981 gts secondary cats
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    981 gts secondary cats

    Question is there a set of secondary cats in the rear manifold ? has anyone tried the Carnewal GT exhaust 981 Carnewal GT Exhaust - Carnewal
    Last edited by RichardCH; 02-08-2019 at 02:41 PM.

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    Re: 981 gts secondary cats

    The cats are only in the main header portion. Past the exhaust collector flange (two bolts) are only mufflers.

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    Re: 981 gts secondary cats

    Quote Originally Posted by RichardCH View Post
    Question is there a set of secondary cats in the rear manifold ? has anyone tried the Carnewal GT exhaust 981 Carnewal GT Exhaust - Carnewal
    What is your definition of a rear manifold?

    Prior generation Porsches had two cats in the main exhaust manifolds. There was the first cat, the O2 exit (secondary) sensor and then the secondary cats. These were individually smaller than the single cats used in 981 manifolds. I am not sure if this change was due to emissions, life of the cats, or cheaper to produce. Some 911 turbos had the cats in the exhaust after the turbos.

    V6

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    Re: 981 gts secondary cats

    thanks it seems as though Gert @ carnewal offers HJS 200 cell cat swaps on OEM headers for circa $1200, this is probably the holy grail

    The P81007 “200 Cell Cat Upgrade” is a pair of original manifolds of which the original elements are removed and are replaced with HJS 200 cell sport cats, the HD version which is especially developed for Porsche DFI and high performance engines.
    This upgrade gives your 981 10 to 12 HP extra and an increase of the sound with 25-30%.

    This upgrade is 100% ok for emission checks and does not throw CEL’s.
    A remap is not necessary.
    On the outside, everything looks like original.

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    Re: 981 gts secondary cats

    Richard,

    HJS 200 cell converters are almost universally used on Porsche aftermarket headers.

    Header design 101:

    The Carnewal headers sounds interesting for people who need to keep the change invisible to the Smog police. One issue is that the runner lengths to the collector are not exactly equal in the stock manifold. Some/most of the aftermarket header manufacturers have equal length runners and they have better collectors. As an exhaust pulse goes down the tube/runner, when it reaches the collector it causes a vacuum in the other tubes, helping pull the next exhaust pulse, increasing flow that goes back and scavages the cylinders better and increases the HP. The length of the runners affects the RPM range that has the most impact. Short tube headers give higher RPM improvement and long tube headers improve low end torque and HP. Unequal length runners can cause one cylinder's pulse timing to change (as it arrives at the collector early or late) and it can affect scavaging in each cylinder differently, causing a slight imbalance.

    HP gains claimed are from before/after at specific RPM ranges and often do not increase the HP at the peak HP RPM by the advertized amount.

    Finally, look carefully at the aftermarket header's collector design and how they go into the converter and the shape of the cone that expands from the collector to the outside diameter of the converter can. I have seen two designs, one that is better to me than the other. One design, that the output of the collector is straight, funnels the majority of the exhaust into the center of the converter. Less gets to the outside edges and I fear that this causes backpressure and tends to heat the center of the core more than the outside edges. The other collector design, twists the three tubes such that the exhaust from each tube is aimed at a different section of the converter face. I prefer this design and feel that the converter will last longer as it will heat more evenly. I doubt there is much HP difference between the designs, but there should be longevity differences in extreme environments.

    Finally, there is an optimum tubing diameter for every engine size. That is why you see GT4 (3.8L) headers advertised as seperate products. The GT4 headers will fit a 3.4L or 2.7L engine, but the tubing diameter will not be optimized for that engine as exhaust pulse velocity and resultant vacuum assist will be less if the tubing diameter is too great. Using a header sized for a 3.4L engine on a 3.8L engine will cause a little more backpressure and less than optimum results.

    So, header design needs to take into account the engine displacement and RPM range that the most power is to be added. As a buyer, you need to consider where in the RPM range (to suit your intended application) you want the boost in HP and that might drive the choice of what manufacturer you choose.

    The Carnewal technique won't have much choice over what Porsche designed in other than reduced back pressure due to larger cells, increasing flow.

    V6
    Last edited by Voyager6; 02-10-2019 at 05:05 PM.

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    Re: 981 gts secondary cats

    thks thats great I was even considering buying new 3.8 GT4 headers/cats off ebay, so Ill throw that idea out of the window. I have changed to equal length headers with optimal collecters so many times in my life with less than noticeable differences but I do know changing the cats is truly significant. I run Kline 100 Cell HJS cats in titanium pipe on my Scuderia and its epic. Generally exhaust gases flow well whatever, its like air in a turbo intake pipe it doesnt have to be perfectly round with perfect bends to work just as well
    Last edited by RichardCH; 02-11-2019 at 11:42 AM.

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