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  1. #26
    FEP Super Member 84StangSVT's Avatar
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    As I have said before, I honestly think you are overthinking this and not taking some of the advice to heart.

    Think about it......as long as you have been back and forth on this, the issues that keep presenting itself with what you want to do both past and present and the advice that has been given, the best answer is clear as a bell.

    Let's run it down a bit.

    1) You are asking for advice due to your current setup smashing the cats and want an alternative.

    IMHO and the general consensus it that the factory catted h-pipe is going to be your best alternative. It was designed for these cars, whether or not it was installed in 84 from the factory.

    2) You state you are not thrilled with having 4 cats when my car originally came with 1 cat.

    Unless you go factory replacement for the 84 model year, any alternative will have more than one cat.

    3) You worry about flow and loss of performance going with the factory catted x-pipe.

    More than likely you will see an increase of power from going with it as it will help maintain a bit of backpressure on the system, which CFI cars tend to like.

    4) You really want to find an aftermarket pipe that you can modify. Current shops state that certain of the bends will be hard to replicate.

    Any modification to the aftermarket pipe to install aftermarket cats in will more than likely end up the same way the pipe you are trying to get rid of did. There is not a good place to modify these pipes to install cats without altering the fit dramatically. If you look at the factory pipe, both sets of cats are offset and there is a reason for that. Trying to stick a straight through cat in them is going to be a pain.

    5) You are looking for a 2.5" pipe for 3" cats.

    You will need to step up/step down after the cats due to the size difference. That is going to make the affected area longer and make fitment even worse.

    The basic point of this is not to be rude, it is to make you see that no matter what you want to do to modify something that was not designed for the intended purpose, it will likely end in a poorly executed scab job unless time and money is not an issue.

    If it was me, I would personally either use the factory catted pipe, or buy an aftermarket catted pipe and not try to scab the existing together. You will be time and money ahead, with a lot less frustration. You can always recoup some of the cost by selling your current aftermarket setup.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  2. #27
    FEP Power Member fgross2006's Avatar
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    Quote Originally Posted by 84StangSVT View Post
    As I have said before, I honestly think you are overthinking this and not taking some of the advice to heart.

    Think about it......as long as you have been back and forth on this, the issues that keep presenting itself with what you want to do both past and present and the advice that has been given, the best answer is clear as a bell.

    Let's run it down a bit.

    1) You are asking for advice due to your current setup smashing the cats and want an alternative.

    IMHO and the general consensus it that the factory catted h-pipe is going to be your best alternative. It was designed for these cars, whether or not it was installed in 84 from the factory.

    2) You state you are not thrilled with having 4 cats when my car originally came with 1 cat.

    Unless you go factory replacement for the 84 model year, any alternative will have more than one cat.

    3) You worry about flow and loss of performance going with the factory catted x-pipe.

    More than likely you will see an increase of power from going with it as it will help maintain a bit of backpressure on the system, which CFI cars tend to like.

    4) You really want to find an aftermarket pipe that you can modify. Current shops state that certain of the bends will be hard to replicate.

    Any modification to the aftermarket pipe to install aftermarket cats in will more than likely end up the same way the pipe you are trying to get rid of did. There is not a good place to modify these pipes to install cats without altering the fit dramatically. If you look at the factory pipe, both sets of cats are offset and there is a reason for that. Trying to stick a straight through cat in them is going to be a pain.

    5) You are looking for a 2.5" pipe for 3" cats.

    You will need to step up/step down after the cats due to the size difference. That is going to make the affected area longer and make fitment even worse.

    The basic point of this is not to be rude, it is to make you see that no matter what you want to do to modify something that was not designed for the intended purpose, it will likely end in a poorly executed scab job unless time and money is not an issue.

    If it was me, I would personally either use the factory catted pipe, or buy an aftermarket catted pipe and not try to scab the existing together. You will be time and money ahead, with a lot less frustration. You can always recoup some of the cost by selling your current aftermarket setup.

