Downpipe design - one for the tuning guys!

[LHD_Kid]

I want to get on with fabricating a downpipe and have read / spoken to a few people on the subject.

I don't have a huge turbo, so the downpipe will be increasing from the size at the turbo flange to the 3" tube I have for the downpipe...

It seems that most people go from the flange straight to an enlarger to get to the dp diameter as soon as possible.

One train of thought that has been suggested by a succesful tuner is to keep the smaller diameter of the turbo flange for around 6 inches, then increase to the larger diameter. The idea being that a little back pressure will help with response or fast spool (not exactly sure on the technicalities)

What are peoples thoughts on this?

For info, I will be running a screamer pipe and standard wastegate. Power level 420 - 450 tops...

[Error404]

The pressure differential pre and post turbine is how the exhaust gas energy is harnessed, so you want ideally to have zero back pressure post-turbine for the best boost response and best volumetric efficiency. In reality you can't achieve that, but get as close as you can.
You want to use a nice conical transition between the turbine outlet and the main downpipe diameter, but keep in mind that with a bolted flange you will need to allow clearance for the nuts.

HTH

[K Simmonds]

I'd seriously reconsider the screamer pipe idea on a road car unless your going to run it under the car and put some sort of muffler on it...I ran one on mine when I first put it on the road and the noise isn't in anyway pleasant.

As Alex says use a conical transition from the flange, a bit of thought here makes installation and removal much easier than stock.

[LHD_Kid]

Interesting... so we're saying a nice smooth transition asap to the larger pipe (allowing for the nuts) is the way to go...

BTW, screamer pipe will be silenced under the car... fell in love with the noise such a pipe made on an R32 GTR, really wanted to know what the I5 sounded with the same treatment...!

[Error404]

The answer is LOUD!

[LHD_Kid]

The answer is LOUD!

This is the closest I can find on YouTube to illustrate the ripping noise that was so good... not great footage and camera phone seems to get a bit overwhelmed- but you get the idea... check out about 3:30 ...

Linky

[diesel des]

Yep go as as big as you can as soon as you can on a turbo car for best response and power. A 15 degree expansion is said to be about perfect.

I'm running a separate system for the wastgate on my urq with a silencer etc. Not sure how loud it is yet. There isn't much room for a 3 inch down pipe and a 2 inch wastgate exhaust I can tell you...

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