Wondering if anyone in here has any experience designing/building headers/extractors/exhaust systems.
Ive got a 2nd hand set of extractors to copy for a 4cyl TR magna (motivator MPH 641) with:
-10mm thick flanges
-4-2-1 design
-O2 sensor on the middle 2 cylinders at the 2->1 piece
-39mm inner diameter tube, 52mm ID at the flange to the flex.
-press-bent, appears to be 2mm wall thickness
-no heat shield mount points, appears to be un-coated

The outside cylinders (no O2 sensor at join) are welded into a joiner on the block side, and have definite cracks halfway around, and it looks like there might be cracks starting on other joins too, and the welds look pretty bad.

I measured each pipe length along the middle of each pipe (painted and scraped a ruler across to get a center line to measure down) and from end to end of each pipe, flange thickness included, got 654, 685, 701, and 673mm.

The flanges bolt holes aren't a straight through hole, but more of a cut-out, or a keyhole shape cut in from the side, so more of a notch than a hole. I'm not sure if this was for ease of manufacture if they were CNC flame/plasma cut, or if its for special thermal expansion properties?

Questions:
1- Which material to use? Stainless or steel? Pros and cons? Stainless is more expensive, but i can do much much nicer welds in stainless than i can with mild steel, TIG welded, with gas inside to stop the metal burning up/going black inside the pipe as i weld. I don't have any experience with titanium, but id imagine its too expensive. Has aluminium ever been used in an exhaust, after the cat or middle muffler for example? How far back does the temp. stay above 400ish degrees?

2- Overall design: How should i work out which lengths are best to use, pipe diameters, 4-1 or 4-2-1 type, I've read that 4-1 gives high horsepower and short lengths give high rpm, but also heard a opposites of a lot of things.

3- Joining 2 pipes into one, or 4 into one, how good are the joiners that look like a piece of squashed pipe compared to cutting pipes at angles and getting a smoother flow perhaps?

4- O2 sensor position, does it matter if its placed on one cylinder, after 2 join up, or after all 4 are together, how far from the engine for temperatures, etc, and can you buy O2 sensors with a longer wire or should i try to reposition the plug for it if i need to get it further back in the system?

5- bends: Ive heard its fine to use a bit larger pipe in a press bend to get the flow of a mandrel bend of the smaller diameter due to the squashing effect, but this was only a cat-back project. Does the same rules apply to extractors, or is it mandrel bends until the cat? How good, or bad, would notching out V shapes and welding them back up to create bends be? It would be a lot of welding, but i could create some nice compound curves, a lot of headaches, and it would fix my not having access to a bender problem.

6- Flanges: Is drilled bolt holes fine for them, or do i need to notch them out to the edge, and in what direction is it best to notch? Is a stock surface finish fine, or should it be machined or surface ground? Is there any advantage of going thicker than a 10mm flange anywhere?

7- Coating: Paint- probably not. Chromed? Heat wrap tape/glass cloth? Ceramic coat? And how does ceramic coating affect threaded holes, are they blanked off before coating or made oversize so the coating brings them back to the right size? I was thinking of adding heat shield mount points, and still need to be able to screw in an O2 sensor as well.

8- Flex join between extractors and cat- will i need to get a larger diameter one? Is there any special instructions for welding them in? I'll probably be doing the rest of the exhaust too so I'm guessing i can move it and the cat as far back as i want if i need more length before the pipes meet up?

Bonus points- The rest of the exhaust system, any tips, anything i should do? What types of muffler are out there and how many do i need? Do many aftermarket systems still use a middle muffler?

Sorry for the long post, I'll edit and add anything i forgot when i remember, and while i might not use every bit of top performance information right now, ill probably go for a bit more performance with a bigger budget when i go to a v6 2nd gen :P

You could probably pick up a set of 4G54 aftermarket extractors from the wreckers for like $50. Hardly worth making your own.

I believe if you contact ARS55 on the forums here, he has a friend who makes custom manifolds and all that. I have seen photo's of his work on the 2.6 making turbo manifolds so if you get incontact with him, he may have all the answers you require as I was very impressed by what I saw in his design methods.

Ben

Wondering if anyone in here has any experience designing/building headers/extractors/exhaust systems.
Ive got a 2nd hand set of extractors to copy for a 4cyl TR magna (motivator MPH 641) with:
-10mm thick flanges
-4-2-1 design
-O2 sensor on the middle 2 cylinders at the 2->1 piece
-39mm inner diameter tube, 52mm ID at the flange to the flex.
-press-bent, appears to be 2mm wall thickness
-no heat shield mount points, appears to be un-coated

The outside cylinders (no O2 sensor at join) are welded into a joiner on the block side, and have definite cracks halfway around, and it looks like there might be cracks starting on other joins too, and the welds look pretty bad.

I measured each pipe length along the middle of each pipe (painted and scraped a ruler across to get a center line to measure down) and from end to end of each pipe, flange thickness included, got 654, 685, 701, and 673mm.

The flanges bolt holes aren't a straight through hole, but more of a cut-out, or a keyhole shape cut in from the side, so more of a notch than a hole. I'm not sure if this was for ease of manufacture if they were CNC flame/plasma cut, or if its for special thermal expansion properties?

