eallanboggs

iTrader: (1)

GMs brochure says VVT allows "the cam profile to change" and "at high engine RPMs the intake valves remain open longer". This implies the exhaust valves are also staying open longer too since this is a single cam, but they don't make mention of that. It goes on to say that "at lower engine speeds VVT closes the valves sooner to lessen or eliminate any unburned fuel exiting the cylinder". I assume they mean the exhaust valves here to help improve scavenging. The cam profile can't really change as they say it does, but they can dynamically advance and retard the timing. This doesn't cause the intake valves to actually stay open longer does it as they say does it? The only way I can see this happening is if they adjust the timing during the intake stroke and with 8 cylinders you've always got those 8 cylinders on any of 4 different strokes at any point in time. The VVT would have to work double overtime to do this properly and while it's advancing or retarding the timing on one cylinder it is also changing the timing on other 7 cylinders too. Does VVT work as they say considering this is a single cam engine unlike the Japanese motors which use dual overhead cams which can be independantly controlled?

Daez

VVT is VTTTTTEEEEEC!!!!!!!!!!!! but for real cars.

helicoil

iTrader: (104)

Yes, VVT works as they say it does and has been working on the LL8 Vortec 4200 engine quite well since 2002 in the Envoy and Trailblazer. Although this engine only uses an intake cam phaser. It has the ability to change the cam timing by 25 degrees which is 50 crankshaft degrees. This gives them the ability to close the intake valve early at low engine speeds creating alot of engine torque, minimizing reversion and valve overlap. And at higher engine speeds they can delay the IVC allowing sufficient cylinder filling where the time duration is shortened by the RPM. The LL8 4 valve inline Vortec 6 cyl makes a tremendous amount of power and torque because of this, I believe almost 300HP

The movement of the camshaft takes place at the cam gear assembly and is controlled by oil pressure to move the phaser on a helical cut gear rotating the 'entire' camshaft counterclockwise, when the oil control solenoid is turned off a spring forces the phaser back the other way. When this occurs all cylinder valve events are controlled the same (adv or rtrd) at the same time because the entire camshaft moves.

There are several GM engines out there now with this technology, not to mention other manufacturers (domestics) are using it, and have been, for awhile now.

Many also control the exhaust camshaft as well, and by doing so, many manufacturers have been able to do away with EGR system hardware all together thus reducing problems associated with inoperative valves, clogged passage ways from carbon build up, etc. They are simply controlling the overlap period to trap 'inert' exhaust gases (EGR) in the chamber without having to recirculate them through a redundant plumbing system such as an EGR valve that has proven problematic over the years.

This is also why you don't see EGR systems on many Hondas. VTEC

Sometimes it is funny to see how much the Domestic OEM's learn from the Japanese and European car manufacturers and then apply years later. Makes you wonder who is chasing who technology wise

eallanboggs

iTrader: (1)

That's very interesting, but since this is a single in block cam (unlike many overhead cam engines which have up to 4 camshafts) how is it possible to only phase the intake side without effecting the exhaust? I can't see how. On a 4 cam setup you have the ability to independently control the intake and exhaust on the even AND odd banks, but not on this motor. How quickly is this solenoid being pulsed to accomplish this intake only phasing? It better be near the speed of light. Or is the solenoid just turned "ON" and "OFF" depending on LOAD conditions? The spring on the phaser must be one hell of a strong spring because it's pushing against 16 springs that are trying to keep the valves closed. If some manufacturers are controlling the exhaust events too with cam phasing doesn't it follow that the intake event timing is being altered on this form of VVT since it uses a single in block cam? The intake and exhaust events are married to each other in a single cam application even with the use of a cam phaser.

briannutter

When the aftermarket programmers give us the ability to change the phasing, it will allow us to run higher compression...Too much? just retard it a bit and wah-law...no more detonation at low r's.

P Mack

iTrader: (6)

The solenoid isn't pulsed for every valve opening. It retards or advances all valve events. Instead of saying it keeps the intake valve open "longer", they should have said "later". As for the independent intake/exhaust issue, the LL8 he was talking about is DOHC.

GOaT Cheese

Cam phasing on GM engines only moves the exhaust cam. GM, at this point anyway does not phase the intake cam.

gun5l1ng3r

Will this be the saving grace for us guys stuck with 91? Allow us to run 91 during the week and race fuel on the weekends?

Alvin

iTrader: (2)

The VVT used in the 4.2L I6 is pretty awesome. It took alot of dyno time, but I was able to clean a good 25 plus hp out of our TB and even more than that in some spots.

The VVT used in the new cam in block engines should also be about as powerful.

cantdrv65

iTrader: (3)

Do any of you guys actually think a Trailblazer with the 4200 I6 has low end torque? I can tell you one thing my 4.3L V6 blazer has ALOT more low end grunt than that 4200, though it doesn't have the top-end power. The VVT doesn't impress me on the TB...

Alvin

iTrader: (2)

Get a tune on it. I promise it will change how you feel about the 4.2L I6. They are so held up from the factory its unbelieveable.

eallanboggs

iTrader: (1)

I don't know about the design of the other GM VVT vehicles, but I was speaking of the 07 Silverado with VVT which is a single in block cam. That means there is no way to only phase the intake or exhaust events since all the lobes ride on the same cam. With a single cam when you advance or retard the timing on one valve(be it intake or exhaust) you by default make that same change on the other 15 as long as the phaser is active. You just don't have the same flexibility or control with a single in block cam as you do with a overhead quad cam set up. This phaser must be load driven which is everytime you stop and start or go up a steep hill pulling a trailer. The more I think about it the more I want to stay away from the VVT system in the single cam in block Vortec. If it were a quad cam engine it would be different, but one of the main reasons so many people like the LS series is its' simplicity. It's old school technology, but it still works well. They added AFM(DOD) and now VVT. I wish they would have left it alone and just put the techno stuff on their overhead quad cam engines like the Pontiac quad four which every mechanic loves to hate.

cantdrv65

iTrader: (3)

How so? Timing and fueling or does it have some kind of invasive torque management?

phantom meneSS

If they controlled the lifter operation on the VVT like they do with the DOD that could change the intake to exhaust opening times to some extent...

