Is the VGT restricking air flow in error?

[zzman]

I am no diesel mechanic just a long time Dodge diesel rider. Please read my story / thoughts.

I had a 97 3500 with straight exhaust and the fuel plate slid all the way up. Great truck no real issues,
pretty loud exhaust.

I had a 01 2500 with straight exhaust and smarty jr tuner. Great truck no issues too bad. Loud exhaust.

I had a 04.5 3500 with 4" exhaust and muffler and smarty jr tuner. Super truck no engine issues.
Exhaust a little loud with muffler, too loud without muffler.

Now, I had a 08 with 5" straight exhaust and smarty jr w/me no emissions. The truck was quiet and
just whiseled with VGT exhaust brake on. This truck needed no muffler and was still quiet as stock.
But this truck blew the head gasket. Had it repaired and better head studs installed. A couple months
later, blew the head gasket again.
The diesel shop mentioned this was happening to many of the 6.7 and they were reseaching the issue.
Later they mentioned the design was not letting the pressure exit the cylinder properly. I think that is what
they said. This I believe would be with TUNED engines.

My thought is the turbo is creating the back up because the lack of sound from tail pipe. It does seem to
produce more air intake due to the lack of smoke compaired to the 5.9s that smoked like crazy.

So now to the point, I have a bone stock 2011 with terrible fuel milage (compaired to 5.9) and was
wondering if the turbo is in fact the weak link and if so is there a replacement turbo that will help this
truck and still stay emission legal?
I'm averaging 9.8 mpg for a year long term. My 5.9 got 11.5.
The truck has enough power with the Aisin to get by, so not concerned for more power.

Running exhaust through the intake is just WRONG, so I think maybe the turbo may be wrong too.

Any thoughts would be appreciated here. Thanks

 

[cerberusiam]

Yes, deleting the emissions on 6.7 with the VGT is a sure way to have HG problems. Not really the turbo's fault, more the program that runs geometry. It is programmed to have the EGR functiuonal as a blow off valve and to keep cylinder temps low enough. When you delete the emissions portion of it it does not change the turbo variable collar operation and the HG's will not hold.

If you have to meet emissions there really is not other choice but to retain the factory operation. Some help is available with a blow valve in place of the EGR but that is only part of the equation, can still have problems depending on the tune. Theother choice is put a better turbo on it that does not have those control problems but it won't pass a visual emisisons exam then.

 

[zzman]

Thank you Cerberusiam, That makes great sense to me. So I guess the valve (plunger) on the intake sorta acts like a waste gate
in a way too limit boost.
Thanks again, I've always thought highly of your thoughts in the past years.

I'm dis-likeing this 6.7 more and more.

 

[cerberusiam]

The problem is not too little boost when deleted, it is too much boost or too much air.

The emissions design of the fueling system is running stoich rich to keep cylinder pressures and temps low plus leave fuel for the DPF to do its job. The EGR is displacing part of the incoming charge with dead air to maintain the balance. When you delete the EGR you now have the same amount fo fuel but more combustible air. The program on the VGT turbo closes it down fast to push more exhaust gas into the intake offestting the boost increase of a smaller housing. Without the exhaust gas there is more combustible air in the cylinder with enough fuel to bring the mixture closer to stoich whihc jumps the pressures an temps in the cylinder. All the deletes are trying to achieve a closer to stocih mixture which increases power and efficiency but if you don't change the other operations there are unintended consequences.

The 6.7 inherently has issues with the head gaskets becasue of the engine design of simaesed cylinders and the very narrow spacing of the cylinders. Siamesed cylinder create differnet expansion rates around the fire ring of the head gasket which is essentially walking the gasket and tearing it. The operational range of the combination of engine design, HG, head bolt TQ, fueling mixture, and planing of the head just leaves a lot lower margin for error.

There a re things than can help the porblem and it may take more than 1 to accomplish a decent result. The blow off valve on the EGR circuit helps as it drops the drive pressure on the turbo when the collar closes and helps limit the amount of air ingested. Whne the head is off it need to be machine very cloase to flat and as fine as possible on the grind. If a standard smootheness and accuracy is a 30, it needs a 10-15 grade to to be adequate. Standard head planing just does not work in these conditions. Better head bolts help as well as getting rid of the VGT turbo.

Everything is just so close on tolerances on these engines that when that is disturbed other problems ensue. Definitely not like the 5.9 that had a much broader operational range and modding pattern. Unfortunately, that is history and this is what we have to deal with today.