Fabricating a 2.5" Downpipe
By Matt Beazer
Last Updated 5/19/05
Last Updated 5/19/05
DISCLAIMER:
DO NOT ATTEMPT THIS MODIFICATION IF YOU DO NOT HAVE THE
TECHNICAL EXPERTISE TO DO
SO SAFELY.
WE TAKE NO RESPONSIBILITY FOR THE RESULTS OF THIS MODIFICATION.
Introduction
The stock downpipe on your average first generation SOHC or DOHC Neon had it's good sides and bad sides. The plus is that the section that mates to the exhaust manifold is 2.5" in diameter. The bad is that it kinks into a 2.5" catalytic convertor that necks it down to 2.25", then has a big kink in it where the second O2 sensor bolts in. Here's a photo looking up into the stock downpipe with the O2 sensor removed.
(Click For Larger Image)
As you can see, it narrows down more
than a little where the O2 sensor bung is welded up. It's not
quite as bad as it looks since the exhaust "kinks" a bit, but it does
narrow down to perhaps 1-5/8" or so, with the O2 sensor sitting right
in the middle of the narrowest part. So the exhaust comes in at
2.5", squeezes down to 2.25" before exiting the cat, goes into a funky
kink, narrows down to 1-5/8", kinks again, then flows into
2.25" exhaust from there. Here's a picture of it from the outside.
(Click For Larger Image)
(Click For Larger Image)
You can see the nice kink, resembling
more of a "fold" in the pipe, likely put there mostly to make a flat
spot for
the O2 sensor bung to weld on easily.
Most people just get a long tube header and scrap the whole mess. Well, I'm cheap and don't have the money for a header right now, but I do have a 2.5" 45 degree mandrel bend, a 2.5" dynomax catback exhaust, and access to a welder. The 2.5" catback is pretty much useless considering what's upstream of it, especially on a car with only a few bolt-ons. By the way, the muffler included with the Dynomax cat-back is crap. The "dual tips" only really flows out of one tip, with the other one being nothing more than a hole hacked in the side of a standard dynomax turbo muffler; it's not even connected to the interior paths of the muffler. You end up looking silly driving around with one soot-blackened exhaust exhaust tip and one shiny one. Finally you you get embarrassed and spray the interior with black spray paint, and then feel like a riceboy for doing it! That, and it doesn't seem to flow much more than a stock DOHC muffler while being much louder at freeway cruising speeds. Buy the piping, don't buy the muffler, and put something better flowing on there. I've not done that yet, because I'm poor.
Anyway, I figured that since I had 2.5" exhaust from the cat back, and the exhaust is 2.5" until it goes into the cat, why can't I just chuck the whole mess in between using the stock flange and a couple inches of straight pipe to make my own downpipe? One without the restrictions of the stock unit, all for about $20 (excluding cost of 2.5" high-flow cat)? So my friend and I did one weekend, and this is the result.
Next, hack off the exhaust in the first straight section after the cat using a sawzall or similar tool. The stock downpipe will come out with a clang, so watch your head. Take the stock downpipe to a bench and cut the exhaust right before the first bend in the 2.5" pipe before the cat. Take out the second O2 sensor. If you got rid of the second sensor using an O2 simulator, whee, you just got a free spare O2 sensor. Toss the stock downpipe. If you want to hear how a Neon sounds with open exhaust, start it up and rev it. I'd recommend earplugs first though, since you'll go deaf and your neighbors will egg your house while you sleep.
On a workbench, take the 2.5" bend, some of the straight pipe, the stock exhaust flange, and one of the 2.5" couplers. Start with about 10" of straight pipe, slip-fit it all together, the coupler mating the straight piece with the stock flange, then the flared end of the bend to the straight pipe. Slide under the car and do a test fit, at first it'll rest against the k-frame and steering rack, the goal is to get a bit of clearance by cutting the straight section shorter. Note I'd recommend a solid bobble strut for this mod, since either way you look at it, clearance will be tight. You should be able to manager 1/2" of clearance if you cut it right, I ended up with more like 1/4". It doesn't hit the k-frame, since the motor doesn't move enough with the mounts I have, but with stock mounts it might.