I think i would prefer the engine to stay simple, i like the idea of having 7 litres and DOD but with 7 litres who gives a cracker about the increase in low end torque alternating cam phasing will give.

In fact id like a 70kw electric motor mounted in the front wheels to do the the cruising and the 427 to take care of the rest, green and mean? sure.

(can i patent an idea?)

Alvin

iTrader: (2)

The torque management isn't bad. The trucks just flat out have very terrible tuning in them.. When I'm able to dyno tune one and pick up over 25rwhp in spots on 87 gas you have to think what the hell they where thinking when they did the tune.

traumadog

My understanding of the VVT for the "cam-in-block" engines is that the camshaft is phased vs, the crank - and since it's a single camshaft, both intake and exhaust are changed.

That being said, I think there was a patent now out there for a concentric camshaft - where intake and exhaust can be handled independently.

2002_Z28_Six_Speed

iTrader: (19)

Maybe the fact that pre-production engines are not the same cast as production parts and the fact they dont want the vehicle to perform too well out of the box? Just a guess.

eallanboggs

iTrader: (1)

They don't control intake and exhaust event seperately with either VVV or AFM(which used to be called DOD). With AFM they simply shut 4 cylinders down under low load situations while with VVT they phase(advance) the intake timing using a helical cut gear and a spring to force it back again. Of course when you phase(advance) the intake timing on a single in block cam engine you also phase the exhaust events too. There's no way around it. Maybe they should use a cam within a cam so that the intake and exhaust lobes could turn independantly of each other. You could use seperate driven sprockets and phasers for each one and choose to phase or not to phase them as you so desire. Now you have almost as much flexability and control as an overhead quad cam engine with a clutch on the end of each cam. Hell lets just make the LS series into something so complicated no gearheads will want to mess with it anymore.

GOaT Cheese

Almost, the 4.2L (2.5L and 3.5L inlines as well) VVT RETARDS the EXHAUST camshaft. And the Silverado uses a VEIN type actuator that is controlled via a pulse width modulation solenoid that routes oil pressure to "hold" a certain cam position.

helicoil

iTrader: (104)

OOooopz, goatcheese is right the LL8 4200 ONLY controls the exhaust, it was late that night I posted, my mistake. The LL8 engine used a spline phaser up to 2005 and now they are using the vane phaser as mentioned because it is faster and lighter and it is more of a compact design.
The PRIMARY function of this system again is EGR. The exhaust camshaft events are only delayed, never advanced. With the exhaust valve closing delayed, the low intake manifold pressure and exhaust backpressure cause exhaust to be drawn back into the combustion chamber. When EGR (exhaust gas recirc.) is not desired, the CPA (Camshaft Position Actuator) would advance the camshaft back to 0 (straight up-not really advanced). The 2.8L, 3.5L and 4.2L Vortec engine family will ALL do this.

The 3.6L High Feature HFV-6 and the Cadillac 4.6L LH2 Northstar, now on the road, will control BOTH the Intake and Exhaust camshafts with phasers as well. The phasers on the Cadillac LH2 are controlled with both oil flow and electromagnets mounted to aluminum housings on the front of the cylinder heads. Max range of control on these engines is 40-50 camshaft degrees. However, these engines are all OHC style engines. BTW, the 4200 LL8 is a 4-valve twin cam, not four cam engine such as the HFV-6 and 4.6L LH2 Caddy.

I do know the Gen IV 5.3L LH6 used in the Trailblazer EXt, GMG Envoy XL and XUV had DoD (Displacement on Demand) which was very similar to the 2005 Chrysler HEMI 5.7L which Chrysler called MDS (Multiple Displacement System) which is only cylinder deactivation not VVT (Variable Valve Timing) same as the GM . These systems use a two-piece collapsable lifter, or more specifically a sliding outer sleeve lifter, locked in place by spring loaded pins to make it a conventional operating valve lifter. When HIGH oil pressure is applied to the lifter the pin is moved inward allowing the inner portion of the lifter to move independently from the outer portion therby cancelling valve actuation. The programmers then disable the injector from discharging fuel and the cylinder is cancelled, nothing comes in and nothing goes out. I know the Chrysler system does not do this all at one time for the 4 cylinders with the special lifters, they do it in succession based on firing order, one at a time. So is it really 4-cyl mode......

From what I have read on the Gen IV 5.3 with DoD is that they do go into V-4 mode cancelling cylinders 1&7 and 4&6. F.O. is (1-8-7-2-6-5-4-3)

I suppose this may have been off-topic from the original posters thread because he inquired about the in-block single cammed LS engine with a cam phaser. I would only imagine it advances the camshaft for increased torque/cylinder pressure when desired since the intake and exhaust lobes are married together. With only one camshaft the benefits for trapping exhaust gas or even changing valve overlap from a SINGLE cam movement event are not really possible. Hmmmm. Not familiar yet with this engine.

Just wanted to clean up some things on this thread that were muddy'd initially by me

Interested to hear more about this engine from the general.