Understading VVT

[mikez71]

Looks very active in the daily driver rpm's to me, but I don't know anything about it ...
Below is the high baro 94kpa map, with mid(84kpa) and low(75kpa) having less effect at higher airflows.
(Low baro map does not affect anything over .36g cylinder airmass until over 3800 rpms, and maxes out at 20 degrees)

Being near sea level, my baro reading is 103kpa. Looking at my logs, I can hit .60g cyl airmass as low as 2000rpms at 50% throttle... In which case the VVT should be inactive...? (until 4000rpms)

 

[Noggles]

There are 2 other tables like that. Would you fall into either one of them in that case?

 

[hagar]

Depends how you think of "inactive." The number 0 means the cam is fully advanced. The camshaft is designed to make maximum cylinder pressure from idle to peak torque at full throttle, then once it crosses that threshold (4 thousand rpms,) it starts retarding the cam because it is now changing over from trying to maximize engine torque, to trying to maximize engine hp.
The full throttle values are straight forward, have a look at your spark tables in the spark correction area under the vvt spark tables. You will see a ton of vvt based spark adjustments to go with those big part throttle vvt adjustments for fuel economy reasons. Just like you can run higher ignition timing for light loads to increase engine power, and thus increase part throttle efficiency, they are also running the cam super retarded in conjunction with piling on additional vvt cam timing.

 

[mikez71]

There are 2 other tables like that. Would you fall into either one of them in that case?

I think the table shown would be the one I am using, being near sea level.

I say inactive because I heard the cam is fully advanced at rest. Was a little surprised that I would be out of it by 2000 rpms with just light driving. (I think) I'll try and log cam phase to verify...

But it's always retarding above 4k rpms, so I guess it should always help up top? Would you lose peak HP disabling VVT? (with a stock vvt cam)

 

[hagar]

Depends how you look at it. Are you inactive if you are pushing against a wall that isn't moving and you aren't moving?

The most important thing to keep in mind is that the cam is designed as a vvt cam first, it isn't a normal cam that is being moved around. Take for example the reason why the tune is purposefully killing cylinder pressure at light throttle by retarding the cam. The cam is designed to make as much cylinder pressure down low as possible at higher loads and full throttle to make more torque, thus helping a heavy vehicle to move easier down low. When you have that same extra cylinder pressure in lower load situations where you don't require the extra torque, you need to retard the ignition timing in order to keep the engine out of knock. To get around that, you retard the cam so the cylinder pressure drops, then you ram timing into it to maximize the efficiency of the engine.
When you see that the cam is "inactive" you are basically seeing the true nature of the design of the cam. When you see the numbers, you are seeing the adjustments used to make that base design as efficient as possible in the spots it was not designed to be it's best. They aren't using the numbers to make the cams performance, they are using the numbers to make the cam less ****** where it isn't optimized in the base design of the cam.

 

[Marky Dissod]

When you see that the cam is "inactive", you are basically seeing the true nature of the design of the cam.

When is the variable cam timing 'at rest'?
The zeros in the table in this post?
Is there another table that shows when the Variable Cam Timing is or is not in effect?

Hagar, is the word you were looking for
schidty?

 

[hagar]

When is the variable cam timing 'at rest'?
The zeros in the table in this post?
Is there another table that shows when the Variable Cam Timing is or is not in effect?

Hagar, is the word you were looking for
schidty?

Haha, that was the word. The cam is fully advanced anytime you see a 0. It can not possibly go any further advanced than 0, but it is being held at 0 so it's not really at rest. Just semantics i am sure, but it is being forced to stay at fully advanced when it is showing a 0.
You can log the vvt position to know when it is moving off fully advanced, but it's movement comes from meeting certain criteria, not a table itself that activates it. It blends with other tables.

 

[mikez71]

Here are the MAX values driving around.. Was thinking there must be other maps/modifiers..


Averages from the same log below, mostly normal driving with a couple heavier throttle sessions.
(80% throttle at most)

 

[hagar]

You can look at that log of cam position and see in it how the engine works just by the way numbers change vs load and rpm. The points where the numbers are the highest, are the heaviest loaded points under normal light throttle driving, or even could be into a super efficient zone where the intake flow just stacks up better because of the manifold design.If you take data point .20 load at 2,000 rpm, it is the highest cam retard because that would be the point the engine is having a hard time taking timing. If you open up your vvt timing adder table, it will be adding extra timing at that point.As the rpms increase at the same .20 load point, you can see the cam timing starting to advance again. It is advancing again because the load on the engine goes down once the vehicle is getting its kinetic energy up to speed, but also the engine is getting into its efficiency range at that point, so the computer starts telling the cam to advance and bring the cylinder pressure back up.

The computer is basically swapping cylinder pressure and ignition around at different points to make the engine the most efficient it can be. At certain points, it is better to have more cylinder pressure and less timing like at full throttle or heavier load, and sometimes it's better to have less cylinder pressure and more ignition timing for best results, like at max fuel economy light throttle range.

 

[Marky Dissod]

You can log the vvt position to know when it is moving off of fully advanced (zero),
but its movement comes from meeting certain criteria, not a table itself that activates it. It blends with other tables.

Interesting, that there is not ONE table that simply says, 'this is where VVT will always advance, this is where VVT will always retard',
although I'm fairly sure that it can be predicted by RpM & throttle.
(I'm a mid-90s LT1 tuner who genuinely thinks that the vast majority of the increasing complexity of ecm & tcm tuning is akin to when a Dr could say
he had a heart attack which knocked him out and stopped his heart, but we saved him with an injection instead of electrically
but instead says
he suffered a myocardial infarction, during which he suffered a syncope and a near-terminal arrest,
but we cardioverted him chemically instead of defibrillating him)

If 'full advance' = 0, which the ecm has to command,
where is the cam when the VCT either fails, or is simply 'relaxed' because it is momentarily passive / doing nothing?
Is it when the cam is fully retarded, or somewhere in between?