Cat-back Mods

[jwintersteller]

So here is an interesting update, at lease I found it interesting. Since my turbo back swap, my MPG's have dropped about 3 miles. I was expecting to get an increase not a loss in milage. Everything I read said my mileage would increase. I tried a few things to get it back to where it was (averaging 16 to 17 MPG's) but nothing got me over 15. I'm pretty easy on the throttle, mostly staying between 65 and 70 MPH.
I was very happy with the EGT's being between 600 to 700 and rarely going over 1,000 even on long climbs. So I was hesitant to put the stock down pipe and cat back on, thinking that these temps would go up significantly.
A few weeks ago I had to pass a smog test so I reinstalled the stock down pipe and cat and left the 4" system after the cat (as I described in my post above). I decided to leave it on for a few trips to see what kind of a difference there would be.
As expected, my milage went up. I just drove back from Nevada, a 300 mile trip with no tailwind even some strong head wind for a stretch. I was back to 17.3 MPG.
What I didn't expect was that the EGT's only went up about 50 degrees (if that) across the board. When I put this whole system on it dropped those temps about 200 degree across the board. I really thought that the most exhaust flow blockage would have come from the cat and 3" down pipe. But it would appear that the muffler was the bigger problem.
I have noticed some loss of power but I don't need power as much as I want 17 to 18 MPG.
The exhaust smell is a bit better as well. So I will not be reinstalling my custom down pipe, which is a little tuff for me because it was a lot of work and had turned out so nice. And the cat will stay as well.
So it seems that a 4" cat-back exhaust system is an overall better system then a turbo back system for these 6.0 PSD vans, at lease for me.
Just thought I should share my experience incase someone was thinking of going the turbo back rout.
Cheers,
John

[JoeH]

This is really interesting. Could it be that the decrease in backpressure on the turbo actually decreases mileage by impacting fuel/air combustion efficiency? You would think that it would be the opposite and that allowing the engine more breathability it would be more efficient but instead it's the exact opposite.

[BroncoHauler]

John,

Maybe you need a more open air intake to go with your open exhaust?


Herb

[Viejo]

Originally Posted by JoeH

This is really interesting. Could it be that the decrease in back pressure on the turbo actually decreases mileage by impacting fuel/air combustion efficiency? You would think that it would be the opposite and that allowing the engine more breath ability it would be more efficient but instead it's the exact opposite.

Is it possible that you would need a custom tune chip to take advantage of the changed engine geometry?

Is this cat-back change effective on a 7.3 as well?

[jwintersteller]

Originally Posted by BroncoHauler

John,

Maybe you need a more open air intake to go with your open exhaust?


Herb

I tried that Herb. Here is that project; viewtopic.php?f=39&t=11277
I did think that once I got that air intake installed that it may balance thing out but it didn't help.

Originally Posted by JoeH

This is really interesting. Could it be that the decrease in backpressure on the turbo actually decreases mileage by impacting fuel/air combustion efficiency? You would think that it would be the opposite and that allowing the engine more breathability it would be more efficient but instead it's the exact opposite.

This is what all the info I found lead me to believe. Even if the MPG's stayed the same and the power went up, I would not have been surprised. But for the milage to drop like that? The only explanation I got from a few diesel shops was "yeah, sometimes that happens". What?

Viejo, I do have a tuner with custom tunes. The guys at Innovative Diesel (their the one that did the tunes on my SCT Tuner) suggested that I try their Supper Street Tune. They said sometimes this can bring up the milage if you're easy on the gas pedal. I did drive around for a few months on the standard Street Tune but that didn't help (it didn't make things worse either). So I thought that the Supper Street Tune wouldn't be the right fix.

I also know that getting better exhaust flow is not only about bigger pipe. Sometimes this can cause turbulence that can cause more back pressure. But because my EGT's are lower with my custom down pipe, I have to believe that the flow is better.
And not enough back pressure can cause a loss of low end power. Which did not happen in my case. I absolutely have less power now that the stock down pipe and cat are back. But the EGT's are only slightly up.