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Thread: deciding between 4 or 5 inch exhaust

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  1. #1
    Join Date
    Jul 2008
    Location
    Lansing ,MI
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    254

    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!
    1991 Chevrolet Silverado 3500, 6.2L. 125,000 miles -Sold

    2007 Chevrolet Silverado LBZ - R.I.P

    2001 Chevrolet Silverado LB7-- Sold

    2011 GMC Sierra, LML- 39,000 miles. All stock

  2. #2
    Join Date
    Mar 2000
    Location
    Loyal WI US
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    10,792

    Default

    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.
    Kennedy Diesel-owner
    More than just a salesman-I use and test the products that I sell on a daily basis!
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    2002 Chev K2500HD D/A CC Long LT 11.77@ 124mph at 7700# fuel only-e.t. needs help
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  3. #3
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,573

    Arrow

    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.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  4. #4
    Join Date
    Jul 2008
    Location
    Lansing ,MI
    Posts
    254

    Default

    thank you both for your replies. i think ill go with a muffler with whatever size i decide.
    1991 Chevrolet Silverado 3500, 6.2L. 125,000 miles -Sold

    2007 Chevrolet Silverado LBZ - R.I.P

    2001 Chevrolet Silverado LB7-- Sold

    2011 GMC Sierra, LML- 39,000 miles. All stock

  5. #5
    Join Date
    Feb 2000
    Location
    Montana
    Posts
    11,382

    Default

    I vote for a 4" system, good free-flowing muffler and a 5" chrome/polished tip... .

  6. #6
    Join Date
    Feb 2000
    Location
    Maplesville, AL - Home Base
    Posts
    536

    Default

    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.
    Ed
    KM4STL

    '06 Sierra LBZ 4x4 Crew SB, Titan 52 gallon fuel tank, TTT/Schefenacker Mirrors
    '98 Suburban, 245,000 - sold 7-4-06

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