Questions:
1- Which material to use? Stainless or steel? Pros and cons? Stainless is more expensive, but i can do much much nicer welds in stainless than i can with mild steel, TIG welded, with gas inside to stop the metal burning up/going black inside the pipe as i weld. I don't have any experience with titanium, but id imagine its too expensive. Has aluminium ever been used in an exhaust, after the cat or middle muffler for example? How far back does the temp. stay above 400ish degrees? Use steel. Stainless isnt really worth it, and is more prone to cracking. Any coating you put on stainless will flake off earlier then steel due to expansion. the flange thickness is all about keeping it flat faced when heated. if you can weld the primaries on without warping 6 or 8mm plate use it, it will be cheaper but you may need someon with a plasma cutter to cut the flange as most places only pruduce 10mm flangeplates. Al will be ok for a catback, i had one for a period on my 180sx. While temps may get over 400 around the cat converter, the fact is the Al is exposed to air, and while driving has a lot of air to cool it down. Its good thermal conductence means it heats up easily, but also spreads the heat quickly, and then can cool down quickly (why its such a PITA to weld)

2- Overall design: How should i work out which lengths are best to use, pipe diameters, 4-1 or 4-2-1 type, I've read that 4-1 gives high horsepower and short lengths give high rpm, but also heard a opposites of a lot of things. Length and runner style are less important then size really. Its something you do when you are chasing the last 10th from all possible performance from the modification. Easiest solution is to make the ID of the pipe the same as the id of the exhuast port. Use 2mm wall steel. WHat ive read is 4-1 are horspower headers, and the length of the primayrs is a reflection of what rpm range you want to make peak power (longer primarys = higher rpm HP) and 4-2-1 headers are designed to create more low down torque, while trying to avoid high end losses. Either design will be better then stock.

3- Joining 2 pipes into one, or 4 into one, how good are the joiners that look like a piece of squashed pipe compared to cutting pipes at angles and getting a smoother flow perhaps? Unless the car is heavily worked, this amount of detail is not relavant for your 4g54. The squashed pipe 4-1 joiners are good, and still require a bit of fab to mate up with the primarys. THe only downside is getting the size and length you want may be hard.

4- O2 sensor position, does it matter if its placed on one cylinder, after 2 join up, or after all 4 are together, how far from the engine for temperatures, etc, and can you buy O2 sensors with a longer wire or should i try to reposition the plug for it if i need to get it further back in the system? personally iw ould put it after all primarys mate up, wther it be 4-1 or 42-1 design. as i see it, if 1 cylinder goes rich the one you use for your 02 sensing (say stuck open injector) the ecu will wind back fuel to ALL injectors, and this could be a bad thing as it will lean the rest out. But if you put the sensor after all 4 primarys the gasses from the other 3 cylinders wil dilute the emissions and although richer then standard, the ecu wont be inclined to wind back fuelling as much.

5- bends: Ive heard its fine to use a bit larger pipe in a press bend to get the flow of a mandrel bend of the smaller diameter due to the squashing effect, but this was only a cat-back project. Does the same rules apply to extractors, or is it mandrel bends until the cat? How good, or bad, would notching out V shapes and welding them back up to create bends be? It would be a lot of welding, but i could create some nice compound curves, a lot of headaches, and it would fix my not having access to a bender problem. you can buy doughnuts from a lot of places now. ANy wall thickness, any pipe ID. Use these and a mitre to cut to required angles, and then just join with straight section. Its what i do.

6- Flanges: Is drilled bolt holes fine for them, or do i need to notch them out to the edge, and in what direction is it best to notch? Is a stock surface finish fine, or should it be machined or surface ground? Is there any advantage of going thicker than a 10mm flange anywhere? See 1st reply for flange question. generally the notching of the bolt holes is a fitment thing. Its easier to locate on the studs and they dont have to be as accurate when cuttin gthe flange.

7- Coating: Paint- probably not. Chromed? Heat wrap tape/glass cloth? Ceramic coat? And how does ceramic coating affect threaded holes, are they blanked off before coating or made oversize so the coating brings them back to the right size? I was thinking of adding heat shield mount points, and still need to be able to screw in an O2 sensor as well. if i got steel i would just paint them. But of the options, ceramic coating is the only one with benefits other then asthetics.

8- Flex join between extractors and cat- will i need to get a larger diameter one? Is there any special instructions for welding them in? I'll probably be doing the rest of the exhaust too so I'm guessing i can move it and the cat as far back as i want if i need more length before the pipes meet up? the ID of the flexi should be the same as the pipe you are using. You weld them in just like pipe. THe further the cat goes back the longer it will take to work efficently, and the more like it will be to block sooner from carbon build up. to close on the other hand will burn it out very quickly

Bonus points- The rest of the exhaust system, any tips, anything i should do? What types of muffler are out there and how many do i need? Do many aftermarket systems still use a middle muffler? depending ho w good you are, when you have to put pipe close to body work, say around the rear axel of a 2ng gen, wrap it in fibreglass cloth, not for heat, but because it cushions and light tapping on the body work or suspeion which will drive you insane. Most off the shelf systems use 2 mufflers, one before the axle the other at the back. As long as you dont use cannons noise shouldnt be an issue without a middle muffler.

Sorry for the long post, I'll edit and add anything i forgot when i remember, and while i might not use every bit of top performance information right now, ill probably go for a bit more performance with a bigger budget when i go to a v6 2nd gen :P

As above