3.5 inch dump .....yay or nay

[juggernaut]

Hi,

I'm just in the process of fitting my 3076 52 trim turbo setup (replacing 3071) in twin scroll format to my S15. I made 388rwhp with the 3071 and expect to make 400rwhp+ with the 52trim 3076.

I currently have the Full-Race 3 inch dump that came with the Full-Race twin scroll kit. Will there be any benefit from fitting a flared 3.5 inch dump off the back of the turbo hsg.

Reading from Maximum Boost, Corky Bell made mention of using 4 or 5 inch dump off the back of the turbo will provide some benefit as it allows the hot gasses to expand. Obviously there is no way I am going to be able to get a 4 or 5 inch dump in there....3.5 inch tops.

Cost is not an issue.

Also is there a point where I should taper it down so as to maintain ex gas velocity.

I actually have a 3.5 inch cat back system built and ready to go on....however, I'm not sure whether this will now be detrimental and whether this would create turbulence as the gasses cool along the ex pipe. My main concern is loosing mid range torque.

Opinions appreciated.

Oh yeah ....the gates plumb back into the exhaust .....not interested in running a screamer.

[Lump]

yay

[Pav]

On the subject of exhaust/dump pipes, I'm eventually upgrading to a larger turbo that requires a 3.5" dump, but the rest of my current exhaust system is 3". If i just get the dump fabricated up in 3.5" and joined to the rest of the 3" system, will this restrict the flow or have any negative effects at all?

[Bomber]

On the subject of exhaust/dump pipes, I'm eventually upgrading to a larger turbo that requires a 3.5" dump, but the rest of my current exhaust system is 3". If i just get the dump fabricated up in 3.5" and joined to the rest of the 3" system, will this restrict the flow or have any negative effects at all?

Not really as long as there are no silly joints or harsh edges inside the pipe. I used to have a 4" dump running off a VF35 turbo in an 02 STI, 3.5" the rest of the way. The reduction from 4" to 3.5" was very progressive which certainly helped. Only wish I had of got one with a tuned length spliiter instead of a bell mouth design.

As you may or may not be aware, exhaust gases (after the manifold) will be hottest coming out of the turbo and will expand quickly upon entering the dump. As the gases get pushed further down the exhaust they both cool and slow down. So if you've got a nice big area for the gases to expand into as in a bigger dump pipe you will be further reducing back pressure and making the turbo run a bit more efficiently as there would be less of a bottleneck right where you need gas to be flowing away as fast as possible.