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Old 10-06-2013, 10:23 PM
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Stealing down from 4" down pipe to 3.5"

I feel like I've lost some bottom end and it's been suggested that its because of my 4" down pipe, so I'm considering dropping down to a 3.5" down pipe. Has anyone else done this? What kind of difference in EGTs do y'all think I would be looking at?
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Old 10-07-2013, 05:15 AM
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I don't see where it's going to help you. It's not that much "gain" with the tuning you have already installed.
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Old 10-07-2013, 08:27 AM
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The turbo is definitely the bigger restriction in the exhaust, the common thought that higher back pressure can aid in lower torque doesn't always hold true, especially in the diesel world where they don't typically produce intake vacuum like a gasser would. I would evaluate your tune/consult with your tuner and see if they can do anything to help. Ported intake has been reported to help spool up and responsiveness tremendously. The only way having an extra touch of exhaust back pressure helps is during scavaging, helping to draw air out of the cylinder...but once the turbo begins to spool there's back pressure in the exhaust too, before the turbo. Adding restriction after the downpipe I would imagine would only hurt spool up, thus hurt its ability to build boost on the intake side. I don't think adding restriction after the turbo would aid in scavaging exhaust out of the cylinders at all...if that's what you were hoping to achieve with the smaller down pipe.
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Old 10-07-2013, 02:02 PM
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Quote:
Originally Posted by Gearhead2012 View Post
The turbo is definitely the bigger restriction in the exhaust, the common thought that higher back pressure can aid in lower torque doesn't always hold true, especially in the diesel world where they don't typically produce intake vacuum like a gasser would. I would evaluate your tune/consult with your tuner and see if they can do anything to help. Ported intake has been reported to help spool up and responsiveness tremendously. The only way having an extra touch of exhaust back pressure helps is during scavaging, helping to draw air out of the cylinder...but once the turbo begins to spool there's back pressure in the exhaust too, before the turbo. Adding restriction after the downpipe I would imagine would only hurt spool up, thus hurt its ability to build boost on the intake side. I don't think adding restriction after the turbo would aid in scavaging exhaust out of the cylinders at all...if that's what you were hoping to achieve with the smaller down pipe.
If I'm interperting what you're saying correctly, that is the way I understood it. I knew, for the most part, back pressure helps out with an NA application, but with an FI application you wanted the exahust to be more free flowing. I've spoken with Gearhead and Quicktricks to no avail.
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Old 10-07-2013, 03:10 PM
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I just want to make clear that I'm not affiliated with matt at gearhead at all, I have tunes of his waiting to be loaded, but I don't work with him or anything, just a name coincidence.

Anyway, yeah...that's the whole concept behind like equal length headers and all. The air leaving one cylinder will help pull the air out of the next cylinder because of the scavaging effect in tube headers...obviously our cast manifolds don't really follow suite. But, really they don't have to, after the combustion event, the next up stroke will be exhaust, the piston drives out the spent gases, but theres a small amount of overlap where the exhaust and intake valves are open at the same time before the exhaust close and the piston travels down with the intake open. The intake boost rushing into the cylinder helps force all the spent exhaust out of the cylinder...that's the difference between NA and FI...the incoming boost also flows out the exhaust some before the exhaust valves close, making scavaging, and the need for smaller exhaust tubing to 'tune' the scavaging effect ('proper back pressure') almost totally irrelevant, especially after the turbo.
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Old 10-07-2013, 07:22 PM
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Originally Posted by Gearhead2012 View Post
I just want to make clear that I'm not affiliated with matt at gearhead at all, I have tunes of his waiting to be loaded, but I don't work with him or anything, just a name coincidence.
Haha, I figured.
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