What stainless steel grade should a performance exhaust be?

304 is the industry-standard minimum for a performance exhaust. 321 is the engineering-correct grade for turbo manifolds and high-heat sections. 409 will develop surface rust — never sell it as a premium system — and 201 is the bait-and-switch grade to watch for.

What grade of titanium are aftermarket exhausts made from?

Most real aftermarket titanium exhausts are commercially pure titanium — Grade 1 or Grade 2. Grade 5 (Ti-6Al-4V) is roughly twice as strong but harder to form into tubing; a bargain-priced 'full Grade 5 titanium exhaust' is a red flag. Always ask which grade and get it in writing.

Is Inconel 625 or 718 better for exhausts?

625 is the exhaust alloy — it handles ~982°C service temperatures and welds well. 718 is stronger on paper but its strength falls off above ~650–700°C and it's built for aerospace hardware, not exhaust gas. 'Inconel' with no grade named is a marketing word, not a spec.

Exhaust Metallurgy: 304 vs Titanium vs Inconel

The difference between a $400 exhaust and a $1,400 exhaust is usually three things: the metal, the welds, and the design. This guide covers the metal — so you can read a supplier spec sheet like an engineer and ask the questions that expose a bad deal.

Stainless steel — 304, 316, 409, 201, and 321

Not all "stainless" is stainless. Here's the lineup:

Polished stainless steel dual-muffler cat-back exhaust with visible TIG weld beads at the pipe joints — A polished stainless-steel dual-muffler cat-back. Look for TIG weld beads at the pipe joints — the sign of real fabrication. Real supplier-listing photo.

304 — the industry standard. 18% chromium, 8% nickel. Corrosion-resistant, welds well, sounds right. The minimum acceptable grade for any performance exhaust worth selling. If a supplier won't confirm 304 in writing, you're done.
316 — marine grade. Adds molybdenum for extreme corrosion resistance. Better than 304 on paper, overkill for most exhausts — but premium if offered at a sane price.
409 — the budget grade. Cheaper, and it WILL develop surface rust. Fine for an OEM replacement on a budget car. Never sell it as a premium system.
201 — the bait-and-switch grade. The lowest tier. Rusts fast. This is what shows up when a supplier quotes 304 and ships something cheaper. Your contract should name 304 explicitly.
321 — the turbo-manifold answer. Standard 304 can suffer weld-zone cracking at sustained turbo-manifold temperatures. 321 adds titanium, which grabs the carbon first and leaves the chromium free to protect the metal. For turbo manifolds and high-heat exhaust sections, 321 is the engineering-correct grade — 304 is acceptable, 321 is right.

The magnet test (rough field check): 304 and 316 are only weakly magnetic. 409 is strongly magnetic. If your "304" exhaust grabs a magnet hard, ask questions.

Titanium — grades, myths, and the questions that matter

Titanium exhausts are where the most money gets made — and the most nonsense gets sold.

Titanium exhaust system with burnt-blue and purple heat coloring and close-up pipe weld detail insets — Burnt-blue heat coloring is the honest look of real titanium. But color alone doesn't tell you Grade 1, Grade 2, or Grade 5 — ask for the grade in writing. Supplier watermark kept as-is.

Grade 2 is the honest standard. Most real aftermarket titanium exhausts are commercially pure titanium — Grade 1 or Grade 2. Good balance of formability, weldability, corrosion resistance, and cost. (Grade 1: tensile ~240 MPa minimum, excellent formability — confirmed against ASTM B265 datasheets.)

Grade 5 (Ti-6Al-4V) is the alloy everyone names — roughly twice the strength of Grade 2 (yield ~880 MPa / 128 ksi), but harder to form and weld into tubing. It shows up in fasteners and high-stress hardware far more than in full exhaust systems. Rule of thumb: a "full Grade 5 titanium exhaust" at a bargain price is a red flag.

