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Ok just having a read...

Can someone clear a couple things up for me....

CPSI = cells.... so 100, 200 or 400. 100 flows alot more then the 400? correct?

I have a 3inch front pipe and a 3inch titanium cat back. so for best flow for mine a 4inch cat with 3 inch flanges, with 100 cells is the way to go?

CPSI, cells per square inch.

400 CPSI, means for every square inch, there is 400 cells. Which means for every square inch, air has to flow through those 400 cells. On the other hand 100 CPSI, for every square inch there will only be 100 cells. 400 CPSI will have tiny little cells which is where the restriction will be, 100 CPSI will have cells that are 4x bigger in size than a 400 cell cat. Picture it like 400 CPSI cat breathing through a straw, and 100 CPSI cat breathing through a garden hose.

Ok just having a read...

Can someone clear a couple things up for me....

CPSI = cells.... so 100, 200 or 400. 100 flows alot more then the 400? correct?

I have a 3inch front pipe and a 3inch titanium cat back. so for best flow for mine a 4inch cat with 3 inch flanges, with 100 cells is the way to go?

Correct about the cells, obviously the less cells there are, the bigger each cell will be so the more the cat will flow. With regards to the size, you want to go with the biggest body you can, whilst making sure that the inlets and outlets are the same size if not slightly bigger than the rest of the exhaust, ie. If your 3" front/cat-backs are 77-78mm, you don't want a cat that has 76mm inlet/outlets. But if your front/cat-backs are 76mm then a cat with 77-78mm inlet/outlets would be perfect.

As Wolverine said, get a Venom cat. I'm using one and my car only made 5rwhp more when run without any exhaust, so it flows well and they aren't expensive http://www.ebay.com.au/itm/3-VENOM-High-Flow-Metal-Cat-WRX-XR6-Turbo-Skyline-VL-/250768882992?pt=AU_Car_Parts_Accessories&hash=item3a62fd7d30

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