When it's all cut properly it should look something like this, before you weld it up:
(Click For Larger Image)
Most people just get a long tube header and scrap the whole mess. Well, I'm cheap and don't have the money for a header right now, but I do have a 2.5" 45 degree mandrel bend, a 2.5" dynomax catback exhaust, and access to a welder. The 2.5" catback is pretty much useless considering what's upstream of it, especially on a car with only a few bolt-ons. By the way, the muffler included with the Dynomax cat-back is crap. The "dual tips" only really flows out of one tip, with the other one being nothing more than a hole hacked in the side of a standard dynomax turbo muffler; it's not even connected to the interior paths of the muffler. You end up looking silly driving around with one soot-blackened exhaust exhaust tip and one shiny one. Finally you you get embarrassed and spray the interior with black spray paint, and then feel like a riceboy for doing it! That, and it doesn't seem to flow much more than a stock DOHC muffler while being much louder at freeway cruising speeds. Buy the piping, don't buy the muffler, and put something better flowing on there. I've not done that yet, because I'm poor.
Anyway, I figured that since I had 2.5" exhaust from the cat back, and the exhaust is 2.5" until it goes into the cat, why can't I just chuck the whole mess in between using the stock flange and a couple inches of straight pipe to make my own downpipe? One without the restrictions of the stock unit, all for about $20 (excluding cost of 2.5" high-flow cat)? So my friend and I did one weekend, and this is the result.
Tools/Parts Needed
- 2.5", 45 degree mandrel bend. I bought mine at JC Whitney.
- 12" or so of straight 2.5" tubing.
- Sawzall
- Welder (MIG welder used here)
- Misc. Ratchets, sockets, 18-24" of extension for said rachet.
- O2 sensor bung if you're going to keep the second O2 sensor, otherwise an O2 Simulator
- 2.5" glasspack or spiralflow muffler, or 2.5" high flow cat (not required, but the car will sound nasty if you don't have one)
- If you have stock 2.25" exhaust, a 2.5" to 2.25" reducer
- 2-3 2.5" coupler pieces to fit the stock pieces together.
Fabrication
First up, you need to remove the stock downpipe. First, spray down the exhaust flange to exhaust manifold bolts with some kind of penetrating lube. If you still have the second O2 sensor connected, disconnect it. Get a friend to put an open end wrench on the "spring bolt" nut that holds the exhaust to the exhaust manifold, and climb under the car with a rachet, and a whole bunch of extensions. I used 18" worth, but 24" would be better. Air tools are nice here. It should pop off with no problem, my bolts had 80k on them and came off fine, but I don't live in a road-salt area either. If they're tight, spray 'em some more with lube and let it sit. Don't snap them off or you might have a hell of a time getting half the bolt out. Note that these are identical to the ones used on the old 2.2/2.5L turbo cars, so if you buy new ones, get the ones for a 1990+ Turbo Daytona. They have welded on tabs that will make it a lot easier to install since you don't need a wrench on the nut.Next, hack off the exhaust in the first straight section after the cat using a sawzall or similar tool. The stock downpipe will come out with a clang, so watch your head. Take the stock downpipe to a bench and cut the exhaust right before the first bend in the 2.5" pipe before the cat. Take out the second O2 sensor. If you got rid of the second sensor using an O2 simulator, whee, you just got a free spare O2 sensor. Toss the stock downpipe. If you want to hear how a Neon sounds with open exhaust, start it up and rev it. I'd recommend earplugs first though, since you'll go deaf and your neighbors will egg your house while you sleep.
On a workbench, take the 2.5" bend, some of the straight pipe, the stock exhaust flange, and one of the 2.5" couplers. Start with about 10" of straight pipe, slip-fit it all together, the coupler mating the straight piece with the stock flange, then the flared end of the bend to the straight pipe. Slide under the car and do a test fit, at first it'll rest against the k-frame and steering rack, the goal is to get a bit of clearance by cutting the straight section shorter. Note I'd recommend a solid bobble strut for this mod, since either way you look at it, clearance will be tight. You should be able to manager 1/2" of clearance if you cut it right, I ended up with more like 1/4". It doesn't hit the k-frame, since the motor doesn't move enough with the mounts I have, but with stock mounts it might.
When it's all cut properly it should look something like this, before you weld it up:
(Click For Larger Image)
Note I replaced the exhaust doughnut,
the stock one was looking a little worn around the edges. If
you're planning on keeping the second O2 sensor and the cat, you'll
want to weld in the cat here, then after it drill a hole and weld in an
O2 sensor bung. You'll probably have to extend the wiring of the
O2 sensor to do so. Have fun, I think an O2 sim is easier.
