connecting 3" DP to 2.5" cat-back????

[jongraack]

I'm probably just thinking about this too hard but I'm also not an exhaust expert so I figured I'd ask.

Dealing with flat flanges here. What is the best way to connect a 3" downpipe to a 2.5" cat-back? (Downpipe is cat-delete) Flanges being different sizes of course but the holes match up for the bolts. The holes match up because the downpipe's flange is drilled to accept different sizes. I could see just throwing a gasket in there and tightening it up real good but since the holes on the downpipe flange are bigger, I would think the cat-back pipe could slide off center.?.?

Any help would be greatly appreciated!!!

 

[T is for TURBO]

I would get the proper flange welded to the catback.

But the real questions is, why are you connecting a 2.5" catback to a 3" down pipe?

 

[jongraack]

I would get the proper flange welded to the catback.

But the real questions is, why are you connecting a 2.5" catback to a 3" down pipe?

trust me, it's not for me. I just don't have the answers and figured someone here could help. I also told him to do the same thing but he insists there is a way to connect them without changing the flange. he's stubborn


plus the 2.5" cat back is free to him, so he wants to try and use it as is

 

[stacked1G]

a 3" dp to 2.5 catback will cause turbulence...the other way around though, is not bad...

tell him to just buy a 2.5" dp .

but to answer your question, it will bolt up fine.

 

[jongraack]

a 3" dp to 2.5 catback will cause turbulence...the other way around though, is not bad...

tell him to just buy a 2.5" dp .

but to answer your question, it will bolt up fine.

What do you mean by causing turbulance? I know what turbulance is but how will it affect the drivability? Curious to learn something new. I've heard of people doing this and it not causing any problems but I'm not sure of the problems that would be caused from this.

Thanks

 

[WES_393]

It won't cause the car to run poorly, but it will make the entire exhaust system flow as much as a full 2.5" system. Bolting on the 2.5 Catback will make the 3" downpipe useless. But, there should be no issues with the car actually running and driving.

 

[stacked1G]

What do you mean by causing turbulance? I know what turbulance is but how will it affect the drivability? Curious to learn something new. I've heard of people doing this and it not causing any problems but I'm not sure of the problems that would be caused from this.

Thanks

the hotter gases flow better(as they enter the downpipe) & As the gases travel down the pipe(through the testpipe to catabck) they expand, & the speed decreases. there for a lager pipe diameter twards the end(catback) can "help" this flow as the gasses cool and expand...therefore bigger to smaller really isnt the best idea, and can cause turbulance..

it will not actually effect driveability, but will definatley effect the flow of his exhaust.

 

[jongraack]

It won't cause the car to run poorly, but it will make the entire exhaust system flow as much as a full 2.5" system. Bolting on the 2.5 Catback will make the 3" downpipe useless. But, there should be no issues with the car actually running and driving.

Awsome. Big help. The only other thing is the connecting of the 2 together? Use a 3" or 2.5" gasket?

the hotter gases flow better(as they enter the downpipe) & As the gases travel down the pipe(through the testpipe to catabck) they expand, & the speed decreases. there for a lager pipe diameter twards the end(catback) can "help" this flow as the gasses cool and expand...therefore bigger to smaller really isnt the best idea.

i dont think it will actually effect driveability, but will definatley effect the flow of his exhaust and possible power gains from it.

This is just temporary until he can afford a 3" cat-back. Thanks alot for the help.

 

[stacked1G]

no problem, and honestly unless your friend is making around 350-400 he'd probally just be better off buying a 2.5" downpipe.

and to better answer your question, the gases in the dp fill a 3" diameter, while entering the catback those gases need to then fit into a smaller diameter(2.5") as they are expanding. thats what causes the turbulance..but as stated above shuldnt effect driveability.

use the 2.5" gasket, the 3" may not make a good enough seal on the smaller pipe. you will just have to see how they line up with eachother tho and make the decision then.

 

[WES_393]

I'd go with the 2.5 just to make sure it's not to big. The restriction is already there so no harm done.

 

[19BLACKGST98]

Im going to be connecting a 3" dp to the stock exhaust but im not sure how im going to do it yet. Im welding a 3" electric cut out onto the down pipe though.

 

[5150DSM]

You could also just buy or fabricate a neck-down pipe. 2 1/2 flange on one side, bell mouth pipe about 3-4" long and a 3" flange at the other side. Or just mount it up as is, providing you can get the bolt holes to line up. If not, slot the 2 1/2" flange holes and match them to the 3" gasket/flange and run it that way. You'll have a little more back-pressure in the exhasut system but it shouldn't affect the car much at low boost, low to moderate power levels.