    The big concern I have with using the stock H with stock cats is that they are ODB1 cats while the MagnaFlows I picked up are ODB2. The MF's don't need the thermatic air system but wont be negatively impacted by running air. The problem with the stock cats, if one goes bad, cannot be replaced without pulling the entire H and replacing it. Not one muffler shop I went to said they could cut and weld new cats into the original H so for the sake of argument, if one (or 2) cats goes bad I cant replace them. I'd have to pull the H and replace it. Im trying to get a setup that works and ill be happy with in the long term. That's why I'm trying to get the best performance with an aftermarket H with hi flow cats welded in.

    Aftermarket H with cats already on are super expensive. And if they can fit a pair of cats onto a BBK aftermarket H from the factory, then why can I not weld a pair of nerf football sized cats into a BBK that ships without cats? The stock setup had 4 cats. im thinking I could get my 2 Magnaflows to go in the spot where the pre cats were on a stock H.

    The reason I even got these cats is because the lack of backpressure made my car run badly when I converted to a X pipe and dual flowmasters. I agree that backpressure is needed with CFI set up. it even says so in the Mustang shop manual. I can easily put the stock H in and try it but im sure later on I'm still gonna want a set up with an H pipe and 2 magnaflow cats. I can always get 2.5 inch cats to make the weld easier to do. They're not that expensive and I could always sell my 3 inchers online.

    I know your suggestion to stay stock makes sense. I just know in the long term im gonna wind up pulling it again.

  3. #28
    FEP Super Member 84StangSVT's Avatar
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    I understand where you are coming from on this, believe me.

    Unless you happen to luck out and find good used stuff, you might be better off to look at this solution. It may be an x-pipe but it is a fairly cost effective means to do what you want.

    https://lmr.com/item/SVE-5250X-CAT/m...mini-cats-8693

    Looking at buying a new h-pipe and the correct size cats for the pipe, looks to be around $349

    One thing I will also put in there is in all honestly, I would steer clear of the BBK stuff. It generally fits like @ss and requires more tweaking and modification than it is worth, even without modifying its design even more. My car runs BBK shorties, 2.5" off road x-pipe and into Dynomax mufflers and all of this stuff was put on by a P.O.. I have had more problems trying to get the damn x-pipe to fit to the headers as it was bent wrong from the factory, which seems to be a common issue with them. It took a lot of time and multiple attempts at heating, twisting, install and remove until it fit well enough to finally seal and quit leaking from the header to x connections.

    Whichever way you decide to go, I hope you find a solution that works for you, eliminates this issue, makes your car run well so you can enjoy it.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
    1984 Mustang GT Hatchback 5.0 V-8/5 Speed

    I'm an FEP Supporter and proud of it. Are you?

  4. #29
    FEP Super Member erratic50's Avatar
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    Last time in a circle on this...... The stock headers you have in place, not this X pipe or that H pipe or another magniflow this or BBK that or solid pipe or rusted one or factory H pipe is the bottleneck!! Do what you want after it and it might make more noise but it won't make more power!

    If you have not upgraded to aftermarket headers or extensively modified the factory 85 units you sourced then none of us here believe there is ANY performance to be lost or gained messing around with cats.

    there is slight performance improvement to be had by going up in size on the cat-back but 2.5" is beyond adequate. That kit paired with headers and aftermarket 2.5" H or X is great up to about double the power you have.

    so if you don't have upgraded headers forget that you are running the factory H. It flows far more than the headers do! You'll only see marginal gains even if you HAVE upgraded the headers because what you have is enough to support your motor.

    With a mild speed density friendly cam, E6 factory 86 heads, a stock 55MM TB, lightly port matched intake, etc, and ported stock headers and a factory H and 2.5" cat-back my car pulled to around 5500 and made enough power past 5500 that it could hit the rev limiter in 3rd and 4th with a 2.73:1. Best cold weather pass on street tires was a marginally traction limited 13.26. I've heard of guys going well Into the 12's with stock H with other mods and am absolutely certain with slicks mine was one that could as my 60' was a joke on that pass.

    All of this being said....

    Listen to to the many extremely knowledgeable guys here that have been modding foxes their entire adult life. We all know a few that bought theirs new in 79 and have grand children old enough to drive- albeit they won't ever get their young hands on the foxbody.

