85_305

http://www.jegs.com/cgi-bin/ncommerc...92&prmenbr=361

Should I get the one made for the 5.0 f-body, or should I just get the basic
2.5'' In/Out, 13.25'' Long for $54?


My exhaust is 3", will having the cat 2.5" in/out really kink me up, or will it be un-noticeable?
Thanx all,
Matt

85_305

And btw.. is the stock cat oval or round (I mean the outlet/inlets)?

doug791

you should keep whatever piece of crap cat you have on your car for e-testing and remove it and put on cat covers for the rest of the time

krow

Do what I did. I found a county that dose not do emissions testing and take out the cat. You will gain about 20 to 25% more horse power. If you can't, go with one that is 3" in/out otherwise it will not flow out the way it should.

CamaroJosh

There is no way i can gain 40-50 horsepower just by removing my cat.... I thought it was only 10 mabye 15 hp boost

V8Rumble

Yeah, 25% is just a bit... "optimistic" I'd say...

Depends on the engine, but if your cat's in decent shape, it's not going to cost much HP at all. As a matter of fact, I just replaced the cat in my V-8 powered truck about 4 or 5 weeks ago because it was obviously clogged, & didn't notice a huge difference afterward - and I replaced the stock unit with a Catco "high-flow" unit too! (BTW, this is the 2nd one of those I've put on one of my cars, & they seem to work fairly decently {from what I can tell} without costing a whole lot...)

Good luck.

85_305

Well it's not so much the emmsions testing I'm worried about; it the fact that the local exhaust shops cant LEGALLY weld up anything exhaust-wise if yoru car is supposed to have a cat, but doesn't.
BUT.. since my exhaust is 3", going w/ a 2.5" cat would be a bad idea?

EDIT: But by what V8 Rumble is saying, it wont much matter if the cat is 2.5" or 3", right?

V8Rumble

Whoa, whoa... I hope I didn't come across as saying that! When I replaced the cat on my truck, I went from a 2.5" inlet/outlet to (you guessed it) a 2.5" inlet/outlet...

Flow through an exhaust (or intake) system is going to be limited by whatever the most-restrictive item is - in other words, if you've got a 3" exhaust, but a 2" muffler, you won't be able to flow any more than what the 2" muffler allows.

Now that would normally lead you to think "Well I'll just get a 4" exhaust system, & that'll solve all my problems!" Makes sense, except that a certain amount of backpressure in the exhaust is actually a good thing, it helps to scavenge the cylinders (which allows more fresh air/fuel to get in), so going with a "too big" exhaust can be harmful as well.

As you can probably tell by now, there's a fair bit of science involved with this - much more than would fit in a message board post like this. Best "concise" advice I can give you is "If in doubt - or if you're not regularly winding your engine to 6500 or more - you're probably just fine with a 1-5/8" header, with the rest of the exhaust being 3" (if single) or 2-1/4" to 2.5" (if dual)." (Note: I've read somewhere that a properly-sized single exhaust is actually a bit more efficient than a properly-sized dual exhaust, because of frictional losses along the wall of the pipe - I don't know, & I suspect that the difference is probably marginal.)

I've also seen information to the effect that there's a LOT of power to be found in the right header collector - meaning 15 to 20 horsepower with that change alone! Needless to say, I'm trying to get more (definitive) data on that...

I guess what I'm trying to say is "Don't cause an artificial restriction in your exhaust, but don't go nuts hogging it out either." (Sorry I can't give you a specific, detailed answer, but it really depends on the car...)

Tru2Chevy

Matt, grab a Carsound or Catco 3" in/out on eBay for $40-$50. Some people say that the Catco's don't last, but mine came with a 5 year warranty as long as it was installed by a shop.

- Justin

85_305

Well, thanks for the help v8-rumble and Justin I will look up the 3" cat on ebay.

EDIT: Justin, I just found a 3" Catco for like.. 48 dollars WITH shipping. Plus, that is a damned nice looking cat
Is carsound and catco any good though (over a stock convertor..)?
Do these prices/auctions happen often w/ these low prices?

87DJP2001

Forget about E-bay and just order a direct fit Catco Cat from Summit Performance for your model car and motor. This will ease any installation problems with a one size fits all.

85_305

Wow.. that's a nice and cheap way to go (BTW.. it's like 130$ for a 3" catco off of summitracing.. whereas on ebay it is 45 bucks or so w/ shipping )
But I am having an exhaust shop do it for me; they can dick around w/ making the ebay universal catco fit, lol.

sparafucile17

Matt,

Are you using the factory y-pipe on your car? If so, I was wondering if you new what it's outlet diameter was. I had posted another thread regarding y-pipe size, but no one really replied. I also want to replace my cat, but would like to mate it to a new 3" flomaster cat-back setup.

But now I just need to figure out what the cat inlet (y-pipe outlet) diameter needs to be so that I can buy the right cat before ripping out the old cat.

Thx
Jeff
------------
'91 Z28 305 TPI, AC Delco Rapidfire2 Plugs, Smog pump removed, TB Airfoil, new paint job, and many more projects planned...

85_305

No, I am not using hte factory y-pipe. The factory y-pipe on our 305 cars are HORRIBLY designed, and will rob power like ****'s. I will be using a hedman y-pipe
Honestly, I dunno what the outlet size on the y-pipe is. But usually you have to buy a y-pipe designed for your exact headers.