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Discussion Starter #1 (Edited)
I'll be picking up some JBA headers soon and I was wondering if I could gain a little more power by replacing everything behing the headers with 2.5" piping and 2.5" inlet/outlet high flow cats. I already have a 2.5" exhaust welded on right past the second cat...but I figure stepping up to 2.5" right out of the headers would be better than 1.5" to 2.25" to 2.5"...any comments on that?

Looking at these cats http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=130005714006&ssPageName=STRK:MESE:IT ...replacing mine since I'm pretty sure they're toast.
 

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I am getting ready to do some exhaust work also. My exhaust manifold has a 3-4 inch crack in it. Going with a LCE header. Also doing a custom catback with an Aero Chamber muffler. Not sure of the condition of my converter. If it looks to be bad, I will probably go with the Magnaflow. I haven't heard anything about the Flowmaster converters. Did you notice the warning?
*Not legal for use in California OBD2(96/up)
If you are not concerned about that, I think it will be a good cat. But, I don't have any first hand experience.
 

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Discussion Starter #3
yeah i dont really care about that as long as it passes smog
 

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I cant see that giving you more power, or at least noticable power, it would more than likely make the truck sound better, but no noticable power. I ran my truck with headers to stock cats to borla w/ turndowns then after my cats went to shit and melted shut I cut them out and ran headers with a straight shot back to a borla w/ turndowns on stock piping. The only difference was alot more noise and a little more top end proformance. I would kinda like to see you do it and prove me wrong because if it does give you a noticeable amount of power it would be a cheep mod for me to do. :2cents:
 

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YoTRacer158 said:
I'll be picking up some JBA headers soon and I was wondering if I could gain a little more power by replacing everything behing the headers with 2.5" piping and 2.5" inlet/outlet high flow cats. I already have a 2.5" exhaust welded on right past the second cat...but I figure stepping up to 2.5" right out of the headers would be better than 1.5" to 2.25" to 2.5"...any comments on that?

Looking at these cats http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=130005714006&ssPageName=STRK:MESE:IT ...replacing mine since I'm pretty sure they're toast.
top-end power gain, yes but you will lose some low-end.
the interesting thing is that both 2.7s and 3.4s use the same (OE) cats with slightly less than a 2.0" ID at the ends but either 2.25" or 2.5" pipe before and after the cats...pretty stupid if you ask me :rolleyes:

if you go to a completely 2.5" system including the cats I'd be currious what the dyno differences are compared to stock.
 

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Discussion Starter #6
Well I wont be doing before and after dyno gains but I'll let you know if I notice a difference if/when I get around to doing it.
 

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YoTRacer158 said:
Well I wont be doing before and after dyno gains but I'll let you know if I notice a difference if/when I get around to doing it.
I imagine you know your truck well enough by now to detect much of a change ;) Keep us posted.
 

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I don’t have headers but I recently upgraded my exhaust with a manifold back system. I went with the long 27” Magnaflow muffler, OBD-2 cat, (the tech told me that the markings “OBD-2 compliant” make it California legal) changed to size of the pipe to 2.5” from the inlet of the cat to the end of the run.

I don’t have any dyno results but I can say for sure that it feels stronger and smoother at all RPM’s. Wow, every time I drive the truck I am amazed by the performance gains. And real quiet, just how I wanted, the tone sounds a little sportier but the truck is not any louder.

As hytenor said, you may loose some low-end. Yep, it’s true it is possible to make the exhaust path to large and hence, not enough “back pressure” If you have high flow headers and do the rest of the system in big high flow components you risk losing some low-end power. Not good as when 4-wheeling we need all the low-end torque we can get.
 

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Discussion Starter #9
Bent Wheel said:
As hytenor said, you may loose some low-end. Yep, it’s true it is possible to make the exhaust path to large and hence, not enough “back pressure” If you have high flow headers and do the rest of the system in big high flow components you risk losing some low-end power. Not good as when 4-wheeling we need all the low-end torque we can get.
Actually making the exhaust too big causes an increase in backpressure, not a decrease. And since my truck is built for the desert I'm more concerned with top end than low end.
 