    Ask yourself what more do you think you'll see with some brand X cat setup when you haven't addressed the area of restriction in the factory parts- we all already know the answer. I'll give you a hint - it's ZERO.

    Don't believe us? put it on the drag strip or the dyno and prove us wrong, but it will need to be conclusive. I'm not talking some crappy clogged up factory H vs something else - I'm talking one in good condition behind your 85 headers.

    Believe me I was damn disappointed when swapping to my 2.5" X pipe didn't make a damn bit of difference and I had to throw headers and throttle body A9L and MAF and ultimately an intake at mine to get more. Just the facts of uncorking a fox for performance upstairs where the fun is.

    Best of luck getting what you want out of your combo.

  5. #30

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    Quote Originally Posted by fgross2006 View Post
    The reason I even got these cats is because the lack of backpressure made my car run badly when I converted to a X pipe and dual flowmasters. I agree that backpressure is needed with CFI set up. it even says so in the Mustang shop manual. I can easily put the stock H in and try it but im sure later on I'm still gonna want a set up with an H pipe and 2 magnaflow cats. I can always get 2.5 inch cats to make the weld easier to do. They're not that expensive and I could always sell my 3 inchers online.
    Substitute "flow velocity" everywhere you see "backpressure", and this will all make a lot more sense. This was the
    argument when you first put that 3" X-pipe on a stock '84 CFI engine. But it was never about backpressure. I have
    yet to see an engine that runs better with backpressure than without, but I've seen a LOT of engines that run way
    better with reasonably sized exhaust tubing, because just as with intake systems, excessively large tubing reduces
    flow velocity, and THAT reduces both cylinder filling, and scavenging.

    Going all the way back to the stock '84 system is not necessary, but your engine will likely run better than it does with
    the oversize system you have now. But it would run even better with a stock '86-'93 system. Ford demonstrated as
    much when they put the tubular headers and larger exhaust on the mid-'85 CFI cars.
    Last edited by JACook; 01-19-2017 at 06:19 PM.
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
    2012 F150 S-Crew 4x4 5.0 "Blue Coyote"... 65 coupe, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ...
    '68 Vert, Mexican block 307 4-speed... '71 Datsun 510 ...
    And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune

  6. #31

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    WOW. Are we STILL having this conversation, LOL. All I have to say before I never check this thread again is why does a garden hose have 5X more water pressure (velocity) with a small diameter nozzle on it then it does with nothing on the hose end? It it possible it flows any more water? NOPE, Impossible, it's the same faucet. Is it possible the pressure coming in is higher? NOPE, it's the same water line going to the same faucet.
    Liberty once lost is lost forever.

    John Adams
    July 7, 1775

  7. #32

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    Setting up strawmen at either extreme doesn't do much to advance the discussion, so yeah, guess we're STILL
    going to keep having this conversation. For the record, I am not suggesting that there's any advantage to be
    gained by trying to achieve anything resembling jet thrust coming out the tailpipe.

    But I stand by my assertion that the reason an engine gets lazy with oversized exhaust has nothing to do with
    a lack of backpressure. To the extent that backpressure is observed in an optimally sized exhaust system, it is
    nothing more than a secondary effect. And it ignores the more important factor that, with an optimally sized
    exhaust system, the cylinder pressure toward the tail end of the exhaust event will actually go slightly negative.

    This is no different than the concepts of ram-tuning the intake side of the engine, where you're using the weight
    of a column of air to promote cylinder fill. Both concepts are about the balancing act between flow capacity and
    flow velocity. Exhaust sizing is no more about an engine needing backpressure, as ram-tuning is about engines
    needing to keep some vacuum in the intake at WOT.

    Hopefully this will make some sense to those who do actually read it...
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
    2012 F150 S-Crew 4x4 5.0 "Blue Coyote"... 65 coupe, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ...
    '68 Vert, Mexican block 307 4-speed... '71 Datsun 510 ...
    And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune

  8. #33

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    ^ ^ ^
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

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