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Necking down a down pipe

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Old 03-15-2013, 12:46 PM
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To add to what I wrote about earlier, When my car was necked down it would only make 8-9 psi on the line on the transbrake, with the 5" downpipe cap opened it will do 12psi foot braking @ 3200 and 16psi on the transbrake @3600-3700. That was worth two tenths to the 60ft.

Looking back at the my logs the only two I can really compare to be fair are the off the trailer 10 psi runs and my car made 10psi over 600 RPM earlier with the 5" DP vs the 3.5.

I did what most have suggested and use the 5" like a cutout and run the car through 3.5 out the back when I am out on the street.
Old 03-15-2013, 03:43 PM
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Thanks for the great insight guys!

If one was to take a 4" pipe, and gently squeeze it to "ovallize it", it would still have the flow of a 4" pipe correct? Maybe I can flatten it abit through the tight spots and then let her flare back to 4" where she can.

The big question is this: If I could make the 4" pipe work (or 3.5" at worst), would you guys run a s475 t6 with the 5" connection OVER say a tc76 with a 3" pipe on a street 5.3 mostly stock, th400, 3.27 rear?????
Old 03-15-2013, 04:30 PM
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no way you need even a 3.5" pipe on a 'mostly stock 5.3' save your money.
Old 03-15-2013, 07:37 PM
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Originally Posted by 69-chvl
Thanks for the great insight guys!

If one was to take a 4" pipe, and gently squeeze it to "ovallize it", it would still have the flow of a 4" pipe correct? Maybe I can flatten it abit through the tight spots and then let her flare back to 4" where she can.

The big question is this: If I could make the 4" pipe work (or 3.5" at worst), would you guys run a s475 t6 with the 5" connection OVER say a tc76 with a 3" pipe on a street 5.3 mostly stock, th400, 3.27 rear?????
Stay away from the TC76 turbine is on the small side, It will spool up quick though. S475 will make at least 150 more rwhp, I guess it all depends on your goals.
Old 03-15-2013, 07:48 PM
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Goal here is a very responsive street ride 1st, dyno/track performance secondary. I did an exhausting (ha ha, no pun) search and the tc76 gets rave reviews for spool, but flattens out up top. The s475 is much bigger and the sky is the limit as far as hp, just not sure it will spool as well being on a 5.3 street car as compared to the tc76
Old 03-16-2013, 05:12 AM
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Originally Posted by Preston99WS6
Stay away from the TC76 turbine is on the small side, It will spool up quick though. S475 will make at least 150 more rwhp, I guess it all depends on your goals.
They have the new tc7675 and tc7875. I would guess its gonna be as successful as a pt7675 and those have been known to make 800+ whp and spool nice and fast.

If its a fun street car than one of those turbos is your best bet. They will max a stock motor out well before they are maxxed out.

75mm is right at 3 inchs so a 4inch dp would be over kill. 3.5 inch would be ideal.
Old 03-16-2013, 08:12 AM
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Originally Posted by 95bowtie
They have the new tc7675 and tc7875. I would guess its gonna be as successful as a pt7675 and those have been known to make 800+ whp and spool nice and fast.

If its a fun street car than one of those turbos is your best bet. They will max a stock motor out well before they are maxxed out.

75mm is right at 3 inchs so a 4inch dp would be over kill. 3.5 inch would be ideal.
I would agree as long as he gets the 75mm turbine, Didnt know the 75mm turbine was available for the TC yet.
Old 03-16-2013, 08:27 AM
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Originally Posted by Preston99WS6
I would agree as long as he gets the 75mm turbine, Didnt know the 75mm turbine was available for the TC yet.
Yeah the only ppl I've seen with it is huron speed. That's where I got mine at.

Whats crazy is if you look at the specs of their mid frame turbos like the 91mm it has the 75mm wheel in it....wtf talk about lop sided as ****.
Old 03-16-2013, 10:41 AM
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Originally Posted by 95bowtie

75mm is right at 3 inchs so a 4inch dp would be over kill. 3.5 inch would be ideal.
Dude you crack me up
Old 03-16-2013, 03:06 PM
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Actually some good info here, but my vote is to get a AJE k member and you will have a ton of room
Old 03-16-2013, 05:01 PM
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Originally Posted by zapp109
Actually some good info here, but my vote is to get a AJE k member and you will have a ton of room
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.

