Mandrel bends maintains the same ID all the way through the bends.
Crush bends are just like they sound; the piping is crushed slightly while bending,
decreasing the ID of the pipe.
Mandrel bent is the preferred method, but requires special
equipment. Virtually all of your local muffler shops can only crush bend.
A truly excellent exhaust can make a big difference to performance.
You want to maintain velocity of the exhaust gases with as little restriction as possible.
The reason is the scavenging effect on your engine. What this means is that with an
excellent system, each exhaust pulse, as it races through the piping, leaves a slight
vacuum behind it to help suck the exhaust out of the next cylinder when the exhaust valve
opens. This helps make your engine more efficient. You want to pump in all of the air you
can, and pump all of the exhaust out. This is especially important at high rpm, when
things are happening more quickly. My understanding is F1 engines are so efficient they
can pump *more* (slightly) than 3.0 L through their NA 3.0 engines.
Now, as you introduce restrictions to the exhaust, you begin to
reduce the scavenging effect because you don't maintain the same velocity through the
system. That is why mandrel bends are preferred.
Think of your exhaust system as freeway. The larger your exhaust
system, the more lanes in the freeway for traffic (or exhaust) to travel. Now lets say
your exhaust system is two lanes of a 4 lane freeway. With a mandrel-bent exhaust, you
maintain both lanes as you go around a bend. With a crush-bent exhaust your freeway goes
from 2 lanes on the straights to 1 lane on the bends. Not very good, eh? Both lanes of
traffic have to squish into one to go around the bend. Traffic already has to slow down to
go around the bend, but having to go down to one lane too slows it down further. This
isn't much of a problem when traffic is light, but traffic is heavy (i.e. your foot is
down on the throttle and rpms are up) things start to back up before you reach the bend.
In this case, though, tempers don't flare and horns don't honk - you just lose
performance.
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