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i have a complete 2 1/2 exhaust headers cat muffler and all, i cant see doing it for just a gain, i only did the cat because it got smashed :2cents:
 

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Bent Wheel said:
I don’t have headers but I recently upgraded my exhaust with a manifold back system. I went with the long 27” Magnaflow muffler, OBD-2 cat, (the tech told me that the markings “OBD-2 compliant” make it California legal) changed to size of the pipe to 2.5” from the inlet of the cat to the end of the run.

I don’t have any dyno results but I can say for sure that it feels stronger and smoother at all RPM’s. Wow, every time I drive the truck I am amazed by the performance gains. And real quiet, just how I wanted, the tone sounds a little sportier but the truck is not any louder.

As hytenor said, you may loose some low-end. Yep, it’s true it is possible to make the exhaust path to large and hence, not enough “back pressure” If you have high flow headers and do the rest of the system in big high flow components you risk losing some low-end power. Not good as when 4-wheeling we need all the low-end torque we can get.
well, that 'tech' isn't exactly correct. a CAT is Cali legal if it is certified for that particular vehicle, not just with a obd-2 stamp on it. It is a complete bs racket, IMO. the aftermarket cats do everything the OEs do when it gets to the tailpipe :rolleyes:
 

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YoTRacer158 said:
Actually making the exhaust too big causes an increase in backpressure, not a decrease. And since my truck is built for the desert I'm more concerned with top end than low end.
yoTracer, Thanks for schooling me.

I guess I have been wrong on this one. The Headers I installed on my 69 z-28 took away some of my low end power. Everybody told me that the larger size pipe reduces backpressure. Made sense to me, bigger pipe = less backpressure. If possible could you please explain how the bigger pipe increases backpressure?
 

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hytenor said:
well, that 'tech' isn't exactly correct. a CAT is Cali legal if it is certified for that particular vehicle, not just with a obd-2 stamp on it. It is a complete bs racket, IMO. the aftermarket cats do everything the OEs do when it gets to the tailpipe :rolleyes:
Thanks hytenor, Shucks, I hope I don’t run into trouble at the smog station.
 

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Discussion Starter #14
Bent Wheel said:
yoTracer, Thanks for schooling me.

I guess I have been wrong on this one. The Headers I installed on my 69 z-28 took away some of my low end power. Everybody told me that the larger size pipe reduces backpressure. Made sense to me, bigger pipe = less backpressure. If possible could you please explain how the bigger pipe increases backpressure?
Basically you want the exhaust to get out at the best/smoothest rate of flow as possible. Too small of piping means it cant get out fast enough, causing too much backpressure. Too big of piping leaves too much extra room causing more turbulent air that doesnt know where to go, causing more backpressure.
 

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Bent Wheel said:
Thanks hytenor, Shucks, I hope I don’t run into trouble at the smog station.
It would be very rare if you had any problems. Smog places just check to make sure you HAVE the damn things, notwhat brand they are ;)

my front cat is upside down and backwards...they didn't even notice.
 

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YoTRacer158 said:
Basically you want the exhaust to get out at the best/smoothest rate of flow as possible. Too small of piping means it cant get out fast enough, causing too much backpressure. Too big of piping leaves too much extra room causing more turbulent air that doesnt know where to go, causing more backpressure.

Ahh..

Now I see, turbulent air flow from too big pipes. Yes I can see that.

I know this is your thread and still you spend your time helping a newbie.

Thank you…
 

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hytenor said:
It would be very rare if you had any problems. Smog places just check to make sure you HAVE the damn things, notwhat brand they are ;)

my front cat is upside down and backwards...they didn't even notice.

Eeeeexelent…. My evil plan is working.
 

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i like my LC header with 2.5 LC cat and muffler. i had to make my own pipe from the cat to muffler with a flange and O2 mount but it worked out fine. as for performance i think it runs much better on the highway but no difference wheeling. but it is fawking LOUD
 

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Discussion Starter #19
My new exhaust is about the loudest I've heard on a Tacoma...so I'm not worried about a little more noise. I'll definately swap out to larger cats and piping after I get the headers and let everyone know how it turns out.
 

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Discussion Starter #20
Picking up my headers in the next few days and I'll pick up the high flow 2.5" cats as well. Have two questions though...

1. JBA's instructions say that certain parts need to be welded (I have a 2004) but a lot of people on here have said that there's no welding involved...which is it?

2. I saw a thread on here about people stealing cat converters off of Toyota's lately...what for and is there anything i could do with my stock ones to make a few bucks?
 
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