Necking down a down pipe
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.
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.
Thread Starter
Joined: Nov 2009
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From: THORNTON, PA (NEAR PHILLY)
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?????
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?????
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?????
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?????
Thread Starter
Joined: Nov 2009
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From: THORNTON, PA (NEAR PHILLY)
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
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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.
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.
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.
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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 ****.
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Thread Starter
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From: THORNTON, PA (NEAR PHILLY)
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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?
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?
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?
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?
Thread Starter
Joined: Nov 2009
Posts: 1,621
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From: THORNTON, PA (NEAR PHILLY)
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?
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.
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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.
Teching In
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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"?
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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"
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; Mar 16, 2013 at 10:35 PM.
