View Full Version : Best downpipe??

29th February 2004, 23:25
Could anyone tell me which is the best downpipe to use e.g open neck or twin dump?? It's for an STI 5 type R,

Thanks in advance

1st March 2004, 10:50
opinion is still divided, but in my opinion, twin dump is still the better option.

If you get the mild steel bpm version, its also 3db quieter.

Both are an incredible advantage in terms of flow and spool up.

some feel the twin dump is not ideal because of the slim angle to which the wastegate opens meaning in an enclosed pipe there is not much room for the gas to escape past, however in practise it didn't cause a problem. It may make boost control slightly more difficult but I think it is a valuable trade off rather than agitate the air flow exiting the exhaust wheel of the turbo. I think it is more important that this stays clear.

It is also beneficial to lag the pipe.

Avoid the hks as it keeps the wastegate exit blocked.

1st March 2004, 10:55
oh, and welcome to the board.

4th March 2004, 20:10
Cheers for the answers Adam, I have bought a twin dump downpipe, now just waiting to get it fitted.

Thanks again


4th March 2004, 21:46
I found that moving from a twin dump setup to a open mouth design with splitter helped with boost control on my car (but see below).

Some of the big problems with the twin dumps are the wastegate pipe is too small in diameter and it actually hampers boost control through excessive back pressure behind the wastegate.

Just my 2 bobs worth.


6th March 2004, 21:31
I ran open neck and made my own splitter mod to it. Was a lot better than a HKS d/p for spool and low end responce, std d/p is like a stranglehold on the cars breathing and spool in comparrison.

I never did get over the boost creep problem on WOT at high revs in my car with high flowing exhaust and d/p running std boost control or actuator pressure. Although scoobyecu was only thing that kept it controlled. Wastegate porting would have been the answer.

Can't comment on the twin dump, but like Adam says both make great advances to spool up.

30th March 2004, 22:17
Boost creep ?

I assume by this you mean excessive boost fall off at high revs, i.e. boost reducing with increasing revs ?

Rob G
30th March 2004, 22:43
The opposite mate

As exhaust gas flow increases the boost increases due to flow restriction out of the wastegate.


31st March 2004, 17:25
As Rob said.

Rob G
31st March 2004, 18:32


p.s. want to make your skylark faster??

31st March 2004, 19:38
Cheers - thanks for the clarification.

1st April 2004, 06:44
The latest Prodrive DP for the STI PPP is a twin dump design IIRC. Theirs has a spliter too.


1st April 2004, 17:02
p.s. want to make your skylark faster??Hi Rob, it goes as fast as it can on the standard turbo's...

what you got in mind?? :D

5th April 2004, 16:03
I.M.H.O. there is no "Best D/P" but I have enjoyed good results with both the RCMS and H+S 3" open neck but with a splitter fitted into the tubo to separate the WG gases. Of the two, the workmanship on the RCMS was noticably tidier.