DIY Divorced Down pipe 7mgte DDP. What metal alloys do I use?

[D51208]

Hi there,

So I am looking at fabricating my own divorced down pipe for my 7mgte with a stock CT26. I have contemplated just buying a down pipe, but I really want a divorced one. The reason I am not buying one from BIC is because 1. I cannot afford it and 2. I can potentially make this for a school project and get credits for it. I believe I have the potential to build this with pre-bent mandrel tubing and using my cutting and welding skills to put it together. I'd rather build then buy.


The idea is to save some money by building it myself without sacrificing quality.

My main question is what type of metal should I use? Mild steel or Stainless steel 304?
(this will not be driven in winter with salts on the road so the only corrosion I am worried about is from moisture, heat and atmosphere).

What gauge do I use?

Does anyone know of any place that sells piping cheap, preferably already bent?

Lastly does anyone know of where to buy a CT26 flange that will work for a divorced settup or will I need to make one myself?

 

[755hp]

On choosing metals: I'd go with 304 stainless steel to start.

 

[585Soup]

Buy an ebay recirculating divorced, have a shop weld a divider in it where the exhaust port and wastegate are... then just cut open the wastegate down tube and weld shut the exhaust side.

 

[585Soup]

I debated doing this, but from what i read, the only real difference you get it just more sound. the HP numbers don't seem enough for me to put in that effort.

 

[toyotanos]

IMO, the bellmouth downpipes are a better option. I had significant trouble trying to control boost when I had a DDP, which was alleviated when I borrowed a friend's bellmouth. They're also much easier to install and remove than the DDP's, something I did many, many times.

 

[suprarx7nut]

Definitely go for SS. If you're concerned about quality i think mild steel is out of the question.

With pre-bent tubing are you planning to do additional pie cuts and welds? I'm not picturing that being easy to line up well. The downpipe dictates the orientation of the rest of the exhaust system. Even with a flex section, a small error in the downpipe dimensions translates to a bigger pain in the ass downstream at the muffler. Without a flex section you have to be incredibly accurate.

 

[coinball]

I used mild steel on the 4 or 5 DDP's I made back in 2007/2008; if you want it to look pretty, don't used mild steel since it'll rust, but if you just want to go fast it'll be fine. Also, machining the DDP CT26 flange is easier with mild steel than stainless (in my experience).

 

[GC89]

Stainless would be great if you have a tig. With a mig it can be done but is a bit of a pain imo. The flange will also be more costly to have made in stainless. I personally opted for regular mild exhaust tubing and coated it.


FYI Randy's DDP's are just aluminized steel and painted unless you opt for ceramic coating.......

 

[FernandoDLT]

I just checked ebay for a Chinese DDP, I paid $190-$230 for mine a few years ago, they're now $760 - $1000!!!! WTF. Are people actually paying these prices??

http://www.ebay.com/itm/TOYOTA-SUPR...EEL-/141140190560?hash=item20dc9c6560&vxp=mtr
http://www.ebay.com/itm/Toyota-Cres...ipe-/170816203666?hash=item27c5706b92&vxp=mtr

 

[suprarx7nut]

The hard part about a coated mild steel is that it will deteriorate and soon show through paint and coatings. I had a ceramic coated "SS" cat back start rusting badly after only a couple years due to a "bad batch" of SS. I'm very weary of mild steel (or poor SS) for exhaust, especially right up on the turbo where the factory system was a cast iron elbow followed by a mild steel downpipe that commonly cracked and rusted.

I just checked ebay for a Chinese DDP, I paid $190-$230 for mine a few years ago, they're now $760 - $1000!!!! WTF. Are people actually paying these prices??

http://www.ebay.com/itm/TOYOTA-SUPR...EEL-/141140190560?hash=item20dc9c6560&vxp=mtr
http://www.ebay.com/itm/Toyota-Cres...ipe-/170816203666?hash=item27c5706b92&vxp=mtr

Holy crap! I hope nobody is paying that. Yikes. A really good SS and ceramic coated downpipe should be $700-800 max. That ebay Chinese crap with questionable materials should be a couple hundred.

Stainless would be great if you have a tig. With a mig it can be done but is a bit of a pain imo. The flange will also be more costly to have made in stainless. I personally opted for regular mild exhaust tubing and coated it.


FYI Randy's DDP's are just aluminized steel and painted unless you opt for ceramic coating.......

Randy's were really aluminized steel? Color me surprised.

I'd definitely opt for good SS. What kind of cost difference are we talking here? $50? $100? Worth more than $100 in my eyes. My Raptor Racing ceramic coated SS downpipe looks brand new after 4 years. Even with a coating or paint mild steel will not look that good. If you want it to be something you can be proud of in a few years use SS, hands down.

 

[fromgehenna]

The 321H (UNS S32109) alloy is best stainless alloy for manifolds and downpipes. Standard 304 or 316 is good for rest of the exhaust. 321H is heat resistant so is does not live so much when it heats and cools down so it does not crack easily. It's also stronger since it has more carbon. You can also get 304H and 316H alloy but 321H is stronger.

Also when welding stainless remember to use pickling gel to welds so they stay stainless. No one likes brown welds on stainless.:biglaugh:

 

[D51208]

Thanks for the input everyone. Ideally I would like to go stainless, if I can acquire some tubing at a fair price and can find someone who has a reasonable charge with a mandrel bender, I will do that. As of right now it looks like it won't be cost productive to get pre bent pieces and fabricate them together. If this route becomes do-able and I can't find an SS flange, I could probably whip one out on the CNC machine...


Right now I am thinking of doing the alluminuzed steel because it will be cheaper and I can confirm that BIC uses aluminzed steel with high temp paint. That's what it says on BICS sight.

 

[D51208]

I have spent a few hours trying to design the flange on mastercam. It is much harder to do than I originally thought, whoever designed the back of the turbo must have wanted to make things difficult. The whole thing is angular, with all sorts of different tangents and whatnot....

 

[black89t]

id just run a ebay bellmouth and call it a day. i had a cooleeze bellmouth on my car when it had the 7m in there and it worked awesome! the only thing the ddp does is sound like you have a external wastegate. i doubt theres any difference it power capabilities between the two.

http://www.ebay.com/itm/7MGTE-Turbo...pra-/121159756250?hash=item1c35af29da&vxp=mtr

 

[SideWinderGX]

I have spent a few hours trying to design the flange on mastercam. It is much harder to do than I originally thought, whoever designed the back of the turbo must have wanted to make things difficult. The whole thing is angular, with all sorts of different tangents and whatnot....

http://www.bicperformance.com/products.htm

Near the bottom of the page, $30 gets you a proper CNC laser cut flange. You'll have to weld the tab in of course.

 

[GC89]

^ that flange wont work for a DDP, you could however email randy and ask if he would sell you a DDP flange instead.


or pickup one like THIS and drill your own second waste gate hole.

 

[D51208]

Well an update. Here is the flange I made. Just looking to find someone with a mandrel bender now to bend some pipe!

 

[GC89]

Nice work, any chance you would share that file? I built my last downpipe but its just a standard one, building a new ddp style one has been on my list of things to do for a while.

 

[D51208]

Sent you a private message.

 

[Backlash2032]

Did you make a CAD file for it? I'm honestly not a fan of how Mastercam makes programs. Way too much clutter for something that could be done in (I shit you not) 20 lines. Most of that would be the periphery cut though.