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Under or over the axle?

This is a discussion on Under or over the axle? within the External Engine forums, part of the LSx Technical Help Section category; Just got my QTP headers in today... I'll be putting them on this weekend and hopefully monday of next week ...

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    Senior Member M-Ten's Avatar
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    Under or over the axle?

    Just got my QTP headers in today... I'll be putting them on this weekend and hopefully monday of next week I'll be getting the rest of my exhaust made.

    My guy told me he would do a 3" mandrel bent X pipe over the axle for $300

    The question is do I really want to run it over the axle or do I want it under?

    The way I see it, being over the axle will have more bends in it, but have better clearance than if it was ran under. Would the bends and the fact it would need to be brought down to 2.5" from the axle out hurt performance too much?

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    Member 180ls1's Avatar
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    Quote Originally Posted by M-Ten View Post
    Just got my QTP headers in today... I'll be putting them on this weekend and hopefully monday of next week I'll be getting the rest of my exhaust made.

    My guy told me he would do a 3" mandrel bent X pipe over the axle for $300

    The question is do I really want to run it over the axle or do I want it under?

    The way I see it, being over the axle will have more bends in it, but have better clearance than if it was ran under. Would the bends and the fact it would need to be brought down to 2.5" from the axle out hurt performance too much?
    Damn that is a good deal go over the axle for sure. and use the 2.5 inch pipe i think one guy on here did a dyno with 3in vs. 2.5in and thay were close to the same but the 3" weighed more so it turned out to be about the same with power to weight ratio figured in. Unless you have a seriously built motor or spray the hell out if it 2.5 will be fine. And over is the way to go it is much cleaner and the bends will not cost much of a gain in horsepower. Not to mention every time you need to jack your car up or put it on a lift you will not have to be worried.

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    The Bandit Wesman's Avatar
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    Quote Originally Posted by M-Ten View Post
    Just got my QTP headers in today... I'll be putting them on this weekend and hopefully monday of next week I'll be getting the rest of my exhaust made.

    My guy told me he would do a 3" mandrel bent X pipe over the axle for $300

    The question is do I really want to run it over the axle or do I want it under?

    The way I see it, being over the axle will have more bends in it, but have better clearance than if it was ran under. Would the bends and the fact it would need to be brought down to 2.5" from the axle out hurt performance too much?
    Run it over the axle for sure. Under the axle just isn't an optimal setup, because not only do you have to drop the exhaust whenever you jack up the rear of the car, but it significantly reduces ground clearance as well. Plus is doesn't look all that great.

    If you run 2.5" dual pipes, that will be more than enough flow. 2.5" duals are good to about 550WHP, above that you would need the 3". I have 3" H-pipe reduced to 2.5" duals before it goes over the axle, and I've been very happy with my setup.

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    2.5 inch dual pipes going over the axle is the only way to go as far as I am concerned. I wouldn't do it any other way.

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    Moderator Firebirdjones's Avatar
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    These guys are right,,,,,NEVER go under the axle. It would be nothing but a nuisance. The rearend has to move up and down,,,,the amount of clearance needed to keep the pipe from hitting the axle would leave you no ground clearance at all,,,any maintainance would require removal most likely,,,,not to mention it looks completely horrendous.

    Reminds me of the clunkers running around town where people get cheap flex pipe from the local store,,,and run it under the axle holding it with coat hangers,,,,,Ugh!

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    Senior Member M-Ten's Avatar
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    that's what i was thinking as well but I just wanted to make sure...

    so 3" then 2.5" from the axle out the back then?

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    The Bandit Wesman's Avatar
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    Quote Originally Posted by M-Ten View Post
    that's what i was thinking as well but I just wanted to make sure...

    so 3" then 2.5" from the axle out the back then?
    You can actually go with 2.5" from the headers back without losing much of anything. If the exhaust shop is willing to do 3" to 2.5" though, might as well do that.

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    Slow'er'Ass Mr. Luos's Avatar
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    Mandrel bent for $300??
    Let him do mine first please...

    Mine go over the axle. I have a 9" rearend though, so I had to run 2.5" over the axle.

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    Senior Member M-Ten's Avatar
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    Quote Originally Posted by Mr. Luos View Post
    Mandrel bent for $300??
    Let him do mine first please...

    Mine go over the axle. I have a 9" rearend though, so I had to run 2.5" over the axle.
    Lol yeah I know the guy pretty well.. I brought my old car to him and he put some mufflers on my dads car as well so he's givin me a good deal

    Yeah he already told me he would do 3" to 2.5" so I guess I'll be going with that.

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    I ahve mandrel bend 2.5 over the axle and it's TIGHT in there, I really think 3" will eventually cause clearance issues and hit things on bumps

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    Senior Member mrr23's Avatar
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    dual 3" over the axle will require the upper panhard brace from BMR or UMI. dual 2.5" over the axle will work with the stock upper panhard brace. i have the bassani 2.5" tru-duals on both of my cars.

