jobambo

"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

y8s

iTrader: (8)

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

Stein

iTrader: (46)

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.

ThePass

iTrader: (3)

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

saitrix

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.

Laur3ns

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.

saitrix

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?

Laur3ns

@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.

gompers

iTrader: (4)

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

y8s

iTrader: (8)

no, it flows less than that.

94mx5red

iTrader: (14)

Thanks again for that Tony

TurboTim

iTrader: (9)

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.

ZX-Tex

iTrader: (15)

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.