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Old 02-09-2013, 11:07 AM
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Cat size suggestion?

I'm about to pick up a pair of cats to go with some long tube headers for my truck. I understand the cats need to be as close to the heat source as possible, so I'm going to bolt them right at the collector.
Question about size though; I'm going 2.5" dia. for the mufflers and exhaust bu t my collectors are 3". Should I go 3" cats and reduce to 2.5" after? Does it matter?
Engine is a 5.7 stocker now, but have plans for a 383 stroker down the road.

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Old 02-09-2013, 12:32 PM
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use 3 inch cats and 3 inch exhaust and an X or H pipe.X pipe usually flows better,both sound similar
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Old 02-09-2013, 01:00 PM
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Originally Posted by vinniekq2 View Post
use 3 inch cats and 3 inch exhaust and an X or H pipe.X pipe usually flows better,both sound similar
thanks - I was thinking of 2.5" as I thought 3" might be overkill for my application.
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Old 02-09-2013, 01:10 PM
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I would think 3" in from the collectors with 2 1/2 outs to keep the exhaust velocity up. The exhaust will cool and slow down through the cats and the 2 1/2 will help with the scavaging. Then again cats and hi performance don't really go hand in hand. IIRC h pipe for low end tq and the x pipe is best suited for hi rpm hp. Additionally the H or X pipe ideally would be as close as possible to the headers.
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Old 02-09-2013, 01:19 PM
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Originally Posted by TMF Lindbeck View Post
I would think 3" in from the collectors with 2 1/2 outs to keep the exhaust velocity up. The exhaust will cool and slow down through the cats and the 2 1/2 will help with the scavaging. Then again cats and hi performance don't really go hand in hand. IIRC h pipe for low end tq and the x pipe is best suited for hi rpm hp. Additionally the H or X pipe ideally would be as close as possible to the headers.
Thanks - so how about 3" cats at the headers, 3 to 2-1/2" reducers directly after the cats followed by an H pipe? Or should the H pipe be before the Cats? My understanding is that the Cats be as close to the heat source as possible to keep them from going bad (??)

I must run cats to be compliant I'm afraid
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Old 02-09-2013, 01:26 PM
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3 inch cats,x or h pipe then exhaust,Ive never seen a reduced system ever make more power,but it is cheaper,

put flanges on both sides of the cats,in case they need to come out and be replaced with straight pipes on occasion?
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Old 02-09-2013, 01:41 PM
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Quote:
Originally Posted by TMF Lindbeck View Post
Then again cats and hi performance don't really go hand in hand.
Somebody should let the LS7 know about this so it can tone itself down a little.

To the OP, for a 350 or 383 you're not going to see a huge difference between 3" or 2.5" exhaust off the cats as long as it's quality mandrel-bent exhaust. There's many much bigger and meaner engines out there making due with a dual 2.5" system.
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Old 02-09-2013, 02:09 PM
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Thinking about this I'm trying to visualize all this stuffed under your truck. If full leingth headers are used and then bolted to the collectors cats, this would put you roughly at the tranny crossmember. I would go out and take some measurements and see if you'll be able to have room for everything.

With regards to cats and hi-po engines the similarity of a gen I,II sbc and a dry sump LS7 end shortly after discussing 16 pushrods and 8 pistons
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Old 02-09-2013, 02:23 PM
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Originally Posted by TMF Lindbeck View Post
With regards to cats and hi-po engines the similarity of a gen I,II sbc and a dry sump LS7 end shortly after discussing 16 pushrods and 8 pistons
I'm sorry, I was unaware that you couldn't put a modern catalytic converter behind a generation 1 small block. the LS7 probably has a special air filter element that couldn't be used on a gen 1 small block. I'm sure the mufflers couldn't be used either, right?
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Old 02-09-2013, 02:56 PM
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Quote:
Originally Posted by Valkyrie5.7 View Post
I'm sorry, I was unaware that you couldn't put a modern catalytic converter behind a generation 1 small block. the LS7 probably has a special air filter element that couldn't be used on a gen 1 small block. I'm sure the mufflers couldn't be used either, right?
I'm just getting at the notion that a catalytic converter in general will not be to conducive to taking advantage if full leingth headers and 3" exhaust in a truck with a stockish 350 or mild 383 and that he smaller piping after the cats will help with tq for towing and down low grunt meeting the op's needs both now and later.
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Old 02-09-2013, 04:04 PM
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Excellent - thanks guys
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Old 02-09-2013, 07:49 PM
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never ever heard of sma ler exhaust adding torque.
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Old 02-10-2013, 02:43 AM
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