Bending a tube is not a neutral operation. The outer wall of the curve is stretched and thinned, while the inner wall is compressed and pushed inward. Every bending method is essentially a different answer to the same problem: how to move material around a radius without letting the inside of the curve flatten.
The section that matters most is the one through the middle of the bend. An exhaust pipe flows only as well as its tightest restriction, and on most cars that restriction sits at a bend rather than on a straight. Two pipes of the same diameter can therefore behave quite differently once they have been shaped.
In mandrel bending a flexible mandrel is drawn through the inside of the tube and held at the point where the bend forms. The mandrel supports the inner wall from within while the tooling pulls the tube around the radius. The curve that comes out keeps a nearly circular cross-section, and the wall thinning on the outside of the bend stays modest.
That support is what makes the process slower and more demanding. Tooling has to match the tube diameter, the wall thickness and the radius, and the mandrel has to be set accurately for every bend. On a titanium exhaust system the extra effort is usually judged worthwhile, because thin-wall titanium has less material to spare when a wall starts to stretch.
Crush bending forms the curve by pulling the tube around a die with nothing inside it. The outer wall still stretches, but the inner wall has nothing holding it back, so it folds or flattens. The bend is fast to produce and needs simpler tooling.
The cost is paid in flow. A flattened bend reduces the effective area at exactly the point where the gas is already changing direction, and the result is a step that the flow has to negotiate. On a mild steel system running at low output this may hardly matter. On a performance system working near its limit, it usually does.
The difference is easy to check on a finished part. Slide a caliper or a set of gauges across the bend section and compare it with the straight. A mandrel bend will read close to the original diameter in every direction, while a crushed bend will read noticeably narrower across one axis and wider across the other.
You can also run a soft plug through the inside. A mandrel bend feels uniform; a crushed bend has a crease or a narrowing you can catch. On thin-wall material that crease is also a stress point, so it is a place where cracks tend to begin after a few heat cycles.
Crush bending still has a place. It suits low-pressure ducting, cosmetic runs where the pipe is mostly straight, and applications where the bend is gentle enough that the wall barely moves. It also suits production where speed matters more than the last few percent of flow.
Mandrel bending belongs wherever the pipe is doing real work: primary exhaust runs, downpipes, charge pipes and any bend that sits close to the throttle body or a turbo. If you are choosing between the two for a system you intend to keep, the deciding question is whether you can afford a restriction at that particular point.
Material behaviour changes the calculation. Titanium and thin stainless both work-harden and both have limited material to give, so they tolerate a collapsed inner wall badly. Heavier mild steel can absorb a crush bend with less drama, which is one reason the two processes co-exist in the same workshop.
The practical approach is to decide the radius and the material first, then pick the process that can hold the section at that radius. If the tooling cannot support the inside of the bend, the honest choice is either a larger radius or a different method, not a hope that the flow will not notice.
