VVT and our Cam Angles 4.2 I6..

[Mep_q8]

I've played around with a few settings of these parameters.. but to be honest, nothing really showed any difference in gains.. I'm trying to get a couple friends of mine to help me rent out a dyno to tune theirs and my trailblazers. This is so I can actually start really playing with these settings, and get some numbers on paper.

I would like to learn more about them before I move on, so hoping some ppl would be able to chime in and give me some tips/pointers. Looking at the tables, I've noticed that Cam Retard appers when Knock is detected, which to me sounds kinda weird. Do the cam angles add to detonation/pinging? Does it provide more or less spark? Or how about taming the engine down so as not to work so hard?? These questions are in my head, with a million possible answers, but with my lack of experience, I can't pin point why the cams are actually getting retarded!

My trailblazer and my friends all pretty much run in PE mode as fuel is so cheap over here we don't really need to worry about mileage. In the Base PE vs RPM, it shows the cam angles at max around the time the stock factory tune sets the PE to kick in. So this is giving me a hint that by maybe adding to the lower cam angles, I'd get that "turbo lag" feel lower down the RPM ranges.

The problem now is that there are so many different parameters to play with that go hand in hand. These are CAM angles, CAM Modifiers Mult, Cam Retard and they are all crossed with different values such as RPM, Fuel Trim, MAP and Knock Learn (First I heard of it too).

Here's a tune i've done for my ride, I've added the max parameters from '02-'06. So don't worry about them for now.. It's just an idea of what I have to work with.

Any advice you could provide, please let me know...

Cheers!

[gman4dx266]

fuel is so cheap over here we don't really need to worry about mileage.

Where you at? Im packing my things now! lol

[redhotjamedjimi]

All of those parameters for your cams are the same as adding or taking timing from your tables. or having an adjustable cam gear. Adding fuel add timing. And when its stoich in the low end adding timing is good o a point. thats where the dyno comes in and knowing your a/f ratio and commanded.

[Blandmiller]

I would disagree that the parameters listed are the same as adding or taking timing from the tables. The LL8 I6 exhaust cam timing is hydro-mechanically variable. Changing the exhaust cam timing will also require you to modify the spark timing and fueling to get the most benefit. The LL8 I6 'Cam Phasor' operation is similar to the Honda 'V-Tec', but only controls the exhaust cam timing.

I would agree that a dyno is the best way to tune the cam timing tables.

[redhotjamedjimi]

I think those first couple add timing to the cam so when it opens more its closing at the same time. not necessarily spark. that im not so sure after review.

[Alvin]

I will say that you do not nessisarly have to make spark and fuel changes to see immediate improvements in HP and Tq with different cam settings.

I will also say that the changes are completely counter intuitive and if it was not for a dyno I would probally still be loosing power with the wrong advance.

[Mep_q8]

Coming to think about it, at lower rpms the engine can't breath as much as it is not spinning enough, so advancing the cam too much would make me lose power.

I still haven't had the chance to go on the dyno, but I have played around with these settings with a better idea with how to play them. I advanced the cam retard to around the same settings in the upper rpm regions all across the lower and mid range rpm. At WOT, I felt a lot more of that balls in ur stomach feeling through the mid rpms, but it did feel a little sluggish in the lower rpms.

With this in mind, I did research onto what VVT does. At lower rpms it does not open the valves as much to prevent blowback of the air-fuel mixture which improves torque. With this in mind, I've set a table to gradually open from a 7 all the way up to a max of 14 degrees at WOT.

I have saved all the different tunes, and when able to access a dyno, I will put my theory into practise. Hopefully it should be right, I haven't downloaded the final tune to check if the new settings make a difference, but I have a strong feeling it would.

Not too sure how GM came up with these figures, but as it has been said before, they normally spend the least time on calibrations for our engines.. i guess that's why HP Tuners was created! Thank god for HPT's!

[Blandmiller]

Trailblazin, where did you find information that describes the VVT as a variable lift function?

I have seen several cutaways and read several GM tech articles for the cam phasor and I don't think it has any means of controlling valve lift. I remember one article explaining the VVT controls as being a replacement for the PCV valve. That article spoke about controlling the valve overlap as serving this function.

Post a link to any information that is contrary to what I have described. I will search my Helm manual for some more info.

[ds8]

The cam phaser adjusts the phase of the valves, not the lift of the valves; this is where the term "cam phaser" comes from. VVT can't change lift, that is what the lobes of the cam do, but VVT can rotate slightly (advance or retard) where the exhaust valves start to open. Theoretically, if the phaser is fast enough and the VVT drive mechanism's inertia is small enough, VVT could change the valve's open duration on the I6. The cam phaser would have to be adjusted for each cylinder on every exhaust stroke for that to happen, and from what I know about these valves, that it way out of the operating bandwidth.

VVT replaces the EGR valve by creating overlap between the intake and exhaust valves. I don't think it was meant to replace the PCV valve...the I6 doesn't have a PCV valve, but I don't think VVT is why.

DS

[Blandmiller]

Directly from the Helm GM Service Manual:

Exhaust Camshaft Position Actuator Description
The camshaft position actuator is bolted to the front of the exhaust camshaft and is integral with the sprocket. The actuator and sprocket can only be replaced as one unit. The actuator has a hydraulically actuated piston located in the hub. The piston has an internal helical spline that slides in mesh with the gear. As the piston moves, the piston and gear mechanism changes the timing of the exhaust camshaft, relative to the cam drive sprocket. When oil pressure is applied to one side of the piston, the cam moves clockwise and timing is advanced. When oil pressure is applied to the other side of the piston the cam moves counter-clockwise to retard timing. The total range of actuator rotation is 0 to 25 camshaft degrees. At idle, the exhaust camshaft position actuator is at full advance or 0 degrees.

I don't believe the VVT changes anything other than exhaust cam timing. There is no method of varying the valve lift.

[Mep_q8]

this is great news to hear! I searched high and low on the inet for research regarding our VVT and didn't come up with anything, so i searched into the background of VVT and got my concepts from the japanese engines. I thought the trailblazer I6 wouldve been the same concept, but I guess not.

Thanks for the helms explanation of our VVT, its really cleared things up and put things into perspective. Not only do we have to tune our cam angles now, but find the best spark for the changes!

[buktooth]

VVT changes the LSA(lobe separation angle)or valve overlap period plain and simple.
Wider lsa eg 119 DEG= smooth idle, great low end torque, great emissions , reduces reversion due to less valve overlap.
Tighter LSA eg 112 = better top end more overlap to provide better breathing at higher rpms.
The 4.2L LL8 , having two camshafts, one intake camshaft, and one exhaust camshaft, that can advance and retard, allowed the engineers to have thier cake and eat it too. Although thier main goal was to use this to eliminate the egr, it is just like having a street cam when putting around town, and with the stomp of your foot the camshaft turns into a race cam shaft for power in the top end.
If anyone has one of these engines on a stand with a degree wheel handy I would love to see the valve events at zero degrees advance and at the fully advanced position of the exhaust cam.
This is probably the reason why these engines make good power down low and almost as much as the V8s up high. This adjustable LSA cant happen with a V8 engine with one camshaft,you can only change the camshaft centerline.
I would bet it is also the reason this engine won Ward's Communications "10 Best Engines" two years in a row.