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Necking down a down pipe

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Old 03-12-2013, 10:24 AM
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And there you have it huge dp myth debunked....I bet if you had a 4 inch catback tied into that 3.5 inch dp you wouldn't have lost anything at all. Just use straight through glass pack style mufflers or bullet style they are called.

I personally am doing 3.5 dp to a 3.5 bullet then splitting it into dual 3inch with a dynomax bullet on each tail pipe. Cant wait for the car to be quiet again.
Old 03-12-2013, 11:02 AM
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What year mustang? There is a ton of room in that engine bay for an adequate dp....
Old 03-12-2013, 11:10 AM
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figure the catback i removed was 30# or so which removing would make the car faster anyhow!

thats for 364ci and a pt88 at 23psi boost

i still agree 'bigger is better' but id be surprised if you could acutally measure it!

if its costing less than 10 pones may be a good compromise from a 5" downpipe out the front bumper of your 'street car'
Old 03-12-2013, 11:29 AM
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Originally Posted by drumbo87
What year mustang? There is a ton of room in that engine bay for an adequate dp....
96, using the stock K and ls3 manifolds flipped backwards. This is a section of 3" pipe, and it bumps just about everything around it.






Old 03-12-2013, 05:27 PM
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Originally Posted by MIGHTYMOUSE
figure the catback i removed was 30# or so which removing would make the car faster anyhow!

thats for 364ci and a pt88 at 23psi boost

i still agree 'bigger is better' but id be surprised if you could acutally measure it!

if its costing less than 10 pones may be a good compromise from a 5" downpipe out the front bumper of your 'street car'
I am confused what you are saying. Are you talking open dumped DP, full exhaust but with cutout, of full exhaust without any cutout?
Old 03-12-2013, 11:19 PM
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69-chvl...... Why not use a tubular k member and free up some space?
Old 03-13-2013, 02:22 AM
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Originally Posted by rotary1307cc
I am confused what you are saying. Are you talking open dumped DP, full exhaust but with cutout, of full exhaust without any cutout?
Think he's saying he was running a full 3.5 in exhaust all the way out back. Taking it off and running an open dp only made the car .10 faster. At over 1k rwhp...
Old 03-13-2013, 02:27 AM
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Originally Posted by drumbo87
69-chvl...... Why not use a tubular k member and free up some space?
This would be a good option I would think. Cost more $ but like they say time and time again you gotta pay to play.

If you dont want to neck the dp and you do run it out front I have seen it done with a 4inch dp and it had a long dynomax bullet on it that was about 20 inchs long maybe. Could hardly hear it idle thats no lie. Sounded stock when he got on it it screamed tho which is always nice haha.
Old 03-13-2013, 06:18 AM
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Aftermarket k may happen at some point. I think for a 4" DP or if I want to go twins it pretty much necessary. Leaning towards a tc76 turbo which only has a 3" connection, so a 3 or 3.5" dp is all that's happening here - cant see the need to change the k out for that. I agree though that the K would free up alot of room.
Old 03-13-2013, 09:52 AM
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Originally Posted by 95bowtie
Think he's saying he was running a full 3.5 in exhaust all the way out back. Taking it off and running an open dp only made the car .10 faster. At over 1k rwhp...
Yeah.... ooook
Old 03-13-2013, 09:57 AM
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Originally Posted by 95bowtie
Think he's saying he was running a full 3.5 in exhaust all the way out back. Taking it off and running an open dp only made the car .10 faster. At over 1k rwhp...
correctamundo
Old 03-13-2013, 10:35 AM
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here is a pass from years ago, full 3.5" exhaust (stainless works)
thats on a pt88 turbo and 24# boost
my previously posted best ever 8.49 was with a 91mm turbo, 26# boost, and no cat back. the car was also lighter from the skinnies and no cat back. you be the judge.
Old 03-13-2013, 10:57 AM
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3" downpipe here only because I can't snake anything larger through the k-member without re-doing everything, and that's not happening. Yes I know it's a restriction which is why I made it as short and sweet as possible dumping out the side before the rear tire. If I had the space I would go 3.5" but no larger and if you are having backpressure problems at 3.5" something else is amiss.

800+ rwhp for reference, went 5.85 in the 1/8th last time out.
Old 03-14-2013, 01:38 AM
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Originally Posted by MIGHTYMOUSE
here is a pass from years ago, full 3.5" exhaust (stainless works)
thats on a pt88 turbo and 24# boost
my previously posted best ever 8.49 was with a 91mm turbo, 26# boost, and no cat back. the car was also lighter from the skinnies and no cat back. you be the judge.
That car was sick man. I remember seeing that video before I even was on tech or even owned an lsx car and I thought to my self see it doesnt have to look or sound like a race car to run like one haha.
Old 03-14-2013, 06:33 AM
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Originally Posted by NicD
3" downpipe here only because I can't snake anything larger through the k-member without re-doing everything, and that's not happening. Yes I know it's a restriction which is why I made it as short and sweet as possible dumping out the side before the rear tire. If I had the space I would go 3.5" but no larger and if you are having backpressure problems at 3.5" something else is amiss.

800+ rwhp for refere

nce, went 5.85 in the 1/8th last time out.
Can you give some details on your setup? Is this the Mustang?
Old 03-14-2013, 10:52 AM
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Originally Posted by 69-chvl
Can you give some details on your setup? Is this the Mustang?
No the Camaro,
347ci / T76 / TH400 / E85 / ~3400 lb raceweight
Old 03-14-2013, 12:57 PM
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On my setup, going from a necked down 3" to 4" gave me almost 3 more psi of boost and dropped 1/2 a second in the quarter without touching the boost controller. It matters.
Old 03-14-2013, 06:15 PM
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Originally Posted by 98Z28CobraKiller
On my setup, going from a necked down 3" to 4" gave me almost 3 more psi of boost and dropped 1/2 a second in the quarter without touching the boost controller. It matters.
No it doesnt. Shush
Old 03-14-2013, 09:44 PM
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So in the end is it the HP rating that dictates the d-pipe size or the turbo's design characteristics? Obviously the hi hp guys can observe a difference, but what about the <700 whp crowd?
Old 03-15-2013, 10:26 AM
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Really it has to do with airflow more than anything ie 600 rwhp doesn't produce as much exhaust airflow as 1000 rwhp so it's not going to have as much of an impact comparably speaking. The other thing is that the number of bends, etc also plays a big role in how it flows not just the size of the pipe. A full 3.5" exhaust that has tons of bends isn't going to flow as well as a short 3" pipe down pipe that just dumps out the side right away.


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