Twin turbo 2.5 vs 3 inch exhaust

FloStang

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Hey all, I recently bought a 2017 GT premium that came with some unexpected goodies. Gt350 manifold, mishi intake tube, upr catch can, lowering springs, and a set of Vossen hf2 wheels. I also noticed it has a Roush axle back. I know the Roush is a 2.5 in axle back. Now I plan to order a Hellion Street Sleeper turbo kit and my question is should I keep the Roush and just pick up a 2.5 inch x pipe or should I go full 3 inch. How much hp am I leaving on the table if I stick with the 2.5. Now my goals for now are to be around 800hp with e85 with room to grow later. I'll prolly be ordering the kit and other goodies with the next big sale like 4th of July. Thanks in advance
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Furious18

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Turbos create a lot of back pressure. In an ideal world you wouldn’t have anything past downpipe.

coyote heads flow a ton of air. If you have too much back pressure you’ll have issues.
 

FastCarFanBoy

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I’m not very knowledgeable about turbos but what I’ve gathered is the stock pipes Won’t be an issue with twins unless your trying to make mega power. The mufflers however need to be as free flowing as possible.
 

erikkire18

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You might have to go 3 inch catback. My kit only mounts to a 3 inch but maybe the sleeper kit is has an option of 2.5 or 3. i have top mount
 

StickShiftyFifty

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3 inch cat back with H pipe would be ideal for street, or best would be electric cutouts. Im looking into the cutouts when i get extra $$
 

Friesian

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I just bought a cheap mbrp 3" catback. Mufflers are straight through. Turbos drowned out the sound so much anyways you dont need a expensive catback
 

wazslow

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I just bought a cheap mbrp 3" catback. Mufflers are straight through. Turbos drowned out the sound so much anyways you dont need a expensive catback
Do you have the Hellion kit? I am working on installing my street sleeper kit now. How did you connect the 3 inch exhaust from the turbos to the catback? It seems like all of the 3inch catbacks still neck down to 2.5 inches. Do you cut that 2.5 inch portion off and use/buy 3 inch band clamps? I dont see any clamps in my kit to connect to the catback.

@HELLION TURBO any tips? Am I missing clamps?
 

Friesian

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Do you have the Hellion kit? I am working on installing my street sleeper kit now. How did you connect the 3 inch exhaust from the turbos to the catback? It seems like all of the 3inch catbacks still neck down to 2.5 inches. Do you cut that 2.5 inch portion off and use/buy 3 inch band clamps? I dont see any clamps in my kit to connect to the catback.

@HELLION TURBO any tips? Am I missing clamps?
I have the sleeper kit. Not installed yet. But, yes it looks like you would have to cut the front part off the catback and just weld it there or clamp it.
 

Furious18

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I’m not very knowledgeable about turbos but what I’ve gathered is the stock pipes Won’t be an issue with twins unless your trying to make mega power. The mufflers however need to be as free flowing as possible.
I’ve heard of many cars having boost creep or very unstable boost regulation.
 

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I would highly suggest at least the 3inch setup. They do tend to have a bit of back pressure which can in turn cause valve float if its not controlled. Luckily the Sleeper kit is a bit more efficient over the top mount kit. If you have any questions, feel free to PM me. I had a Hellion kit on my 2017.
 

gimmie11s

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Turbos create a lot of back pressure. In an ideal world you wouldn’t have anything past downpipe.

coyote heads flow a ton of air. If you have too much back pressure you’ll have issues.
This.

You need to go 3" turbo-back exhaust with straight through mufflers (no chambers) at the very minimum or you will be running into valve float at moderate boost levels 10+ psi or so.
 

5.oh

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Or boost referenced cut outs. Pretty affordable and allow you to keep your current exhaust. Have the cut outs open at say 5-6 psi and then boom you create a free flowing exhaust while making power
 

99Formula

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I’m running the Street Sleeper kit (62/66 Precision BB turbos) with boost actuated cutouts on my 2019 along with a FRPP resonator delete and factory cat-back. There were no signs of any sort of valve float.

The car made 770 to the tire through a MT-82 on 8.9lbs. It sounds stock (other than some turbo noise at idle) until the car makes around 4lbs and the cutouts open. It’s all in what you want, but I’ve built one too many super loud cars and wanted something more docile until I really laid into it.
 

5.oh

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I’m running the Street Sleeper kit (62/66 Precision BB turbos) with boost actuated cutouts on my 2019 along with a FRPP resonator delete and factory cat-back. There were no signs of any sort of valve float.

The car made 770 to the tire through a MT-82 on 8.9lbs. It sounds stock (other than some turbo noise at idle) until the car makes around 4lbs and the cutouts open. It’s all in what you want, but I’ve built one too many super loud cars and wanted something more docile until I really laid into it.
which boost referenced cut outs did you go with?
 
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