Downpipe Fabrication Question

[Blackfin]

I am preparing to weld up a one piece down pipe/mid pipe for my turbo supra. This is my first real tig welding project but I have been practicing. I will be using stainless steel pipe 304 or 321, depending on availability.

I am built for 650 - 700 hp with the turbo (GT4088R) being the limiting factor and something I can easily change out if I want to change my goals.

The turbine outlet is a 3” v-band. Due to limited clearance between the turbine outlet and the firewall I will need to begin the downpipe with 3” tight-radius elbow. The downpipe will connect to a 3-1/2” HPF catback system. The catback reduces to 3” at the uppermost end.

My first question is where is the best place for performance and a tight fit to transition from 3” to 3-1/2” pipe. Obviously, the easiest approach is to leave the catback alone and use 3” pipe all the way back to the catback. This would also simplify the ordering of pipe. The other apparent options are to transition to the larger pipe approximately 4-6” below the elbow at the turbine or approximately 30” after the turbine elbow as soon as the exhaust is underneath the pan.

My second question is where is the best place to locate the bungs.

I understand that many of you have fabbed exhaust and/or understand how to optimize performance much better than me. Kindly share your opinion.

 

[Clip]

that is one hell of a post it should be used as a format for other posts.

wish i could help, i'm just beginning to weld up a full exhaust for my truck and my supra's still NA.

I'd just leave it 3" dia. all the way back for ease, but with a 700hp goal it might be better to go 3 -1/2" all the way.

 

[Blackfin]

that is one hell of a post it should be used as a format for other posts.

wish i could help, i'm just beginning to weld up a full exhaust for my truck and my supra's still NA.

I'd just leave it 3" dia. all the way back for ease, but with a 700hp goal it might be better to go 3 -1/2" all the way.

Thanks. I think a good answer begins with a properly presented question so I try to make things as clear as possible.

 

[MDCmotorsports]

Either pie cut & weld the elbow yourself, or check these guys out:

www.atpturbo.com

They have all your bends and o2/egt bungs that you need.

BTW, 304 or 308 will do nicely.

1/16" 308L rod is what you'll want to weld it with.

 

[Blackfin]

Either pie cut & weld the elbow yourself, or check these guys out:

www.atpturbo.com

They have all your bends and o2/egt bungs that you need.

BTW, 304 or 308 will do nicely.

1/16" 308L rod is what you'll want to weld it with.

Yes, i was planning to begin with a 3" radius elbow like this one from ATP:
http://www.atpturbo.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=ATP-STL-025&Category_Code=PPB

Elsewhere, I thought I would cut up some u-bend or j-bends and form my bends from them. I want to avoid pie-cuts of straight pipe section as much as possible to optimize flow. Thanks for the sugestions on the pipe material and rod. Not familiar with 308 pipe. I will try to learn more. Does it take the heat and polish well?

 

[jdub]

Ceramic coat the downpipe and the grade steel used becomes moot...I suggest a 2000 deg rated, black coating.

 

[MDCmotorsports]

Ceramic coat the downpipe and the grade steel used becomes moot...I suggest a 2000 deg rated, black coating.

Shush old man, go yell at some kids on your lawn or something.

LOL!

308 Stainless is rare in the form of tube, so I really wouldn't worry about it.

 

[Blackfin]

"Putty and paint make the carpenter what he ain't."

Agree, ceramic coat is most sensible.

Back to my original question. Still looking for suggestions.

 

[jdub]

Shush old man, go yell at some kids on your lawn or something.

LOL!

 

[MDCmotorsports]

"Putty and paint make the carpenter what he ain't."

Agree, ceramic coat is most sensible.

Back to my original question. Still looking for suggestions.

I would just run the 3" all the way down under the car and then transition to the 3.5" tube.

 

[jdub]

Yep...3" to the CAT. The KISS principle applies

Bung wise, a foot or so down from the turbine exit will keep you well clear of any excess heat issues for a WB O2. Just mount in a place that you can get to easy.

 

[Blackfin]

Any idea about how much this KISS will cost me? Specifically, if I could make 700 RWHP on the dyno with the suggested 3” pipe to 3-1/2” catback, how much more could I make with a full 3-1/2” system?

Jdub, gotcha on the O2 bung for the wideband. About 1 foot down and accessible. Thanks!

What about the bung for EGT? I think the outside radius (back) of elbow location is normal for this. But this location may have a clearance problem. Can it be moved up closer to the turbine outlet or to the side?

 

[jdub]

Personally, if I were going for 700 HP+, I'd run 3 1/2" from the turbo back...you'll need a new CAT. I also think you can meet your goal using 3" pipe to the CAT. Depends on what you want to spend...it's going to be more difficult fitting the larger tubing from the turbo down too.

I'd put the EGT bung (or just tap it) on the turbine snail, just above the flange. You'll get more accurate readings...bungs on the downpipe read about 100 deg C lower than just aft of the turbine outlet.

 

[Blackfin]

"I'd put the EGT bung (or just tap it) on the turbine snail, just above the flange."

Sorry jdub , I don't quite understand. Do you mean tap directly into the rear of the turbine housing (above the flange) or somewhere between the v-band flange and the turbine housing, which doesn't look possible.

Also, it would seem to me that flow right behind the turbine is quite turbulent. So placing the 3" to 3-1/2" expander here would seem preferable (for performance maybe not for feasibility) since flow is already disrupted in addition to the more obvious benefit that comes from maximizing the length of larger pipe. However, I am unsure whether or not the change in exhaust gas velocity so close to the turbo would have an adverse impact to performance.

 

[jdub]

In the turbine snail, right above the flange where it mates to the exhaust manifold. This location reads temps as exhaust gas exits the manifold and before it hits the turbine wheel. Once the gas hits the wheel, energy is extracted and the temps drop...i.e. you are not getting a good reading. The best way would be to measure individual cylinder temps like they do on aircraft engines, but that would be a bit much to have 6 sensors on a car motor

Here's the rub about using a 3" to 3-1/2" expander at the turbine outlet...you don't have a lot of room between the turbo and the firewall. It would be a good location, but a real PITA to fit. It would be easier to fit on the vertical part of the downpipe before it curves under the car.

 

[IJ.]

There's actually some power to be had if you can get the 3.5 trans up close to the 1st bend

Turbulant flow off the Turbine wheel likes the room to expand and straighten out.
(It must suck to have the LHD Firewall space restrictions)

 

[jdub]

Yep...3" 90 deg bent to 3" to 3-1/2" expander in the vertical would work well and be a heck of a lot easier to fit!

 

[IJ.]

I have 3.5" right to the turbine

 

[jdub]

3.5" V-band?

 

[IJ.]

rgr