Why titanium exhausts crack (the honest version): thin walls — 30–40% thinner than steel equivalents — plus high notch sensitivity. A rock nick propagates into a crack faster than steel. Every weld needs back-purging (inert gas inside AND outside) or it embrittles, and most local shops can't do it properly. Thin walls also mean more resonance — the exotic sound comes with drone.

The myth to kill: "titanium makes more power." It doesn't — not versus an identical-design stainless exhaust. GM's own titanium-exhaust engineer said the reason for titanium was mass reduction and power-to-weight, not engine power. The engine cares about exhaust DESIGN — diameter, scavenging, routing — not the material. What titanium gives you is roughly 40% weight savings versus stainless. That's the whole advantage. For a street car, a quality 304 system gives you nearly the whole experience at a fraction of the cost — and it won't crack on a pothole.

Inconel — 625 vs 718

When a listing says "Inconel exhaust" with no grade, that's a marketing word, not a spec. Two grades matter:

625 is the exhaust alloy. Nickel-chromium superalloy, service up to ~982°C (1800°F), tensile ~827 MPa. Handles extreme exhaust temperatures without degrading, welds well, no post-weld heat treatment needed. This is what high-end Inconel exhausts are made from.
718 is NOT the exhaust alloy. Stronger on paper (~1375 MPa tensile), but its strength falls off above ~650–700°C — and it's built for turbine blades and aerospace hardware, not exhaust gas. Harder to weld properly than 625.

Titanium vs Inconel, honestly: titanium wins on weight (about 40% lighter than stainless; Inconel is dense — heavier than steel). Inconel wins on heat. For a street car, both are luxury. 304/321 stainless is the rational buy.

Turbocharger basics — read the spec sheet

Twin-scroll hybrid turbocharger with OEM-style housings and visible electronic wastegate actuator — A twin-scroll hybrid turbocharger with electronic wastegate actuator visible. PWG vs EWG is a fitment item — confirm it before quoting. Supplier watermark kept as-is.

Hybrid turbos: OEM housings machined to accept larger compressor and turbine wheels. Keeps factory bolt-on fitment while flowing more air. The sweet spot for street builds.
Compressor wheel sizing: inducer (air inlet diameter) and exducer (base diameter). Bigger inducer = more max power. Bigger exducer = higher boost pressure. 11-blade spools faster with lower peak flow; 9-blade is slower to spool with higher top-end.
Turbine housing A/R: smaller A/R spools faster but chokes at high RPM; larger A/R breathes better up top. Twin-scroll separates exhaust pulses for faster spool versus single-scroll.
Wastegates: electronic (EWG) vs pneumatic (PWG) — a FITMENT item. A 2011 135i with the PWG N55 takes a different turbo than the EWG version. Always confirm which one before quoting.
Connections: T3, T4, or V-band (flangeless clamp — the preferred high-end choice).

Supplier scripts — copy, paste, send

Stainless grade lock-in:
"Confirming the exhaust system is full T304 stainless — not 409, not 201. Please confirm the exact grade in writing before we proceed. If any section uses a different grade, list which sections and which grades."

Titanium grade check:
"You list this as a titanium exhaust — which grade? Grade 2 or Grade 5? Please confirm in writing, and confirm the wall thickness."

Turbo manifold grade:
"Is the manifold 304 or 321 stainless? 321 is our spec for the hot side — please confirm."

Inconel check:
"You list Inconel — which grade, 625 or 718? Please confirm in writing."

Turbo fitment:
"Confirming fitment: [year] [make] [model], engine [N55/B58/etc.], wastegate type [PWG/EWG]. Is this a direct bolt-on for the factory manifold?"

Red flags

"Titanium" with no grade. See above.
"Inconel" with no grade. See above.
"Stainless steel" with no number. 201 ships when 304 was quoted. Name it.
Grade confirmed verbally but not in the Alibaba chat. If it's not written, it didn't happen.
"Grade 5 full titanium exhaust" at a bargain price. Physics has a price. Verify before you repeat the claim.

See the verified exhaust suppliers → partsplugvault.com