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Thread: 2007 LBZ 4” or 5” exhaust?

  1. #1
    Join Date
    Jul 2020
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    Montana
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    Default 2007 LBZ 4” or 5” exhaust?

    I ordered a Diamond Eye 4” straight exhaust with 3” downpipe from Summit Racing. They accidentally sent the 5” exhaust straight kit…

    After watching and listening and reading about either or exhaust size, it seems there is not much of a difference except sound…

    I read that the 4” is better because some back pressure is needed for the turbo to spool faster and I also read that the 5” is better because you don’t want back pressure at all to reduce the chance of head gasket failure…

    So my question is…

    With all the mixed matched info out there….

    Which will be better performance and longevity wise? The 5” or 4”?

    Any more clarification will be helpful.

  2. #2
    Join Date
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    Default

    All of that is bunk. Hogwash.

    While excessive CYLINDER pressure can cause gasket issues, any exhaust restriction is a disadvantage with a turbo. If the cylinder pressure is too great due to exhaust back pressure, exhaust flow is reduced, and less energy is absorbed by the turbo (which, in turn, can't drive the compressor). Excessive back pressure will absolutely SLOW turbo spooling. As the gas cools post-turbo, it contracts (gets smaller), which reduces the pressure to some degree. Larger exhaust, noise aside, reduces the velocity of the exiting gas, retaining heat in the pipe for a greater time. It's a trade-off. A turbo, to operate optimally, should have NO pipe. Just dump the gas right out of the turbine housing. But we don't do that, for some obvious reasons. Choose your pipe size(s) according to the gas you want to flow, generally based on horsepower. Anything in the 500 HP and less range won't benefit with larger than 4". Consider, all the late model HP behemoths are still coming from the factory with 3" and 3.5" pipes.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  3. #3
    Join Date
    Feb 2000
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    Wink Overcompensating

    Depending on lots of factors, maximum total exhaust system backpressure (due to the exhaust system + turbo) can be in the range of a few tens of PSI (stock - backpressure generally equals boost pressure - mostly due to the turbocharger) to a couple hundred PSI (high dollar performance diesel pushed hard). On some high performance diesels, they need high rate exhaust valve springs to fully close the exhaust valves against high exhaust backpressures.

    Maximum cylinder pressure during combustion can be in the range of thousands of PSI... ~3500-PSI max or so for a stock engine to scary high PSI for a maxed out performance application.

    Bottom line... how an exhaust system backpressure might affect the head gaskets is minuscule when compared to combustion pressure... However, exhaust system backpressure can affect EGT, which in-turn can have a profound effect on engine life...

    The 6.5 my daughter and I rebuilt last fall was a result of being pushed well past the max EGT by its previous owner, resulting in the aluminum pistons being pushed into their plastic state. It would have benefited by a larger turbocharger, which would have added power, and reduced both exhaust backpressure and EGT.

    For most, a 5" exhaust system is more about having a big pipe than producing a worthwhile gain in performance. If you're happy with the fit and the sound, you're golden. No worries.

  4. #4
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    Default

    Thanks for the input! I am not sure how far I am going to go with this LBZ, but I did decide to keep the 5” exhaust.

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