piston as it travels upward. By manipulating the timing of the exhaust valve, engineers can control how much exhaust gas is left in the cylinder. By holding the exhaust valve open slightly longer, the cylinder is emptied more and ready to be filled with a bigger air/fuel charge on the intake stroke. By closing the valve slightly early, more exhaust gas remains in the cylinder which increases fuel efficiency. This allows for more efficient operation under all conditions."

Ideally steve99 and/or kodename47 will volunteer their time and sanity to make a spreadsheet which can take information from logs and generate the perfect intake and exhaust AVCS tables to compensate. This spreadsheet will be so perfect that it can compensate for any intake and exhaust configuration, turbo or supercharged.

[KoolBRZ]

Originally Posted by KoolBRZ

I modified the intake and exhaust AVCS tables because I have a theory that a change in exhaust back-pressure makes a change to the engines volumetric efficiency. I believe that if you run an after-market exhaust, your valve-timing might not be correctly compensating for the change in back-pressure. It depends on your tune of course.

Here are pics from my first try at compensating for the difference in back-pressure. The modded tables are the safe tables that are used when the IAM is at or above .95. The unmodded tables are the normal tables used when there is an IAM below .95. So if it becomes unsafe to run the modded tables, it will automatically use the unmodded normal tables. The tables labeled safe are the modded tables, first Intake, then Exhaust, and the tables labeled normal are the unmodded tables.

Athough I've posted this elsewhere, I believe it's better suited to the software tuning forum.

My first cold start with these modded tables was a disappointment. Misfires and an IAM down to .93. It increased to an IAM of 1.0 after it warmed up however, and stopped misfiring, so it must be temp related.

Since I have been on the Phantom ESC waiting list since September, I realized there is no sense optimizing my current tune, since I will be switching to the OFT ESC tune once I get my Phantom ESC in the next few months. I copied the intake and exhaust safe tables from the ESC tune to my current tune, and the normal tables from shiv's OFT UEL 91 Octane tune. This way it will default to a known good table if the IAM drops below 1.

[swarb]

What are you trying to achieve?
Do you have links to any logs?

[Ranatsu]

Originally Posted by KoolBRZ

Very good info in this link! Even if it is slanted towards FI.
Is this talking about Intake AVCS, or both Intake and Exhaust AVCS? I would do one thing further with our cars though, I would adjust and log Intake AVCS, and then adjust and log Exhaust AVCS, Since they each have different effects at different RPM's.

To answer your question, that article was probably written before the USDM 2008 STi came out, that was the first car that had dual AVCS in the US.

[burdickjp]

A lot of that Wikipedia copy pasta doesn't sound right to me.

Looking at power while tuning is less illustrative than looking at torque.

To be comprehensive you'd want to log all inlet timing positions for all exhaust timing positions. If you're clever you can do some design for experiments math and reduce the number of positions necessary, but for the sake of the community, let's not.

How about you load a completely stock firmware, go through all the timing variations with 5 or so WOT throttle pulls along the same stretch of flat road and share your logs?
That'd be really helpful for all of us.

Have we figured out how to limit the throttle angle? It'd be helpful to then do the same thing at lower throttle openings to fill in the rest of the VVT tables. With cable throttle cars I just use a physical throttle stop, but that wouldn't be as helpful here. If we can limit the throttle angles it'd be more helpful.

[nickw14]

What are the repercussions for doing WOT pulls with 0* AVCS angle?

[Kodename47]

Originally Posted by nickw14

What are the repercussions for doing WOT pulls with 0* AVCS angle?

If you're changing the AVCS, be aware that the AFR may change and you may alter the knock threshold. You would want to set the AFR to a known safe value and probably reduce the timing by a couple of degrees.

[s2d4]

O boy...
Back pressured this post out of my rear end quick smart.