I've read from the site the HKS downpipe is closed neck - how does this affect performance gains compared to open neck/twin dump. Have the HKS superdrager exhaust and cannot decide which downpipe to go for to best setup my MY95 WRX.

 

What it means is that the wastegate exit is effectively blocked off on the HKS, and the gas has to escape into the exhaust path from the hot end impellor. This restricts flow, and slugs boost delivery, from a spool up and high end flow perspective.

By going to an open neck design like a Scoobysport DP or a twin dump down pipe the waste gate gases don't restrict flow so much.

If you fit a standard Scoobysport DP it tapers from 3 inch bore down to 2.5 inch where it meets the centre section. Fine for the stock Scoobysport system but potentially a little restrictive with the 3" bore HKS system. Although with a 3 inch bore system you get better performance at high revs, and a 2.5 is better at lower revs.

2.5 inch really suits small turbos like the TD04 fitted on Euro spec cars from 96/7 because there's not a lot going on above 5.5 to 6k rpm. With the larger TD05 or VF sereis turbos fitted to Jap and early Euro spec cars a 3 inch bore system is of more use.

 

There are two versions of the Hayward and Scott (read old Scoobysport) downpipe, one tapers to suit the UK sized exhaust and the other is a straight 3" pipe all the way.

 

see my thread re changing my exhaust, shows the difference between the 2.5" and 3" scoobysport!