Why The Gen-V LT Outshines The LS

[petethepug]

Oh, this has gotta hurt!

This Cadillac Escalade V-8 Failed After Just 4 Miles — Motor1.com

General Motors is under investigation for a string of engine failures. New cars shouldn’t have engines that fail after just a few miles. However, one 2023 Cadillac Escalade suffered a catastrophic failure with just four miles on the odometer. A new video from Dave’s Auto Center attempts to...

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[Marky Dissod]

VVT, DOD & DI are all technologies that provide additional tuning benefits, but that add complexity expense & failure points to the engine without a benefit significant enough to compensate for it.

As these technologies were added to the LS the engine became less reliable but removing that tech was easy & restored the former benefits to the engine;
if the LT is to ever become a staple like the LS it will be because of the ability to affordably ditch the unreliable aspects & simplify the processes back to a reliable engine that can handle significant performance increases without falling apart.

What works is what works, all the claims about advantages with the more involved technology never matter if the added failure points nullify the benefits.

You can hit & maintain a target AFR consistently with computer controlled port fuel injection, the injectors do not need to be introduced to the combustion chamber & its DESTRUCTIVE environment to achieve a reliable AFR;
it just costs more to develop injectors that will work in a combustion environment & it reduces their lifespan to move them to the combustion chamber.
VVT can expand the optimal rpm range of the cam profile beyond a static ground cam but not without adding cost & reducing reliability to a point that nullifies the benefit,
DOD is just a gimmick to please the EPA that provides no real advantage in exchange for its failure prone design

Not sure if I agree with you on Variable Cam Timing (it varies the cam which varies the valves).
Guess the deciding proof for me would be to take an L92 / L9H, disable the Variable Cam Timing,
custom grind and install a cam meant to meet or exceed ALL the performance targets hit by the GM OE cam with its VCT active,
and if the custom grind 6.2L performs as well or better than the GM OE 6.2L, then I'll agree with you that GM's VCT is worthless.

Direct injection and Cylinder Valve Deactivation, yeah, those features are bugs;
I'm convinced they were designed and installed with the express intent of harming the Gen 5 / LT V8s' durability / reliability at or past 175,000 miles.
Put another way: GMT800s and early GMT900s are hurting GM's bottom line.

 

[RET423]

I didn't say VVT was pointless, I said it expands the optimal rpm range the cam can deliver over a cam that has a static grind

But to gain that expanded optimal range comes at a cost of more moving parts & more complex system to apply the changes on the fly to gain that benefit

The gain compared to the cost in dollars complexity & additional failure points/reliability don't make sense in a real life application; the regular person who operates a VVT engine would experience little to no difference if that same vehicle had the VVT removed

If the system never fails while they own the vehicle it's no big deal but over time increasing failure points are chickens that will come home to roost, the manufacturers know this so they delay these things as long as they can; but government regulations eventually push them to implement tech that increases the cost of their products & reduces the reliability at the same time

 

[Marky Dissod]

I didn't say that you said VVT was pointless.
To avoid crisscrossing my inferences with your implications ...

I'm of the EXPERIENCED opinion that
V4 mode is a liability not only because of how it fails, but how it ages - BADLY.

Any-cylinder-deactivation WORKS better than V4 mode, ages a wee lil bit better than Engine Half@$$,
but when it fails, how many cylinders does it even halve left, and for how long?

Don't think VVT is not a liability as it ages, seems to age gracefully with little to no degradation.
Don't know enough about how it fails to render my verdict on it yet.

Direct injection is a disappointment, given that it can't / doesn't keep intake valves clean.
It does not appear to have enough of a history of failure, YET,
but GM can tungwipe my kornwhole for the cost of replacing those direct injectors.
I know that there are port injected versions of these engines in aftermarket development.

 

[rdezs]

As for GM's VVT, it's proven to be quite reliable and effective. Let's not forget that it allows you the best of both worlds to some extent. Not too long ago, if you wanted to pull a heavy trailer you went for the RV cam that gave you the low-end torque, but sacrificed high-end horsepower. The guys wanting the high-end horsepower had no low end torque, and an erratic idle. And good luck getting a heavy trailer rolling with one of those cams. The VVT is a very good compromise for real world drivability and function. And it's one of the reasons you can get 20 miles per gallon out of your 6.2 while having over 400 horsepower.

The LT.... I think they broke one of the most basic rules. Don't mess with something that works real good. At some point you make things so complicated, it drastically affects reliability. I have a generation one SBC in a Jeep CJ7.... And it's the one vehicle in the stable I would trust wholeheartedly driving to Alaska. The electronics on today's vehicles are troublesome to say the least.

 

[RET423]

VVT has the capacity for great things but GM uses it to enable EGR without an EGR system, the hold the exhaust valve open momentarily when EGR is commanded to injest exhaust back into the cylinder

Because of the application I thinks it is a waste of technology that damages the engine like all EGR schemes even as the components themselves are not as unreliable as say DOD lifters

I think that cylinder deactivation is without value even if it could be implimented in a reliable way, it's intellectually offensive to think the same vehicle at the same speed in the same circumstances will require less energy to do the same work if some of its cylinders are shut off while the active cylinders drag them along than it would with all cylinders contributing; it's an expensive gimmick that has damaged GM's reputation & deservedly so.

Had the LT never been developed these technologies would not be doing any better on the LS platform, bad ideas get bad results

 

[Marky Dissod]

VVT has the capacity for great things but GM uses it to enable EGR without an EGR system,
the hold the exhaust valve open momentarily when EGR is commanded to ingest exhaust back into the cylinder.

Sounds like at least some of this 'internally insidious EGR activity' can be tuned out, or mitigated by tuning?

