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Anyone built their own NA downpipe?

HiSPL

Fanking Champion 1993
Joined
Jan 8, 2015
Location
College Station, TX
Would like to see pics or hear about what you done did.

I want to upsize the entire exhaust but will be keeping the stock manifold.
 
Would like to see pics or hear about what you done did.

I want to upsize the entire exhaust but will be keeping the stock manifold.

mikep is building one for an L28 or 26 or whatever it is. Oooooooobviously not the same cylinder count, pipe routing and all that but it's a sight to behold, plus it might give you ideas
 
I just increased the size of the j-pipe with the original flange ported for the new tubes, to a 2.5 inch turbo exhaust. It worked out pretty good, 135whp with a k-cam on a untouched 530 head
 
Why not replace the stock manifold at the same time?

Get a cheapo pair of SBC headers and cut them up. Way back when I did a 4-2-1 and then converted it back to a 4-1, ugly but it worked and still lives on to this day.

rps20210626_083309.jpg


rps20210626_083401_781.jpg
 
For me, it's just not needed. I'm staying NA and the stock manifold flows good enough. It's the rest of the exhaust that is restrictive.
 
Why not replace the stock manifold at the same time?

Get a cheapo pair of SBC headers and cut them up. Way back when I did a 4-2-1 and then converted it back to a 4-1, ugly but it worked and still lives on to this day.
That reminds me of a Teutonics manifold I had in a Jetta many moons ago
 
Why not replace the stock manifold at the same time?

Get a cheapo pair of SBC headers and cut them up. Way back when I did a 4-2-1 and then converted it back to a 4-1, ugly but it worked and still lives on to this day.

Do you by chance mean SBF? The SBCs unevenly spaced exhaust ports would make for a ton of hacking.
 
Do you by chance mean SBF? The SBCs unevenly spaced exhaust ports would make for a ton of hacking.

Either that or its for a LS/LQ, I unfortunately don't remember. I used to have the other half of these headers as I was going to do the same modification but for my Nissan. Though I think I threw it away because I can't find it and I don't want to look very hard.
 
For me, it's just not needed. I'm staying NA and the stock manifold flows good enough. It's the rest of the exhaust that is restrictive.
Agreed! We have over 150whp in our B230 based racecar and it's still using the factory exhaust manifold with a little bit of port work. The factory downpipe isn't horrible, either. Depending on what you want to do, you may want to shorten it or leave it full length, but you ABSOLUTELY want to make the 2-1 merge exit into a larger diameter exhaust than stock. At least 2 1/4" or even 2 1/2".

https://pbase.com/mperry/image/171783471.jpg
Nice! Right now we are looking into building a new downpipe for our setup that isn't basically stock sized with our almost double factory HP goals for the race car and want to go up to 1 7/8" or 2" diameter piping. The 1 7/8" pipe will work better with reusing the original downpipe's flange and should be enough for 200whp with the right supporting mods, Erland Cox suggests, so we'll stick with that for ease of building. We're still trying to find a place to make us the 2-1 merge, though. I couldn't find one online that was reasonably priced. We may have a local fabricator that'll do it, but I like what you're doing here anyway. :)

EDIT: Thanks for the www.coneeng.com link whoever posted that somewhere that apparently wasn't this thread! :)
 
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We decided to not build a larger 2-1 because we didn't have a bender for 1.875" pipe, didn't have much time and weren't sure how great it was going to turn out.

The fact that we had a shipment coming from KG Trimning sealed the deal on getting a complete header from them, so we will try to make the 240 header fit on our cramped 140 setup. Fingers crossed!

I just increased the size of the j-pipe with the original flange ported for the new tubes, to a 2.5 inch turbo exhaust. It worked out pretty good, 135whp with a k-cam on a untouched 530 head
What size pipe did you use and did you bend it yourself? Shave that head 2mm, port it, open up the chamber some and tune the engine management for another 10+whp AND torque. But yes, I expect that was plenty fun as it were!
 
What size pipe did you use and did you bend it yourself? Shave that head 2mm, port it, open up the chamber some and tune the engine management for another 10+whp AND torque. But yes, I expect that was plenty fun as it were!

I used pre bent sections from Simons exhaust, 41mm id so 1.75inch tubing from the flange with a bit more length on the 2 tubes before the merge, straight to a turbo 2.5inch downpipe end I cut off. And yes loads of fun, I still miss my k cam with the current stock turbo one:)
 
Nice. That's basically what we've been running on our race car until now. What diameter is the stock downpipe? I thought it was basically the same.
 
Not sure on the stock diameter, at least for my car I know it was bigger as I had to create some space in the flange with a burr. Stock might be around 38mm maybe? This is what my first attempt looked like with diy bungs for egt and lambda:
<a href="https://imgur.com/Pwt5Mbc"><img src="https://i.imgur.com/Pwt5Mbcl.jpg" title="source: imgur.com" /></a>
 
I like it!

I see you have the tab there to hold the two pipes together. I am wondering if that is something we should add to the KG Trimning header we're getting, or if everything should be left separate. We could also add a flange to help support it to the bellhousing like the factory setup originally was, but I wonder if that will just add a stress area to crack in the future. :shrug:
 
I put it there to make sure it wouldn't flex as much with the turbo stock cat still being there as well, which increased the weight being supported. A tie in to the bell housing would be good to prevent stress, but I would suspend it with some sort of spring/rubber setup to allow for movement.
 
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