Is there a major performance hit with 3" downpipes with 2.5" flanges?

[Lord Khan]

I will be ordering a downpipe next week. I can either order this one: http://www.machv.com/busracracdow.html which is a 3" straight through, but need a 3" slip-on cat with it, http://www.machv.com/busrachigcat.html

or I can get this one:

http://www.dsmparts.com/customer/product.php?productid=446&cat=92&page=1 which is also 3", but tapers to 2.5 inch at the cat side flange.
I was thinking of getting one of these cheap Ebay cats to go with it: http://search.ebay.com/eclipse-cat_W0QQfkrZ1QQfromZR8
Are these even real Cat Converters, or are they straight pipes?


My main question is, for the price difference, is there that much of a performance decrease with dp's with 2.5" tapering sections for the flange, compared to the straight 3" ones? I want good performance, but I'm trying to save a few bucks if I can.

 

[nickrule99]

Those pipes on eBay are for stock replacements, meaning the inlets are only 2" or 2.25". So they would be of no use to you. Also, one says "For competition vehicles use only, not legal for pollution controlled vehicles used on public highway." So I guess it looks like a cat, is advertised as a cat, but doesnt do anything.

Anyways, with the bigT28, you could get away with a 2.5" dp, and then get a highflow cat with a 2.5" flange on one side and 3" on the other to mount to your 3" catback, which is the ideal way to save a buck.

You could get that 3" one that tapers to 2.5", but that 2.5" senction will negate any gains you had before it. So, you'd be better off with eithe r a full 3" setup or do all 2.5", which would be cheaper.

-Nick

 

[Lord Khan]

Actually, the Authority dp has the 2.5 inch taper, and my Thermal cat also has the taper to fit the stock type of cat, so that would actually fit easily with one of those cheap Ebay cats. I think that for the price, I might as well try them out. I can always get the dp, and my catback tapered sections cut, and put on a slip-style 3" catalytic converter. What do you think?

 

[Lord Khan]

Oh well, looks like my hopes and dreams were just shot down. I just got off the phone with DSMparts.com and they said that the Authority dp is a cat eliminator, and won't mate to my Thermal Catback flange properly. Looks like I might have to go with the WAY MORE expensive 3" straight setups anyway.

 

[nickrule99]

What about taking your catback to a shot and having the flange changed? Id assume you will have to anyways, since its bolted up to a stock cat, so the flange is too small even for a 2.5" Cat.