"Truth-If you have 3" tubing that is press bent it will smash 1/2 inch at
the center of the bend. At that point you have 21/2" flow instead of
3". It doesn't matter if you have one bend or five bends you still have
21/2" flow."

source Mandrel Myths-Mandrel Bending vs Pressure Bending

 

only crush bent pipe flows worse than 2-1/2 because the flow restriction increases as area changes occur.

 

It's already too late as I already have this midpipe/cat/resonator setup on my car. Anyone know if these cats are any good? http://www.flyinmiata.com/index.php?...umber=06-58200 doesn't say much about them in the description. I figured I couldn't go wrong for $100.

 

Make absolutely sure you get a cat that is the lowest restriction possible, or have a test pipe. You could have a 4" exhaust and if you have a tight-celled cat it won't help one bit.
I had a 2.25" cat-back exhaust with a high-flow cat and when I went from the stock greddy downpipe to the BEGI downpipe spool went from 4600 to 3900.
Then, I yanked the whole cat and exhaust and did a 3" cat with a 3" glasspack on the end of it and then it dumps by the rear axle. You'd think that would flow super well - no bends, only ~ 3ft. of 3" piping after the cat...
Wrong, the cat I used was a cheaper 'catco' one. Now it spools at 4500. I guarantee when I do give this cat the ol' hammer and chisel port job, spool will drop well below 4k.
Moral of the story - anything after the cat is wasted money/effort if the cat restricts it, and no matter how free flowing the rest of it is, the motor can't flow any better than the cat allows. Get a good 200 cell cat or a testpipe.
-Ryan

 

If you run a small turbo, say like the 2560, is it still advisable to get a full 3" system? I was thinking about building a downpipe (not sure what diameter yet), then just hooking it up to a 2.5" decat pipe and then the cat-back system I am using at the moment is 2.25". Does running a 2.25" backbox render the earlier larger exhaust parts useless? Was just thinking about doing that setup to get me going quicker.

 

I have a 2560 an spool went down from 4100rpm to 3300rpm with the OEM to 3" upgrade. That is with a high flow 3" cat.

 

That's awesome, I wonder how much lower it would go with a 3" decat. By the looks of it, it will be worth the extra effort making a full 3" system. Out of interest are you running EBC?

 

@saitrix: I was running EBC with a 7psi wg actuator can and it sucked due to cold weather overboosting. Then I ran MBC which was ok and not run a 14psi wg actuator can and nog mbc/ebc at all.

 

AKA: A bottleneck is a bottleneck. The exhaust only flows as much as the most restricted point.

 

no, it flows less than that.

 

Thanks again for that Tony

 

Even though they may have mandrel bent sections, any muffler shop will still crush bend when necessary. They will not spend the time on a chop saw getting the perfect angle when they can just tweak it on their hydraulic crush bender.

You get what you pay for; a $180 exhaust is probably fine for you. Chinese price exhaust for the chinese turbo.

 

FWIW I ran a 2.5" crush-bent exhaust with a metallic cat and a straight through magnaflow. It still worked much better than the stock exhaust and was really not much louder if at all. The spool improvement was obvious. So a 3" even with some crush-bent sections will still be much better than your stock exhaust, and at $180 that is damn cheap so I say go for it.

But to reinforce what others say above, an all-mandrel bent 3" would probably still be an improvement over that. However it will be louder, and a lot more expensive. I have a 3" enthuza now, but only because I bought it used at a good price.