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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
FYI, I just measured a 930 J pipe. Thus:

1.5" primaries per Brian.
2" secondaries.
2.25" Y pipe to turbo.
K27/T3 flange 4 sq inch nominal, est. 3.5 sq inch actual.
T4 flange apx 6sq inch or est. 5.5 sq in actual.

These are nominal or OD sizes.

Here is how a stock system seems to play out:

The secondaries seem to have a ID cross section of about 1.8 times the primaries. As we have an exhaust event once every 240 deg per bank that is 1.5 exhaust events per revolution. An SC cam I believe keeps the exhaust valve open for about 220 deg. Thus, there is some reserve for exhaust expansion and collection at the secondary tube.

Then the two secondaries enter the Y or J pipe on a 930. The Y pipe is about 60% of the size of the two secondary tubes. Thus, gasses will have to be accelerated significantly as they enter the Y/J pipe.

The 2.5" Y pipe than supplies the K27/ T3 flange is mostly the same size at the turbo inlet so the velocity entering the turbo hot side dose not change.


On most after market 930 systems they seem to keep the primary to secondary ratio. However, the area of the two secondary tubes where they enter the turbo is much larger than the entry to the K27/T3 flange by close to 1.6 times. Thus, the gasses need to start accelerating before entering the turbo.


A couple of thoughts.

I wonder if we should be working from the turbo back and look at the changes in velocities?

On a stock 930 the Y/J pipe is going to see the highest velocities of any section of the headers and thus be the area that might be most likely to create back pressure. Doing something to retain the heat along that run might be something to consider as that would reduce back pressure to some degree. (wrap, coatings, larger tube?)

On most after market 930 systems the transition just before the turbo might be something to look at as gasses have to accelerate about 60% before they enter a K27 flange do to the cross section difference.

Putting a WG at that point is only going to add unnecessary turbulence and make the WG circuit unreliable. Moving the WG cricut away from the transition up into the turbo should make for an improvement.

A T4 turbine connected to two 2" secondaries moves most of any changes in velocity into the turbine housing as two 2" tubes have the cross section of about 1.1 times the hot inlet size. Seems like a good fit.

Running larger that stock 930 tubes to any turbo using a K27-7200 hot side (HF/K29 quick spool...) is probably not going make for much improvement as the turbine will probably not accept the increased flow potential anyway and WG sizing will probably become even more important. But that is just a guess.

Not an expert, just trying to figure this stuff out.

Last edited by 911st; 01-19-2010 at 03:34 PM..
Old 01-18-2010, 10:08 AM
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