NightmareTA

iTrader: (20)

Like the title says, I have a turbo 5.3 that I'm almost done building. I can only fit 3" exhaust in the engine bay but I was wondering whether I should step it up to 4" just past the framerail or just leave it 3" all the way back? I know smaller pipe ussually means higher velocity but turbos ussually like more exhaust. I just wanted all of your opinions on which would be better for power, or if it won't matter much between the two?

Heres a pic of setup...

tim99ws6

iTrader: (31)

You can never have too big of a exhause/DP on a turbo car. Make 4" fit as soon as possible and carry it as long as possible if you can.

123quattro

Really, it depends on how much power you want to make. 3" pipe will support 600whp. I'm guessing you are shooting for quite a bit more than that. You can make 4" fit in there. If not, convert over as soon as you can.

NightmareTA

iTrader: (20)

For the start, I'd be happy with 550-575 RWHP at whatever boost it takes to make it. Guessing around 13psi or so should do the trick. Eventually though I do want to throw meth on it and crank it up to around 17 PSI for over 600 WHP. I'll do 4" after the frame rail then and maybe see if I can squeeze 4" in the engine bay as well. I think it'll fit but it'll be close between the engine block and framerail.

tiznodd

iTrader: (4)

It's not just about how much power a 3" pipe will support, spool is also affected. I would highly suggest a 4" pipe. If 4" won't fit, look at a 3.5". There does seem to be diminishing returns at a certain point, but going larger than 3" will be a pretty significant improvement.

NightmareTA

iTrader: (20)

Thanks for the info. Seems that 3" is on the smaller side then according to everyone. The T76's exhaust side has a 3" opening so I figure it has to at least be sufficient if thats the size on the turbo hotside, but I know that a lot of people expand it to 4" right out of the turbo. I'll see what I can do for fitment. My existing downpipe is pretty ugly anyways haha.

HydroStream6

iTrader: (16)

People have made over 1000hp on a 3" all the way back. With that said, I'd still do the largest you can. What size and AR is the turbine side?

NightmareTA

iTrader: (20)

Turbine size is .84 AR and it has a 3" outlet on it. Smaller AR for a bit quicker spool on a 5.3. I believe it's a custom option since you can't buy a T76 with that size AR from turbonetics off the shelf.

Johnv

If you can't fit 4" , then try 3 1/2" downpipe into 3 1/2 or 4" exhaust.
Even 3 1/2" will flow considerably more than 3"

slow67

iTrader: (2)

Why not run 3" all the way back, but have a cutout up front for the track?

NightmareTA

iTrader: (20)

Honestly, this is a pure street car. I plan on going to the track maybe once or twice a year tops, but I want full power all the time on the street....so basically I want it to run what it does at the track on the street with no cutout, slicks, etc. Street will be 98% of this cars driving.

jridenour31

I agree with everybody in here. The bigger is better mentality actually does apply to downpipe sizing. Velocity isn't a concern post turbo. You just need to get rid of it with the least amount of backpressure. If you're running it all the way back you should definitely step up as soon as possible since the extra length and bends will be adding more resistance. You may not gain much power on a soft tuneup but it will spool noticeably faster and pay off considerably when you turn it up.

NicD

iTrader: (7)

You should go at least a half inch bigger than the outlet as soon as possible and if you have a lot of bends go a full inch bigger. Bigger is always better on a turbo car.

3" outlet = 3.5" - 4.0" exhaust.

NightmareTA

iTrader: (20)

Yea that seems to be the way to go after seeing everyones responses. I do have one more question though. Because of how my wastegate is set up and how close it is to the downpipe, I really can't fit bigger than 3" until just after where the wastegate connects to the downpipe. If I step up to a bigger pipe just after the wastegate, is that good enough? Really not wanting to re-design half my setup just to fit a bigger pipe in. There would only be about 6" or so of pipe out of the turbo before it got stepped up to a bigger size.

kbracing96

iTrader: (25)

Turbonetics will weld a 3.5" or 4" v-band on a new turbine housing for an additional $75. You could also carry your WG outlet to under the car where you step up to the bigger pipe. Just an idea. Here is a pick of a 4" V-band Turbonetics did for me. Both turbos have the same F1 68mm turbine wheel.

black06g85

iTrader: (6)

well I'm running 3" for now so I'll let you know how that works out in a few week

2nd Gen Fl 'bird

iTrader: (13)

If you look at physics of heated gas(exhaust here) it has expanded. The farther it travels from the heat source it obviously is going to cool. As soon as it leaves the turbine outlet it starts cooling from that. When hot gas is "allowed" to expand, it cools. That's the reason of going immediately to a larger pipe off of the turbine outlet is the best for flow, expanding gas=cooling gas= less volume=less restriction of exhaust flow. Therefor, no back pressure and better spool times. So, to answer the question I think you're asking(IMHO)there will be no gain to increase pipe size if not directly off of the turbo. The exhaust has cooled(less volume) by the time you get 3-4 ft down stream so, no reason to step up to bigger. Go as big as room allows. Guru's chime in please if wording is wrong or if this is wrong period.

jridenour31

Yes, there is reason to go bigger downstream if that's your only option. There is more to backpressure than just diameter. Length and bends also add resistance. A straight two foot section of three inch that steps up to four inch and then runs to the back has considerably less resistance than running three inch all the way to the back with several bends.

NightmareTA

iTrader: (20)

Thats what I originally figured. Unfortunately, I have to follow the stock 3rd gen exhaust routing due to my BMR T56 tranny crossmember blocking any chance at running the exhaust right down the tranny tunnel. It'll go to the pass side floorboard, then take a turn into the tranny tunnel and go to the back. Thats why I figured I'd at least go to 4" past the framerail.

2nd Gen Fl 'bird

iTrader: (13)

Okay, I'll agree on the above but, 5.3 with the 76 is not worth the wrangling over trying to get 4" under the car. Not knocking the combo because I'm in the process of the same motor/turbo. Throw us some pics and power numbers when you're done. JUst curious.