the difference between a twin dump downpipe and open neck downpipe? which is best for performance and reliablility?

 

Just so happens I have an example of each on my garage floor!

The Open neck is a short funnel fabricated into the D/P from the flange where it bolts onto the turbo to the actual D/P tube, about 75mm long. It makes an open chamberallowing the waste gate (when it opens) to flow it's gases into the exhaust main stream relatively un interrupted. This is now a very popular design.

The Twin Dump Pipe design is where the same turbo flange area has the D/P tube welded to it to take the min stream exhaust gases, but the wastegate gases are taken into the D/P via a much smaller tube, about 30mm diameter that is welded (and venting into) to the D/P about 100mm from the flange.

Will send you a pic on this thread if you like tomorrow. A picture speaks a 1000 words!

Generally, most feel the Open Neck to be the better of the two.

911

 

The trouble with the twin dump is that when the wastegate opens it partially blocks the dump pipe and most of the gasses end up going down the main dump leg.

Andy

 

Plus the twin dump tends to cause overboost.

 

Looks like i may be about to make a mistake then:

The choice I have of D/P:


open neck (of sorts) Magnex but 3''


twin tube (with sports cat), 3''


general view:


The tech argument is to dump both and get a new true open neck and sport cat in the centre pipe. Too much $$$ at this point, so which one of these two? I really want to run a cat this year. The 'open neck' has run great for 3 years now.

911

 

I've got a 3" closed neck. Am I missing out?

Bob

 

Maybe not.
It is all about the interruption of the main gas flow when the waste gate opens as boost limit is reached.
I'm not so sure it is worth worrying about to be honest Bob. I'm fitting the Cat anyway for now.
Graham.

 

I'm pretty sure that a good open neck with FLAT splitter is the best combo. Prodrive used a very long open neck design for smooth flow.

Bob, yes a bit.

F