JR-IROC

I'm gonna buy a flowmaster american thunder cat-back soon but There are two options, (17233 or 17234) both are a 3" exhaust system but with different cat size connections. Don't know if my car has a 2.5" or 3" cat. Can anyone help me out. The car is a 89 iroc z with a 350. This might be a dumb question but it took me so long to decide on a flowmaster system. Thanks.

miamivice

Bringing this one back from the dead.... Which one of these is 2.5 vs the 3 connection to the cat? Was on summits site trying to figure out the difference between the two and couldn't find anything

Ron U.S.M.C.

Go down the page of the "application" box and look for = Notes.
Next to that is the description of the cat outlet size it fits.

This one is for the 2.5in cat outlet and the one I have. The only difference I could see between the two sets offered is mine came with an adapter that took the 2.5 in. cat outlet pipe and increased it to fit the 3in. intermediate pipe.That way you’re getting a 3in.cat back.

http://www.summitracing.com/parts/fl...maro/year/1989

Side note = I love the way my Flowmaster cat back sounds.....

Ron U.S.M.C.

If you buy this cat back there’s a little something you should know. The tips are angle cut and are either flush with the rear bumper cover or possibly out a tad. They are not rolled tips and are as sharp as a razor. I do a lot of work at the back of my car and like my legs just the way they are. They don't need alterations done by the razor sharp tips. I cut mine back about 6in. and you would be surprised at how you can still see them even from the side when you're walking up .I think they look great.

dmwracing42

iTrader: (3)

If you buy the 2.5", effectively you've bought a 2.5" catback. The intermediate pipe is 2.5", which then expands beyond that. It's pretty much pointless to buy the 2.5" unless you aren't intending on any major engine upgrades.

Just get the catco 3" cat to go along with it, or go my route and put a piece of pipe in between the y pipe and catback.

Ron U.S.M.C.

I have the stock manifolds and "Y" pipe with a 2.5 in. high flowMagnaflow cat leading into the Flowmaster "3in. intermediate pipe"that travels to the muffler.

This Flowmaster system even has 3in. exit pipes so it is as close to a3in.cat back as you can get.

As far as changing the cat I did mine in a rush to pass smog before the new catback was installed so I went with the stock size unit. Having him change his cat to a hi flow Magnaflow would be a smart move mainly because of the better internal component resulting in a faster/cleaner flow. Even if he did go with the 3in. exit cat he is still getting a 2.25 inlet coming from the stock y pipe. I guess he could find an adapter. I don't want to go into exhaust facts and theory over this but there are a lot of factors that come into play concerning everything from velocity = pipe size, etc.

Going with any system that does not including manifolds and y pipe than youare already in the "choke" zone so he may only receive a 2 to 5 hpgain but I would think it’s more about sound and volume. That's how Ipicked mine. Sound and volume.

dmwracing42

iTrader: (3)

Right, but if you read the Notes, it says the intermediate pipe inlet DIA is 2.5" for the 17234. An exhaust is only as effective as its most constricting point, which in this case is the 2.5" inlet to the intermediate pipe. That being said, if you stick with a 2.5" cat, then the you wouldn't see any benefit (significant) from the 3" vs. 2.5" intermediate pipe.

Ron U.S.M.C.

Lets just dismiss the fact that even everything before the cat inlet is a choke point and what you say is true than what kind of benefits am I missing ?

dmwracing42

iTrader: (3)

I was operating under the assumption that a 3" y-pipe was used. If it's a stock y-pipe, this whole discussion is moot I guess.

But, there's some kind of calculation, I don't know what it is specifically, but it calculates what size pipe you need for proper flow depending on your HP. Anything over like 230-240 HP with 2.5" pipe creates backflow pressure and you can start to lose significant HP.

That's why I'm going with Dyno Don's headers and y-pipe along with the Flowmaster 17233. If you don't replace the stock exhaust completely, you might as well just buy a muffler instead of springing for the whole catback, because the benefits won't be too significant.

Ron U.S.M.C.

That's the whole point. If you don't replace the whole system its difficult to support any real hp. What you are doing when you install the 3in. cat back is more about at least building half of the equation as prep for the rest when the time comes. I still strongly believe that for your average car its more about sound and volume with this cat back until you can do a complete upgrade where you can accomplish the added 20HP or so.
As far as changing the tone and volume of a muffler a 3in. pipe does make the car sound a little different but also the build and displacement of the car will change that also. No matter what he does if he ends up getting the 80 series muffler (that's what comes with the American Thunder cat back)he will get the Flowmaster sound. Some like it, some don't. If they do like the sound than allot feel its too loud. Heres a video of a car that has the Flowmaster sound.

87 Camaro Flowmaster Exhaust - YouTube

dmwracing42

iTrader: (3)

I feel as though you and I are actually arguing the same point.

To OP, just get the 17233, any mods you want to make later will thank you.

miamivice

I am running stock manifolds and a stock size cat. It's an almost all original GTA notchback so I THINK I want to keep it that way if possible.

That being the case, I would really have any performance gains by going with the full 3" one (model 17233) vs the 17234 model right?

dmwracing42

iTrader: (3)

If you know for a fact that you don't intend to ever go with aftermarket from the cat forward, might as well go 2.5". But if you think there's even a chance you will (or if you ever intend to go more than ~250 HP), the 3" is the way to go.

Ron U.S.M.C.

I'm not arguing any point just putting out information that might be considered before he makes a decision.
OK, Im not saying this to contradict what you're saying but If he goes with a 3in. in and out cat he will still need a adapter to the stock Y pipe.
My sujestion would be keep the stock cat until he can afford to replace the cat forward.At that time remove the 2.50 to 3in. adapter on the 3in.intermediate pipe and that's it.
That gives him a couple of choices
If he can afford to buy the 3in. in 3in. out cat than do it.As you have said.

dmwracing42

iTrader: (3)

This is a good idea. OP, really depends on your future plans. This is the more expensive option, but the better one if you think there may be an engine swap or headers in your future. If this isn't in your future, 2.5" it is.