Adapter between Cobb Catted Downpipe (2.5") to 3" cat back exhaust

[ashwinearl]

Hi,

I'm exploring new cat back exhaust to mate with existing Cobb Catted downpipe.  My understanding is the Cobb DP is 3" but tapers down to 2.5" for easier interface with stock exhaust, which is how I currenty have it.

Is this Grimmspeed Adapter the one to get for mating the Cobb Downpipe to an aftermarket 3" catback exhaust?

 

[KZJonny]

I believe that is the one you'd need. I had to go the other way from my 3" DP to the 2.5" fitting on my catback, which wants a doughnut gasket.

Why not trim the taper so you have a true 3" pipe the whole way? Concerns about overboost?

[Tehnation]

depends on which cobb downpipe, old version tapers to 2.5 oem style, while the new dp's are full 3". 

[ashwinearl]

11 hours ago, Tehnation said:

depends on which cobb downpipe, old version tapers to 2.5 oem style, while the new dp's are full 3". 

It is fairly old,4 -5 years. Cobb catted DP for the 5 speed.  When I was looking the other day, I only saw Cobb catted for Automatics available.

[ashwinearl]

11 hours ago, KZJonny said:

I believe that is the one you'd need. I had to go the other way from my 3" DP to the 2.5" fitting on my catback, which wants a doughnut gasket.

Why not trim the taper so you have a true 3" pipe the whole way? Concerns about overboost?

I'll ask the shop's opinion on that. I was just thinking for fitment issues especially with this being a wagon, that it wouldn't be good to shorten it anywhere. But I don't know how easy/hard that is. Might be a non-issue for them.

[jams20]

This is the one I used. I started w a Cobb DP tied to stock exhaust then upgraded to 3” exhaust. Works a charm as long as your current donut gasket is in shape.

You run into fueling and air issues before exhaust becomes a bottleneck. 

[KZJonny]

Either way you do it will be good, I'm sure, but if I were doing my install again, I'd have cut off the flare to the doughnut gasket on the catback and welded in a short 2.5" --> 3" adaptor and 3" flange to match the DP.

Having the donut gasket is nice, and gives you a little flex in the system, but by the time I was done buying the adaptor plate, doughnut gasket, and new stainless bolts to mate the whole thing up (the ones the Grimmspeed sends you with the package are like....2.5" and nowhere near long enough to get through the DP, adaptor, and springs bolts....) I think I would've been about the same price as just making my own 3" direct fit up. You'd also have the advantage of eliminating one gasket from the exhaust system. Although I do wonder about the long term disadvantages of having a more rigid system on the whole and where it ends up moving the strain that the doughnut gasket is intended to take up.

I know that for most people the 2.5" restriction isn't going to be a bottleneck, and my whole catback is 2.5" since I just don't imagine ever needing a 3" at my power goals. I guess I was just thinking that if you've spent the extra cash on a full 3" system, why not go the extra little bit and make sure the whole thing is uniform. At very least you should probably see some increase in laminarity of exhaust flow, although admittedly that won't buy you any measurable power. Might sound better tho? hah!