Next, weld the sucker up. If you don't have access to a welder, any welding shop, exhaust shop, etc. should be able to put a bead around it all for you. If they ask what it's for, tell them it's for an old Camaro or something so they don't think you're nuts and start lecturing you. If you don't have a welder and can afford one, buy one and learn to use it. It opens up a whole new world of fabrication options for you. Here's a shot of it welded up:
(Click For Larger Image)
Next, weld the sucker up. If you don't have access to a welder, any welding shop, exhaust shop, etc. should be able to put a bead around it all for you. If they ask what it's for, tell them it's for an old Camaro or something so they don't think you're nuts and start lecturing you. If you don't have a welder and can afford one, buy one and learn to use it. It opens up a whole new world of fabrication options for you. Here's a shot of it welded up:
(Click For Larger Image)
Don't make fun of our welds please, it
ain't no TIG welder, and we're not professionals. :) I'd
recommend adding a glasspack muffler of some kind after the downpipe if
you're not using a cat, otherwise it'll sound like you're a
Honda-lover-wannabee, complete with pissed off bumblebee
exhaust. A glasspack will take the rasp out of the exhaust note
without hurting flow too much. Note that removing the cat is
illegal for on-road use, <insert EPA loving statement
here>. Here's what I ended up with:
(Click For Larger Image)
(Click For Larger Image)
Get under the cat and mate it up with
the rest of the exhaust, using a reducer if you have the stock 2.25"
exhaust, otherwise using a coupler. It's a pain in the butt to
weld the downpipe to the rest of the exhaust since it's so close to the
body, and pretty much impossible to weld the top. We just ended
up tack-welding it to keep it in place and used a 2.5" stainless band
clamp to keep it from leaking. Works fine, with no ground or body
clearance issues. Here's some pictures of it installed:
(Click For Larger Image)
(Click For Larger Image)
(Click For Larger Image)
(Click For Larger Image)
Note the difference in the weld quality
from the weld closest to the exhaust manifold compared to the one
further down. The nicer weld was done using flux-core wire with
gas, the latter we ran out of gas and had to stick with just flux
core. There's no clearance issues and you keep the stock exhaust
doughnut, letting the engine move without needing a flex-pipe like you
do with a header. Not that it's as good as a header; this will
never come close to a good long-tube header, but it's a lot cheaper if
you're poor like me. Eventually I'll install a header, but this
at least makes the exhaust one consistant size and gets rid of the
major restrictions in the stock downpipe, a concern for me considering
the mods I have coming up that have been sitting in my garage way too
long.
Overall, was it worth it? Yes, it was. I picked up some good midrange and top end power, with a near-undetectable loss in low end torque below 2000RPM. It's probably a bit better than swapping the stock SOHC muffler for a DOHC unit, so I'd put it in range of a 4-5HP bump, all above 4000RPM. Note that your exhaust will get slightly louder, though not at idle. I've only really noticed it being louder while cruising in fifth at half throttle or more while doing things like climbing a hill at 65mph. I think I'd feel it more if I didn't have the crappy dynomax turbo muffler on it, eventually I'll go to something more free-flowing. Is it worth the 20 bucks in exhaust pipe and the welding wire? Definitely. Otherwise, I'd spend the money and go for a long-tube header.
Also, it will likely be felt more with a car with more mods, currently the car has just an iceman, 55mm TB, and MP computer. Once I put my ported head with the oversize valves and crane #12 cam on, it will be more useful with the increased exhaust flow. There are better mods to do on a near-stock Neon before you get to this.
Conclusion
Overall, was it worth it? Yes, it was. I picked up some good midrange and top end power, with a near-undetectable loss in low end torque below 2000RPM. It's probably a bit better than swapping the stock SOHC muffler for a DOHC unit, so I'd put it in range of a 4-5HP bump, all above 4000RPM. Note that your exhaust will get slightly louder, though not at idle. I've only really noticed it being louder while cruising in fifth at half throttle or more while doing things like climbing a hill at 65mph. I think I'd feel it more if I didn't have the crappy dynomax turbo muffler on it, eventually I'll go to something more free-flowing. Is it worth the 20 bucks in exhaust pipe and the welding wire? Definitely. Otherwise, I'd spend the money and go for a long-tube header.
Also, it will likely be felt more with a car with more mods, currently the car has just an iceman, 55mm TB, and MP computer. Once I put my ported head with the oversize valves and crane #12 cam on, it will be more useful with the increased exhaust flow. There are better mods to do on a near-stock Neon before you get to this.
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