LS3 Seizes in GXP

[ls2gto]

Check out this horror story over at the G8 board. A GXP LS3 A6 seizes at 2300 miles. The dealer is not sure why but suspects a cracked block. Now things get even worse. The dealer replaces the engine and then the second engine loses a cylinder at 250 miles. The theory is that when the first engine broke, debris was blown out the exhaust manifolds and into the cats. Then, the second engine sucked the debris in the cats back into the cylinder through the exhaust valve during decel induced exhaust reversion. There is actually a TSB on this posssibility.

http://www.g8board.com/forums/showthread.php?t=21446

Understandably, the owner is now considering lemon lawing the car.

[Saladbar]

Haven't clicked the link..... But... Their theory is crazy IMO. I would think the exhaust is always pushing air, releasing pressure from the cylinders.

Then again, GM said my cam lope caused my M6 tranny to make noise at low speeds and called it "gear rash".

[joelster]

Their theory is 100% wrong guaranteed. No way in hell a motor will suck something back into an engine. I would suspect something in the intake system chunking pieces into the motor, or possible in the wiring harness caused an injector to not fire ie: leaning out a cylinder.

[MyNewSolstice]

not on a manual thats why u never shift in a manual if going through deep water you'll suck water up!

[Saladbar]

Originally Posted by MyNewSolstice

not on a manual thats why u never shift in a manual if going through deep water you'll suck water up!

Umm... that happens in auto's too.

And the water comes in via the throttle body.

[ls2gto]

No, the theory is valid- it is called "reversion" and occurs during the overlap period when both the intake and exhaust valves are open on the piston down stroke. Anti-reversion headers are used to help reduce this effect. Here's a link:


http://www.custom-car.us/exhaust/header.aspx


Reversion is the flow of exhaust gasses back into the combustion chamber when the downward movement of the piston creates a vacuum in the cylinder. As we mentioned in engine tuning basics, the exhaust valves are still open when the intake stroke begins. This presents the potential for exhaust gasses to be drawn back into the combustion chamber when the piston moves down the cylinder. Any exhaust gases that are drawn into the combustion chamber will displace the air/fuel mixture being drawn in through the intakes valves and will increase the temperature in the combustion chamber, thus reducing the volumetric efficiency of the engine, as well as engine power.

Hot cams have more overlap to increase the amount of intake charge at high RPMs, but this is at the expense of reduced bottom end torque and causes the lopey idles that sound so cool.

[The_Blur]

Originally Posted by ls2gto

No, the theory is valid- it is called "reversion" and occurs during the overlap period when both the intake and exhaust valves are open on the piston down stroke. Anti-reversion headers are used to help reduce this effect. Here's a link:


http://www.custom-car.us/exhaust/header.aspx


Reversion is the flow of exhaust gasses back into the combustion chamber when the downward movement of the piston creates a vacuum in the cylinder. As we mentioned in engine tuning basics, the exhaust valves are still open when the intake stroke begins. This presents the potential for exhaust gasses to be drawn back into the combustion chamber when the piston moves down the cylinder. Any exhaust gases that are drawn into the combustion chamber will displace the air/fuel mixture being drawn in through the intakes valves and will increase the temperature in the combustion chamber, thus reducing the volumetric efficiency of the engine, as well as engine power.

Hot cams have more overlap to increase the amount of intake charge at high RPMs, but this is at the expense of reduced bottom end torque and causes the lopey idles that sound so cool.

While you're correct about the physics of a vacuum sucking air back into the chamber, the forces causing this to happen are not strong enough cause debris to go back into the engine since the intake supplies the overwhelming majority of air for combustion. If it were possible for large pieces of debris to experience enough force in such a short instant to fly back into the engine, then even more air would have to be provided from the intake because the air sent into the exhaust has already been used for combustion. It would be a lot like exhaling then breathing the same air back into your lungs. While we all do it, the majority of our breathing air is not the same as the air we exhaled. Cars are the same way. Only a very small amount of air that flows back into the engine comes from exhaust, and that amount is not enough to compel an object with the entire exhaust cycle of force behind it to change direction. It defies physics for any object with the force and inertia sending it in one direction to be so abruptly reversed with so little influence.

In other words, ls2gto is right to point out that exhaust air does return to the engine in small quantities, but anyone proposing that these small quantities is enough to suck damaging pieces of debris into the engine is smoking exhaust fumes.

[CamaroSpike23]

the theory is a valid theory, but not for this.

this looks like he went super lean on the #7 cyl and melted the head which then jammed up in the cyl

[Saladbar]

I would think that a manifold still has one exhaust valve opening at a fairly quick rate (every .75 of a second or so @ idle) on the exhaust stroke, forcing air out, even if another valve is still slightly open for the intake stroke on another cylinder. I think that alone should be enough air being forced out from the to overcome any vacuum the slight intake might be causing. Since it's all sharing the same manifold mind you. Of course, at idle, I'm not sure if the vacuum is more that the exhaling - if you know what I mean.

I'm probably totally wrong, and likely should read the link above... But I can see where reversion may happen more frequently with headers (single tube per cylinder) vs on a manifold (multiple cylinders per tube). <- Not counting the short run from each exhaust port of course

[ls2gto]

Originally Posted by The_Blur

While you're correct about the physics of a vacuum sucking air back into the chamber, the forces causing this to happen are not strong enough cause debris to go back into the engine since the intake supplies the overwhelming majority of air for combustion. If it were possible for large pieces of debris to experience enough force in such a short instant to fly back into the engine, then even more air would have to be provided from the intake because the air sent into the exhaust has already been used for combustion. It would be a lot like exhaling then breathing the same air back into your lungs. While we all do it, the majority of our breathing air is not the same as the air we exhaled. Cars are the same way. Only a very small amount of air that flows back into the engine comes from exhaust, and that amount is not enough to compel an object with the entire exhaust cycle of force behind it to change direction. It defies physics for any object with the force and inertia sending it in one direction to be so abruptly reversed with so little influence.

In other words, ls2gto is right to point out that exhaust air does return to the engine in small quantities, but anyone proposing that these small quantities is enough to suck damaging pieces of debris into the engine is smoking exhaust fumes.

GM's engineers do not agree, but then again, maybe they are smoking exhaust fumes. Check out the TSB at post 126 in the thread:

[Gentry78]

yes the theory is true, but will never cause enough vacuum to suck anything back in because the vacuum from the over lap is out weighed by the force of the other cylinders blowing out the exhaust

[walls]

Well, "this can't happen" folks...how do you explain that TSB?

[DTM]

I'm calling driver mod error, going through 2 LS3 motors like that is insane, was this guy maybe redlining the car as he drifting out of the dealer?

Both times?

[leviticus88]

Originally Posted by walls

Well, "this can't happen" folks...how do you explain that TSB?

GM trying to cover for poor quality control in the motor assembly perhaps...

Physics is physics, GM's TSB isn't going to change Physics...