Old 03-16-2013, 07:08 PM
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Originally Posted by rotary1307cc
Dude you crack me up
Whys that? Atleast I'm not tryn to sell him on just doing a 3 inch because it will do good enough because it will for a stock motor build all day.

Not saying bigger isnt better because we all know no exhaust is the best exhaust for a turbo.

OP doesn't seem to want a dump out the bumper and doesn't want to cut the car all up neither.

That being said 3 inch will do he's currently not looking to make big hp numbers, and when one of the bigger successful shops in the country says they have made 900whp on a pt7675 and a 3 inch dp not to mention mighty mouses car how could you not agree a 3.5 will be just fine for a stock motor hp setup?
Old 03-16-2013, 07:09 PM
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notch and box the frame with some pipe where the dp passes, so that it mirrors the DP, could fit anything
Old 03-16-2013, 07:13 PM
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Originally Posted by 95bowtie
Whys that? Atleast I'm not tryn to sell him on just doing a 3 inch because it will do good enough because it will for a stock motor build all day.

Not saying bigger isnt better because we all know no exhaust is the best exhaust for a turbo.

OP doesn't seem to want a dump out the bumper and doesn't want to cut the car all up neither.

That being said 3 inch will do he's currently not looking to make big hp numbers, and when one of the bigger successful shops in the country says they have made 900whp on a pt7675 and a 3 inch dp not to mention mighty mouses car how could you not agree a 3.5 will be just fine for a stock motor hp setup?
hey whatever you say....go tell the guys on bullet who make actual power and fast passes that a 3.5" through mufflers out the back is "ideal" for a 900hp boosted lsx
Old 03-16-2013, 07:17 PM
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Originally Posted by 95bowtie
Whys that? Atleast I'm not tryn to sell him on just doing a 3 inch because it will do good enough because it will for a stock motor build all day.

Not saying bigger isnt better because we all know no exhaust is the best exhaust for a turbo.

OP doesn't seem to want a dump out the bumper and doesn't want to cut the car all up neither.

That being said 3 inch will do he's currently not looking to make big hp numbers, and when one of the bigger successful shops in the country says they have made 900whp on a pt7675 and a 3 inch dp not to mention mighty mouses car how could you not agree a 3.5 will be just fine for a stock motor hp setup?
]]

I agree with this - if 900 whp was done through a 3", a 3.5" is plenty as suggested. What my thick Italian head is struggling with is the t6 5" s475 outlet being necked down - just wondering if that would foul-up some engineering of the turbo that's all. Kinda like expansion chambers on 2 cyle motors (which I used to work on). They have a optional t4 475 with a 4" connection, but its like 200.00 more, and I'd have to neck it down a touch also, so why not just save 200.00 and neck the t6 right (so long as its a non-issue, which it may not be at this level).

As far as cutting the body, I could. But then the shock tower is in the way, and I could cut that as well. But then again, guys are making huge power with 3" - why should I cut anything right?????

Sorry for the rant guys...just dont want to make a mistake, cant afford to make to many of them.
Old 03-16-2013, 07:26 PM
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Originally Posted by rotary1307cc
hey whatever you say....go tell the guys on bullet who make actual power and fast passes that a 3.5" through mufflers out the back is "ideal" for a 900hp boosted lsx
Never said it was ideal just that it will work and you know that.

This isnt a race car obviously......so why put race car size exhaust thats good for 1500hp when he's not trying to make that much.
Old 03-16-2013, 09:15 PM
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Originally Posted by 95bowtie
3.5 inch would be ideal.
.....
Old 03-16-2013, 10:09 PM
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I was always under the impression that you should run the biggest down pipe possible to reduce back pressure on the turbo that is as long as space allows it. If you have a gt42 with a 5" outlet and you can run 5" pipe shouldn't you or is there no real performance advantage compared to necking down to 4" or even 3.5"?
Old 03-16-2013, 10:26 PM
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Originally Posted by rotary1307cc
.....
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"

Last edited by 95bowtie; 03-16-2013 at 10:35 PM.
Old 03-17-2013, 12:39 AM
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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.


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