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Necking down a down pipe

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Old 03-17-2013, 07:59 AM
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Originally Posted by Martin@Tick
Run the biggest down pipe you can fit. I'd make it as large as possible, I'm running a single 5" downpipe split to dual 4" exits. Size matters.
Of course, but there's some packaging hurdles here.
Old 03-17-2013, 08:17 AM
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Originally Posted by 69-chvl
I dont think that k-memeber will help as the downpipe hits the body, manifold, and sparkplug. K-member would help down below I believe, where I dont have a problem. 3.5" is about as big as its going to get, especially after I wrap the pipe cause its so close to everything.
No your going about this all wrong these are some examples of what im talking about.

Here is a 5" split to 2 3.5"


here is a 4"


Old 03-17-2013, 09:21 AM
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Originally Posted by zapp109
No your going about this all wrong these are some examples of what im talking about.

Here is a 5" split to 2 3.5"


here is a 4"


Thanks for posting Zapp. Problem is, I personally cant do that pro-quality work!!! Come on man, I can barely even sneak a 3.5" pipe through a straight opening w/o an issue let alone snake those pipes around like that
Old 03-17-2013, 09:26 AM
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Im a complete novice as well. But i know I want mine to go out the back so Im going to be doing the 4" setup. However ill have a 5" cutout upfront. If you feel you cant do this id just run the dp out the front fender
Old 03-17-2013, 09:33 AM
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I like the idea of splitting up into two 3.5" pipes, looks like I may do the same for cruising.
Old 03-17-2013, 09:40 PM
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Originally Posted by 95bowtie
For his situation if he were to use a turbo with a 75mm turbine yes, are you done? It doesn't matter as much as you think.

Depending which borg warner 400 series turbo you have all the turbine exits are closer to 3.5 inch so a 4 inch dp will suit your needs no matter what. Like you pointed out tho clearly I wasnt speaking about a s475 anymore.

One persons results compared to another really dont mean ****. There could be many reasons why change in dp size made such a difference in before and after hp numbers or track times.

One guy sees no change and the other sees pretty large change. It's obvious it has more to do with his whole setup more than simply just the dp being too "small"
do you have any first hand experience?
Old 03-18-2013, 07:47 AM
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What ive got out of this so far is that even though the turbine is only 3" if it has a 5" flange on it you need to run the biggest as possible (5") because the guys on YB do?
Old 03-18-2013, 09:00 AM
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yes even if on a mostly stock 5.3 shooting for 700rw max.



-no
Old 03-18-2013, 09:53 AM
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Originally Posted by pwrtrip75
What ive got out of this so far is that even though the turbine is only 3" if it has a 5" flange on it you need to run the biggest as possible (5") because the guys on YB do?
Originally Posted by MIGHTYMOUSE
yes even if on a mostly stock 5.3 shooting for 700rw max.

-no

Holy crap I'm confused now lol!!!
Old 03-18-2013, 10:31 AM
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He is being sarcastic

If you can fit it, run it
Old 03-18-2013, 10:36 AM
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Originally Posted by 69-chvl
I dont think that k-memeber will help as the downpipe hits the body, manifold, and sparkplug. K-member would help down below I believe, where I dont have a problem. 3.5" is about as big as its going to get, especially after I wrap the pipe cause its so close to everything.

Don't forget that there is a plug down there that will need changing from time to time and a starter that is in the way from the bottom.
Old 03-18-2013, 11:16 AM
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Originally Posted by 98Z28CobraKiller
Don't forget that there is a plug down there that will need changing from time to time and a starter that is in the way from the bottom.
Yeah I know - will have to run a 90* plug boot on that one, and and she is close to the starter but doable.

I just got of the phone with the great Jose at FI, he suggested the s475 t4 1.10 AR, he didnt like necking down the t6's to a 3.5 either. The t4's 4" to 3.5" via a smooth transition is much more ideal. I just had to cough-up the extra 200.00 for it.

That may of been the most expensive 1" I ever paid for (200.00 an inch)!!!

Last edited by 69-chvl; 03-19-2013 at 09:32 AM.
Old 03-18-2013, 05:56 PM
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Post turbine back pressure is a huge problem that no one seems to address on this website. Another reason why I'm running dual 4" down pipes.
Old 03-18-2013, 08:10 PM
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Originally Posted by Martin@Tick
Post turbine back pressure is a huge problem that no one seems to address on this website. Another reason why I'm running dual 4" down pipes.
lies, ... all lies
Old 03-19-2013, 02:14 PM
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Originally Posted by rotary1307cc
lies, ... all lies
Drink the kool aid and shut up!
Old 03-19-2013, 03:27 PM
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post turbine back pressure is a red herring.

you guys are fun to watch!
Old 03-19-2013, 05:08 PM
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Dave so you're saying with a 5" down pipe your car would not of picked up at all? I find that hard to believe.
Old 03-19-2013, 06:11 PM
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at 1300 hp, likely.

...for a ''mostly stock 5.3'' per thread topic nooooooooooooooope

wouldn't have done it on my car no matter how much hp it cost because streetcar

anybody want to start a 'best downpipe for 1300+hp' thread i'll stay out of it. until then ill keep fighting for you professionals to not confuse the poor guy.
Old 03-19-2013, 06:55 PM
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Originally Posted by MIGHTYMOUSE
at 1300 hp, likely.

...for a ''mostly stock 5.3'' per thread topic nooooooooooooooope

wouldn't have done it on my car no matter how much hp it cost because streetcar

anybody want to start a 'best downpipe for 1300+hp' thread i'll stay out of it. until then ill keep fighting for you professionals to not confuse the poor guy.
Dave,

I may of missed it, but what its the exit size of your turbo to your current 3.5" d-pipe?
Old 03-19-2013, 06:56 PM
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Ahh I love LS1tech, where 5" downpipes will make so much more power over a 3.5" downpipe on a stock 5.3L street car.


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