Originally Posted by kenbuck
...a regular bender, makes wrinkles when it bends the pipe, reducing the diameter and smoothness of the bend and hurting the flow. ...i dont know of any after-market exhaust company that sells anything "crush-bent."
That may have some truth for race cars, with extremely high-output engines, but it's certainly NOT true for an I6 Grand Chero.
I have the advantage over you here: I have built exhaust systems and tested them on real-world vehicles, on the road, on a chassis dyno, and on an exhaust system flow bench.
The differences I saw between a reasonably well-made "open-bend" system ("open-bend" being the correct technical description for the common non-mandrel-bend exhaust pipe) and a mandrel-bent system is less than the test-to-test variability, less than 2% for most common automobiles.
That means less than plus OR MINUS 4 hp on a WJ 6-cylinder; having done extensive dyno testing on production engines, I defy you accurately measure a difference that small, and I know you'd never detect it in any real-world operation.
To prove this point, I developed an open-bend system for GM for the LT-5 "ZR-1" Corvette in 1992; it tested exactly the same as the mandrel-bend system that GM had in production at that time on a 475 HP Lingenfelter-modified car.
Most of the backpressure in production exhaust systems comes not from frictional resistance in the pipes, but from what for want of a better word I call "disturbance" resistance at the places where a pipe joins another pipe of different diameter, whether larger or smaller, and where it joins to mufflers.
For example, the worst thing you can do for flow resistance is having say a 2.5" pipe join a muffler with 2.25" inlets or outlets - or have a 2" diameter passage tube in a muffler that has 2.5" inlet and outlet nipples.
Mandrel-bending makes beautiful pipes, no question about it, but it also results in a great deal of thinning of the pipe on the outside of the bend, so it is usually not as durable, all other things (pipe material, wall thickness, pipe diameter) being equal. The thinning on the outside of the bend is occurring in part because wrinkling is NOT occurring on the inside of the bend.
And it also results in rather expensive pipes.
Now, if spending money is your objective, by all means go right ahead, but if a hotrod WJ is what you really have to have, that 6-cylinder engine just isn't going to do it - not by changing the exhaust pipe anyway.
Wanna go fast?
Trade it in on a V8, and you'll instantly have something that'll outrun all but a race-built 6-cylinder.
As to aftermarket companies not producing open-bend pipes: that's hogwash, pure and simple. The big aftermarket exhaust commpanies, including Walker (tenneco), Maremont (AP), and Arvin all make open-bends on virtually every pipe they make, as do all the big muffler chains like Midas. The reason?
Tooling for a mandrel bend is very specific to not only the pipe diameter and the bend radius, but also the pipe wall thickness, and to a lesser extent, the material used. This means the guy doing mandrel bends is going to compromise somwhere, either in pipe diameter or wall thickness, or he's going to tool for only a very few specific models.
Save your money and put it towards a good set of tires, and regular thorough maintenance. Your Grand will last much longer, and you'll like it much better.