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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Better late than never.

On a stock 3.2 the SSI system primary tube length and size offers a little bump in efficiency right around 3000 to 3500. As the turbo will be adding back pressure to the system I suspect it will slow down the exhaust rate some so I guess there could be a small bump in tq just before and after 3k. However, the turbo starts coming in at that point so this may actually show any yeald. From the dyno info I saw that compared the SSI to stock 3.2 Carrera there was no help in efficiency any ware else. Thus, I would not expect any improvements from going SSI over the stock 3.3 system for the most part. It migh even be more restrictive at higher HP ranges with its smaller primary size.

However, maintaining a split system to a turbo that has a split scroll hot side is a known opportunity and SSI and 993's can both accomplish this.

The 993's do have a larger primary tube size and may create an opportunity for less back pressure at higher hp levels. However, most street 930's are running smallish hot sides anyway and this may not come to be a benefit.

Wiith the stock and 993HD ithere may be potential to create hotter cylinders do to the short tube design. Again, not an issue for a street 930. However, on the track, an SSI might make for more balanced temps at the exhaust valve.

Again, a fully spilt scroll system may be one opportunity to go after. I do not think having equal length primary tubes is of that much importance on the street as it migh on the track.

The other opportunity is creating an effective waste gate system. The WG style that B&B and others use may have some design limitations. Some have been suspect that this style of WG can trick the WG circuit to delivering a bit higher boost than expected. I suspect the HP gain on this type of system is from the poor WG design and the improved boost onset is from the improved presure differental created with a lower back presure muffer. But people buy it.

Running the more popular HF/K29 style turbos that retain the smallish hot side but add a larger compressor wheel also adds issue to the WG needs. The larger compressor wheel makes for more HP and also slows down the hot side wheel. This creates more back pressure and more gasses that need to escape from the WG circuit.

Now we have the poor WG circuit delivering higher boost than intended and the increased need to release more exhaust gas via the WG circuit. This is being seen as boost creep where we think we have a .8 bar spring but are seeing 1 bar of boost in the higher gears.

I am not an expert in this but I would not expect much changes pre boost between systems with our low overlap cams. I also would not expect any difference in boost onset with changes in HE design. Nor would I expect much difference in HP for the most part if boost is carefully adjusted for at the manifold due to varations in the WG circut (including any restriction after the WG).

I suspect more opportunity lies with the choice of turbo, if it is a full split scroll system, and what happens after the Turbo. Minimizing back pressure likely has more to do with increasing boost onset than messing with the header design for most street 930's.

A full race turbo or one running overlap cams might be a slightly different story.

Must my uneducated opinion.
Old 01-02-2009, 02:19 PM
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