 

[RET423]

I don't think it can be eliminatedwith the stock cam grind but it likely can be reduced to a small amount by disabling the VVT

If the aftermarket ever starts developing VVT performance cams & the tune can be tweaked to capitalize on it the pre VVT engines will be relegated to dinosaur status, done right you can get a near flat torque curve with VVT & a cam ground to use it

 

[rdezs]

I couldn't agree more on the AFM/DOD. I saw an increase of about 1.5 MPG by deleting it. My wife commutes 50 miles a day round trip, on a 55 mph State highway. It was frequently running in V4 mode.

The self-induced EGR. Perhaps a software or hardware limiter to prevent the VVT from retarding the final two degrees? Just a thought...

 

[Marky Dissod]

The self-induced EGR. Perhaps a software or hardware limiter to prevent the VVT from retarding the final two degrees? Just a thought ...

EGR Valves on older V8s operate during low throttle, light load, and UNDER 2500RpM.
While the EGR Valve opens, the timing is also advanced. (Funny, kinda the same thing that happens during Lean Cruise. Hmmm ...

 

[j91z28d1]

yeah, I'm 99% sure I don't agree with any of that. vvt ls and non vvt ls both have no egr valve. egr was removed from the ls engine when the ecm got good enough to make them pass the epa limits without egr valves. somewhere around 2001-2003 I believe, depending on the car and long before vvt. there's not really a way to run exhaust back into the intake manifold using the cam. especially a single cam in block push rod engine just by advanceing and retarding the cam. this is been used for 50 years manually to move power band around. that's all it is.


as for afm, you're forgetting that both valves are left closed when 4 cyl is active. which means there's no air pulled into the cyl. so nothing to compress. you don't have pumping losses on those 4 cyl. so you're not only saving fuel burn from closed injectors, you're saving the tq needed to compress air/fuel. it 100% makes a difference but do the benefit out weight the down side of it breaking. not at all. it's junk because it fails. it would be just fine if worked for 300k without fail.


as for aftermarket vvt cams. this has also 100% been done. tx speed has spent a lot of time developing cams and they will supply a vvt tune for it. the 5 or 6th Gen camaros have vvt in the automatic cars and non vvt on the manuals. they get like 15ft lb of tq better with a correctly tuned vvt aftermarket cam. and it does show up on dynos and track. the liming factory is again the single cam in block. you gotta limit the amount of movement to such a small amount the bigger the cam gets so the valves don't hit the pistons., it almost isn't worth it. now a double over head cam with control of both individually. 100% worth it.

 

[RET423]

To get EGR doesn't require a seperate EGR system if you have VVT, the exhaust valve can be left slightly open at the start of the intake stroke & exhaust will be drawn into the the cylinder whenever the VVT wants it

The EGR effect is using inert exhaust gasses to pollute the air/fuel mixture enough to slow/cool its burn rate, how & when they get that exhaust to return to the combustion chamber is the EGR system

The old EGR system was replaced with VVT but EGR wasn't done away with

 

[RET423]

As for the AFM, having both valves disabled in the closed position increases load on the engine, GM designed it that way.

Take any syringe & place the plunger in any position you like, then put your finger over the end to simulate closed valves & try to move that plunger in & out; it will have vacuum resistance when you try to pull & pressure resistance when you try to push

This is how GM eliminated rattling when in V4 mode, but utilizing the resistance of trapped air to absorb the slack

If one of the valves was left open this resistance would be greatly reduced but it would still exist as a parasitic loss

But the parasitic loss even if removed wouldn't make V4 mode more efficient, the same power is needed to move the same weight at the same speed in the same conditions whether you create that energy with 4 or 8 cylinders; it's just physics

AFM is a gimmick that delivers less efficiency and increases engine maintenance costs & production costs

 

[j91z28d1]

I would disagree with basically all of that. if you have vvt control of dual over head cams you could in theory suck some exhaust back in the cyl. but even those cars have egr valves. it's not a good way to do it and with single cam push rod engine, you could never retard or advance a cam enough to do that without the valves hitting the pistons. there's a limit as how fast vvt can move a cam, and it's not per cycle. it's slowly a few deg over a rpm band. also many non vvt ls engine do not have egr valves. it was simply not needed to pass the epa limits with better ecm's. my yukon is ls hybrid 6.0 with a Miller cycle cam shaft grind. it's a one off late intake valve closing grind to let more fresh air/fuel out thru the exhaust to get a cleaner more efficient charge to compress, it requires more static compression since it's losing dynamic compression by doing that. no way it's sucking dirty air back into the cycle. it's specifically designed to do the exact opposite. and it doesn't have a egr valve.



as for afm pumping losses, do your experiment with the finger over the tip, yes it creates a vacuum and is harder to pull back, but let go and what happens? the vacuum pulls the piston back up. there's almost no actual loss. next time you have you valve cover off, pop the 2 rocker arms off, do a compression test and watch what happens. Basically nothing. you'll always have friction but that's about it.

 

[donjetman]

AFM is a gimmick that delivers less efficiency and increases engine maintenance costs & production costs

Its a transfer of wealth

 

[RET423]

EPA standards escalate, when tuning was able to hit the mark with greatly reduced EGR the factory eliminated the EGR system and ground the cam to allow a very slight EGR effect all of the time; this was sufficient to meet the EPA standard until they increased enough to once again require more EGR; by then VVT coupled with the cam grind was how they increased the EGR instead of reintroducing a stand alone system

Overhead cams are not necessary to create EGR, it can be done statically with a fixed cam grind or dynamicly with VVT or a combination of the 2 which GM has done