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    Slow'er'Ass Mr. Luos's Avatar
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    Quote Originally Posted by mrr23 View Post
    dual 3" over the axle will require the upper panhard brace from BMR or UMI. dual 2.5" over the axle will work with the stock upper panhard brace. i have the bassani 2.5" tru-duals on both of my cars.
    What rearend in the cars?

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    Senior Member M-Ten's Avatar
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    i eventually plan on lowering it.. probably use the strano lowering springs and maybe the koni shocks.. so together.. maybe 1 3/4" or 2" ??

    you think it would scrape even though I'll be doing it over the axle?

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    Senior Member mrr23's Avatar
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    Quote Originally Posted by Mr. Luos View Post
    What rearend in the cars?
    stock

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    i would assume 3" would make it sound deeper and meaner. could be wrong. if you have the money, go 3" over axle with the pan bar relocating kit.

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    Quote Originally Posted by mrr23 View Post
    stock
    I assume the clearance is still tight?

    I had non-mandrel bent 3" pipes over my stock 10 bolt, but once the 9" was in, I needed 2.5" pipes.

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    Member 180ls1's Avatar
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    Quote Originally Posted by M-Ten View Post
    that's what i was thinking as well but I just wanted to make sure...

    so 3" then 2.5" from the axle out the back then?
    yes that is exactley the ticket! read below why starting with a 3" would be better then starting with 2.5 arter the headers. here are a couple of good quotes

    "Yep, sort of…. Those numbers are based on a study that David Vizard did a few years ago. He figured that at some point most tracks will set noise standards and wanted to know how much flow you would need not to loose any performance. He found that above 2.2cfm/hp there was only a negligible restriction that was almost statistically insignificant. You couldn’t measure any change above 2.4cfm/hp."

    "He also found that the required flow drops as you go further back in the exhaust because the gasses cool and take up less volume, so the further back you mount the muffler the smaller it could be. The muffler only has to be that big if it is mounted right after the collector, and that you could make an exhaust that has sufficient flow behave exactly like open collectors if you build a ‘pressure wave terminator box’ (basically an expansion chamber) and mounted it at the end of the collectors and ran whatever exhaust out you wanted. This is somewhat the idea behind some of the funky designs coming out now by Magnaflow and a few others. You’ll see long, 3.5” collectors, a 3” x-pipe with 2.5” outlets and mufflers and then 2.25” tailpipes on a QUIET 600hp car… I did something like that on my truck a while back, the Y comes together into a 3”, and ends with a 2.25” tailpipe "

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    Senior Member M-Ten's Avatar
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    Would I NEED that pan bar relocating kit? Cuz i wasn't aware that I did and I didn't order it or plan to..

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    Senior Member M-Ten's Avatar
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    Quote Originally Posted by 180ls1 View Post
    yes that is exactley the ticket! read below why starting with a 3" would be better then starting with 2.5 arter the headers. here are a couple of good quotes

    "Yep, sort of…. Those numbers are based on a study that David Vizard did a few years ago. He figured that at some point most tracks will set noise standards and wanted to know how much flow you would need not to loose any performance. He found that above 2.2cfm/hp there was only a negligible restriction that was almost statistically insignificant. You couldn’t measure any change above 2.4cfm/hp."

    "He also found that the required flow drops as you go further back in the exhaust because the gasses cool and take up less volume, so the further back you mount the muffler the smaller it could be. The muffler only has to be that big if it is mounted right after the collector, and that you could make an exhaust that has sufficient flow behave exactly like open collectors if you build a ‘pressure wave terminator box’ (basically an expansion chamber) and mounted it at the end of the collectors and ran whatever exhaust out you wanted. This is somewhat the idea behind some of the funky designs coming out now by Magnaflow and a few others. You’ll see long, 3.5” collectors, a 3” x-pipe with 2.5” outlets and mufflers and then 2.25” tailpipes on a QUIET 600hp car… I did something like that on my truck a while back, the Y comes together into a 3”, and ends with a 2.25” tailpipe "
    Interesting.. so it's best to go from big to small then.. because we were wondering if we should go from 3 to 2.5 and back to 3 but I guess we'll just do 3 to 2.5..

    Even though the magnaflow inlet is 3"

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    Member 180ls1's Avatar
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    Quote Originally Posted by M-Ten View Post
    Interesting.. so it's best to go from big to small then.. because we were wondering if we should go from 3 to 2.5 and back to 3 but I guess we'll just do 3 to 2.5..

    Even though the magnaflow inlet is 3"
    well you can do 3' from the headers - 2.5' over the axle - back to 3" out the ass, but that might get a little expensive and time consming. did you allready buy the mufflers if so what are they and what size? i am assuming you are looking at a 3" magnaflow or do you allready have one?

    and no you do not need a different panhard as long as the guy doing your exhaust work is good.
    Last edited by 180ls1; 01-09-2008 at 10:45 PM.

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