2.75 inlet on 3in exhaust

[TimF]

I bought a 3in cat back exhaust but it has a 2.75 inlet, when I called the company they said that most 3in exhaust on the market are 2.75 inlet, and when I looked at other brands like megan, it had a smaller inlet then the rest of the system. I think this is kind of weird considering there are 3in downpipes and test pipes just to flow into a 2.75 inlet? Is this guy bsing me or what.

 

[jtmcinder]

A 3" cat-back that is intended to be used with a cat will often have a smaller inlet than 3". This is actually good, since sudden changes in diameter - even increases - cause resistance to flow. The best 3" cat-backs start at the diameter of the outlet from the cat and gently increase to 3". Of course, if you intend to go to a full 3" turbo-back, this wasn't the cat-back section that you wanted....

 

[romeen]

Yes, this is typically the way most cat back systems are made. The idea being that you can bolt it up to your stock cat. My ebay catback was 2.5" for the first few inches. I had it cut off and a short section of 3" pipe with flange welded on to make it true 3" all the way through. But it's not necessary and I doubt the difference in performance is significant enough to notice during typical daily driving.

 

[TimF]

Well I already got a 3in downpipe, so would it kill it allot to got from 3in to 2.75 back to 3in? I wanted a full 3in, and I cant just cut off the flange cause its 2.75 a good half way. I talked to the seller and He already said that if I want to I can return it for a full refund and they will pay shipping back, but is it worth the hassle. I looked into it and like the magen has a 2.5 inlet as well. I was thinking of returning it and getting a megan, would it be worth the extra 200$ for a extra .25in plus moddifying it to fit. And also I plan on deleting my cat.

 

[romeen]

IMO, the performance loss from having the neck down shouldn't be a major issue. Although not ideal I suspect it would be hard to tell the difference in seat of the pants driving impression. I would keep what you have. I already had a 3" cat-deleting DP when I got the catback which I is why I felt it was worth it to cut off and replace the neck down. But if I had been running a cat with a similar diameter then I would have left it as is.

 

[gofer]

Read this ---> Turbo Exhaust Theory

 

[TimF]

That was long, so essentially having the 3in DP to a 2.75 neck in the cat back then back to the 3in is not that bad, although not as good as a full 3in if I read this right.

 

[TimF]

should I get a 2.75in cat delete for my catback inlet or a 3in test pipe to match my downpipe

 

[turboglenn]

Unless you're in the 500+ HP range that small bottle neck isn't going to matter hardly at all. as for the test pipe. Get what ever size you plan to run in the future, if you've got a 3" DP get a 3" test pipe, keeping more of the area smaller may have SMALL advantages in flow based on bernouli's airflow pricipals, but in a turbo car the back pressure is the main enemy and like has been said, you're looking at minimal loss anyway at this point. I say buy what you plan to run in the future just to avoid having to buy another one later down the road.

I just cut off and replaced the 2.5" bottle neck on my thermal cat-back with a piece of 3" after 10+ years of use (as well as upgrading from a 2.5 to 3" DP) and i had great gaines, but then again i was making over 400hp to the wheels daily for many years with only a 2.5" DP and test pipe

 

[TimF]

Thats what i was thinking. I have the goal for 400, but it will be a while, So I can just swap in a bigger exhuast when I get down the line.