Understading VVT

[hagar]

The table Mike posted above is a fixed table of when the vvt will be moving and to what extent. The left axis is grams per cylinder of airflow, but you can look at that as another way of saying "load." The other axis is engine rpms, and the data points are the degrees of cam retard the engine is running at for the point it's at. On the timing side things get more complicated and blends things together, but as for the actual camshaft moving, that table above has full control of that point. If you put that table to all 0s, the cam would run full advanced at all times and not move.

As for the vvt position, if the hydraulic pressure fails, the worst that can happen is the cam is locked at full advanced at all times. This is because there is a clock spring that holds the cam under tension in the full advance position. If that spring ever fails, which i have never seen, it is a non interference engine, and there are hard fixed limiters built into the cam phaser that only allow it to move so far. Even if the spring failed and the cam was flopping to full ratarded, the pistons would still not make contact at TDC.
When you do an aftermarket version of a vvt cam, you use limiter blocks to reduce how far the cam phaser can move to maintain piston to valve clearances under a worst case scenario.

Lt1 tuner eh? Did you use tunercat? That was one of the first programs I started with.

 

[Marky Dissod]

Used to use TunerCat exclusively. Several years ago, four TunerPro users wrote updated definition files
that access previously hidden switches & constants that TunerCat still can't see,
so now I use TunerPro for switches and constants, and TunerCat for tables.

So, 'zero' is full-advance. What 'number' represents full-retard, and how many degrees after is that?
Curious because I very strongly suspect that, if this GMT821 is not my last vehicle,
it may well be a GMT900 of some kind, so I want to learn about their tuning idiosyncrasies.

But it sounds like, if the VVT hydraulics fail, the cam could wind up anywhere between full advance and Simple Jack?

 

[mikez71]

So, 'zero' is full-advance. What 'number' represents full-retard, and how many degrees after is that?

VCM Scanner (HPtuners' datalog software) displays the desired cam position as degrees.
Maximum I logged (sample of 1 normal commute) was 23.4 degrees. Maximum number found in the vvt maps is 25.

 

[hagar]

VCM Scanner (HPtuners' datalog software) displays the desired cam position as degrees.
Maximum I logged (sample of 1 normal commute) was 23.4 degrees. Maximum number found in the vvt maps is 25.

And while I don't know the hard limit to go with your data, I would bet money that the advance wouldn't go much further than that before hitting the fixed hard part limiters built in, they would just build more advance into the base cam design to keep the range. I know if I was designing an engine for stock stuff, I would make my cam phaser lock out with the restrictor just beyond the point that I know my cam stops becoming effective in the tune. I will try advancing more at part throttle next time I have a stock one on the dyno and report back. Actually, it's a fun experiment. I seriously doubt it will go much further before the stock phaser over clock lock hits, but I will try and report back. I spend most of my time retarding the cam at part throttle in those ranges because an aftermarket cam naturally bleeds off pressure down low... but I will try for fun to see if the range goes further while affecting power.
Part of my tuning process on stock trucks even, is to numb down the cam timing and add advance igntion for maximum timing. It is a balance i dont think gm got right.I have never chased it backwards to see how far it can go, but now I am curious.

The goal when tuning is to get the least amount of throttle input to get the vehicle moving, and keep it moving. And it all depends on the engine and vehicle weight/aero, There is a reason why you guys notice these tunes shifting at 2 grand each gear, while picking up fuel mileage. An ls or Lzt engine at 2 grand is much more efficient at 2 thousand rpms at 50 degrees of timing, than it is at 1200 rpms at 20 degrees of timing like they run stock. Swathdiver actually nailed it on the rpm thing. An engines efficiency is an engines efficiency. It is fully possible to get better mileage at part throttle at higher rpms than lower rpms.

 

[hagar]

The obvious question is why they don't do it stock. The answer is because manufactures get their efficiency ratings based off data points they are told they need to hit. Certain load at a certain speed, then they get their certification for a fuel rating. They don't care that the rest of the points in the tune that are not being tested, suck. When you look at that tune and see all the crazy cam retarding and crazy timing adder at the same exact point... they are nailing certain points to pass emissions. It's like the ecoboost f150s. They pass insane emissions, but ask anyone who tows with one, and your 6.2 is now getting better mileage because they are always in boost to pull that weight. It's such a sham.

 

[mikez71]

According to this article

https://www.hotrod.com/how-to/hrdp-1007-gms-variable-valve-timing-system-performance-test/

There is 52 degrees of adjustment, but it claims 7 degrees of that can be advanced?
45 degrees of retard.

I also wonder if the logged degrees is cam degrees and the article could be referring to crank degrees? (that would equate the 25 degrees in tune to 50 degrees of crank angle, essentially using the full range?)

The article seems specific to GM, and the gen IV's.. And they mention that the closing of the intake valve is the largest gain, which is adjusting counter to what you want for the exhaust valve.

 

[Marky Dissod]

And while I don't know the hard limit to go with your data,
I'd bet money that the advance wouldn't go much further than that before hitting the fixed hard part limiters built in,
they'd just build more advance into the base cam design to keep the range. I know if I was designing an engine for stock stuff,
I'd make my cam phaser lock out with the restrictor just beyond the point that I know my cam stops becoming effective in the tune.
I'll try advancing more at part throttle next time I have a stock one on the dyno and report back. Actually, it's a fun experiment.
I seriously doubt it'll go much further before the stock phaser overclock lock hits, but I'll try and report back.
I spend most of my time retarding the cam at part throttle in those ranges, because an aftermarket cam naturally bleeds off pressure down low ...
but I'll try for fun to see if the range goes further while affecting power.
Part of my tuning process on stock trucks even, is to numb down the cam timing and add advance ignition for maximum timing.
It is a balance i don't think gm got right. have never chased it backwards to see how far it can go, but now I am curious.

GM got it different. They have different goals than you do, including raising their CAFE MpG score despite making their vehicles HEAVIER, and appeasing the EPA, whose primary goal has never been improving emissions (it's nearly always their secondary goal).

The goal when tuning is to get the least amount of throttle input to get the vehicle moving, and keep it moving,
and it all depends on the engine and vehicle weight/aero.
There is a reason why you guys notice these tunes shifting at 2 grand each gear, while picking up fuel mileage.
An ls or Lzt engine is much more efficient at 2000RpM @ 50 degrees of timing, than it is at 1200RpM @ 20 degrees of timing like they run stock.
Swathdiver actually nailed it on the rpm thing. An engines efficiency is an engines efficiency.
It is fully possible to get better mileage at part throttle at higher rpms than lower rpms.

(Hypermilers might disagree, but there are precious few hypermilers who are also into tuning, so, feel free to ignore this one for now,
except to make a note of what the goals might be while the driver lifts off.)
If the goal were best MpG whenever reasonably possible,
Lean Cruise and Variable Cam Timing did not have to be mutually exclusive - til the EPA misunderstood Lean Cruise and said they were.

It'd be interesting to compare GM OE L8T tuning to that of the L96, the last Gen4 Vortec workhorse 6.0L.