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Duramax Diesel Performance Shop What's it take to make big power? From a little to a lot, find out what it takes. Post a question or offer your advice.

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  #1  
Old 09-27-2012, 06:20 AM
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Default deciding between 4 or 5 inch exhaust

I'm planning on putting a bigger exhaust on my lbz. I've heard that a 5 inch straight pipe is so big it doesn't have enough backpressure to spool the turbo. I am planning on running a straight pipe, or maybe a full flow "muffler". I have heard the 5 inch screeches at highway speed at WOT. I don't plan on adding power, and I'm not pulling anything but a boat. Anyone have suggestions? Thanks!
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Old 09-27-2012, 07:12 AM
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I wouldn't run either without a muffler unless you like it loud particularly low frequency drone. A 5" system in my experience (mines 4-5") will not lose response with proper tuning, but the biggest gain (aside from looks) is sound unless you are running high HP.
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Old 09-27-2012, 11:45 AM
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DmaxMaverick DmaxMaverick is offline
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Choose your noise level. Bigger is louder, all else being equal.

The "5 inch straight pipe is so big it doesn't have enough backpressure to spool the turbo" statement is hogwash. Differential pressure is what causes the turbine to spool, and higher backpressure after the turbine decreases that differential. The IDEAL turboDiesel exhaust is a short, straight pipe right off the turbine. Any more than that is gas friction, and friction requires (uses) energy/power. Of course, we don't do this, or we'd have the exhaust dumping into the cab. The actual length, size and shape of the pipe can be tuned to create and optimize laminar flow, which develops the least friction (this isn't usually desirable, because the exhaust noise is slowed less, too). A pipe that's too large, too soon, can reduce power, but only because the gases cool quicker, and cooler gas is more dense, more turbulent, and this equates into friction. All the backpressure for an effective turbo happens between the cylinders and turbine. After that, it's just waste. A N/A engine (Diesel or gas) is different, as they are able to scavenge exhaust, which can increase power, and headers can help optimize this advantage.
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Old 10-02-2012, 06:05 AM
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thank you both for your replies. i think ill go with a muffler with whatever size i decide.
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Old 10-02-2012, 01:54 PM
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I vote for a 4" system, good free-flowing muffler and a 5" chrome/polished tip... .
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Old 10-03-2012, 04:41 AM
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We are certainly all different and I don't mean any disrespect to anyone, but I have never understood the desire for a chrome tip on any exhaust. That just seems like wasted money to me.

Of course at my ever advancing age CRS disease is sneaking up on me and maybe I just don't remember.
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Old 12-02-2016, 10:44 PM
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From what I've been reading, while there is more unrestricted airflow in the larger pipe, it also changes the power curve designed and programed for the vehicle as built. The larger pipe decreases the power at the normal zone we drive in. The "more power" is now both higher and lower than it was at our normal cruise speeds. Therefore, more power applied. I just keep reading the reports from the "regular guy on the street (smile) that while they love the new sound, their mileage always seems to be a bit lower than they hoped.

Since mileage is a significant issue in most buyers, and all brands being basically the same, if it was as easy as giving us just a little extra steel (cheap material cost) in a .5 or 1" larger exhaust pipe and gain significant improvements, would the big maker do it just to stand out as the best? I understand them going cheap on things like shocks, but something so cheap that would make them stand out on mileage just doesn't seem like something they'd miss. Not for something so high on all buyer's minds as mileage.

The only thing I've done lately to improve my truck is adding a new Bradford Built Flatbed after my OEM bed got smashed while parked and unoccupied in my driveway.
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Old 12-03-2016, 09:25 AM
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The decreased MPG is usually due to the increased pressure of the right foot while listening to the new noise.
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