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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
T,

I agree with your conclusions on reducing the hot side back pressure. We are not the first to figure this out and it is a basic turbo trade off. Efficient HP or earlier boost onset.

I think Eddy Bello dose not hit full boost on his 3.6TT 1200hp monster until about 5k rpm. However, when they do come on they are operating at there most efficient range on the hot side as well as the cold side.


I once thought exactly as you that we should try mating of the 7200 with its newer compressor wheel w the 7006's less restrictive hot side. However, there dose not seem to be enough difference on the compressor side to maybe warrant the effort it seems.

As noted above, you might be happy with the 7006. Andial and Porsche were when they took the 1992 C2Turbo into IMSA racing.

However, at the HP levels you are talking about the 7200 and 7006 compressor wheels are moving off there peak efficenty islands.

I once had an HF built on a 7006 hot side. It did build boost a little later and slower but did carry well up top. I did have overboost issues with it if I tryed to run under 1 bar.

Building an HF on a 7006 might be something to look at if they can still come up with the parts or you find a good core. This would be close to an old school K29 hybrid I suspect.

This may seem an odd comment but there seems to be something special about compressor wheels for a 930 when they get to around 60mm. This seems to put a CIS 930 in the sweet spot of the efficiency island. I do not know if much is gained going past 60mm. The challenge is finding the right hot side combination to free up there potential. This is easier done with the GT 35 or Turbonetics 60-1.

As most CIS owners seem so set on reducing lag the market place build turbos with small hot sides and big compressor wheels no mater if they sometimes over-boost or increase resident exhaust pressure. The work and that is all that matters.

Part of the key I suspect might be keeping the size of the hot wheel in proper per-portion to the compressor wheel. It is a leverage thing.

I am wondering if putting a 62mm wheel with a little 7200 compressor wheel is like putting 27" wheel on a car that is geared to work well with 25" wheels. It make the car go faster with each turn of the wheel but it takes longer to get to speed.

My thought is we first need to come up with a proper size turbine wheel to keep the balance we need, then we need to mange the A/R or porting dimensions of the housing around it.

Another approach might be to try to increase the efficiency of the exhaust system its self. I do not think increase WG capacity is going to help. It would help however with over-boost if the compressor wheel is turned to fast by a small hot wheel.

Maybe something like Brian's equalized primary and secondary lengths that better time the exhaust events could help some. Playing with the primary and secondary tube lengths and diameters might have some effect if they are to small.

However, to me the opportunity could be managing the hot wheel leverage working against the compressor wheel load to reduce back pressure.

Not an expert, just what I believe.
Old 01-01-2010, 09:34 PM
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