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Thread: Just a few questions ;)

  1. #1
    Regular Member BlueRob's Avatar
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    confused Just a few questions ;)

    Itīs time for the mexican questions of the day
    I was checking some alternatives to install some exhaust tips on the back of the V6 beast, in the middle of the process I thought about a little farther with the issue (bad idea ), and said to myself….myself, why donīt you change the back boxes too. Thatīs when the questions start to fly all over my head…..As you might know I canīt get my hands on any of the brands mentioned along the forum for this matter, so I have to relay on what I can find here. What should I look for? Are any rules of thumb for this? I might think that to improve the exhaust system the back boxes should be bigger or more efficient than the original ones, how can I know what box is better than the originals? Any specs for this?
    Oh and reading along some threads about this found CPSI, what is this?
    On the other side is it worth the money to change the pipes on the bottom for something bigger?
    Oh! What do you mean when you talk about a cat back system?
    Just a few questions for starters
    Many thanks

  2. #2
    Regular Member Ste's Avatar
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    Vehicle : Jaguar XF 3.0D V6

    Trim : Black

    Engine : 3.0D V6

    Year : 0000

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    CPSI = Cells per square inch. Relates to catalysts and describes the number of channels through the brick per square inch.

    The spec of the back boxes? What do you mean by this? Do you mean max restriction, total noise reduction (ie insertion loss) or noise 'quality' info???

    When designing the rear boxes, the designers look at what the engine exhaust mass flow and temps are for every operating condition, and what the noise would be without the boxes. Then the design for the boxes can start, and it really is a balancing act between noise attenuation and back pressure (restriction). Max back pressure is (very normally) at max engine exhaust flow (most normally full load / rated speed). HOWEVER, aftermarket boxes can have lower BP and hence less noise attenuation, as the law is not as stringent as type approval / homologation tests.

    Cat back - moving the cat from the front exhaust system to the rear. This can have the effect of reducing back pressure, as the volumetric exhaust flow rate will be lower as the gas cools. The mass flow rate will be the same (can't create or destroy mass), but volumetric flow will change due to temperature drops over the length of the exhaust system (this drop can be VERY significant) - result is lower catalyst restriction, as BP is mostly volumetric flow rate dependant.

    Bigger front pipes?? The velocity of the gas should not exceed about 70m/s at any operating condition. Above this 'flow generated noise' starts to occur, and also the faster the speed - the higher the BP (not linear either, ^4). So larger pipes have lower BP, but also increase heat loss as more surface area. Higher exhaust gases have a lower viscosity, so cooling too quickly, can actually be bad ! ! ! ! But see above comments too ! ! ! !
    Bigger pipes are also less likely to be able to be fitted smoothly with large bend radii - large bend radii reduce restrictions, it's much better to have a large sweeping bend, than a sharp turn. So increase is probably good, but needs to be assessed against these other 'attributes'


    Custom exhausts are normally quite expensive, but can be useful in improving potential power, but should be done BEFORE any remap work is done. Air inlet breathing should also be looked at too.


    HTH

  3. #3
    Regular Member rushy's Avatar
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    Not going to even attempt to answer your question after Ste's reply, think he said it all! LOL

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    Regular Member ditchp's Avatar
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    Vehicle : Vectra cdti estate

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    what ste said but in an easy way......

    magnex

    powerflow

    remus

    peco

    sebring ( i think )


    Midlands Mod Shop - NFFC.

  5. #5
    Regular Member BlueRob's Avatar
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    WoW Ste, many thanks for that write up, I really appreciate it as rushy said you covered from A to Z, had to read it 3 times to get the hole thing out
    Here in mexico as far as I know there are no custom made systems (back boxes), you can get universal fit mufflers from different brands, the only one decent that I´ve found so far that I can buy here is Magnaflow which IMO looks decent.
    Here is the link if any of you want to check them out...
    http://www.magnaflow.com/

    Thanks again mates

  6. #6
    Regular Member BlueRob's Avatar
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    Made up my mind....went back to the original idea.....stainless steel tips.....spend the whole afternoon visiting shops that deal with exhausts and must say what a disappointment it was....most of them have no clue of how or what to do...the ones that do.. work like Sh*t and carry low low quality materials. Second of all I must say that being an automatic gearbox it my not sound as cool as an manual....so anyhow going for just the tips

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