Thread: Gt30-76r
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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
I wish we would start checking the back pressure with different turbos. This is such a basic concept.

This post talks about ballanceing the hot and cold side and suggests that there is significant opertunity on the hot side. And that it is not alwas a small turbine that makes for the quickest boost. Nor is it the bigest that makes the most HP.

Originally Posted by TurboKraft View Post

...Long story short, the engineers at Garrett (and BorgWarner, etc.) have the tools to refine their products very well before putting them into production. They don't match one compressor wheel to a different turbine wheel by chance or accident. They look at the airflow characteristics and inertia of the components and put together the best combinations. Rarely will you find a manufacturer's turbo with a compressor wheel that radically out-flows the turbine wheel, as you often find in the aftermarket. (And these are usually targeted for small displacement, high boost, high HP engines that also run massive wastegates to make up for the lack of turbine flow.)
That's not a 930 engine's m.o.

One result of this mis-matching is higher exhaust backpressure at the turbine inlet (header) flange. 930s are bad enough, and the long, undersized tube feeding the wastegate on most aftermarket headers doesn't help matters, either.
Anyone else have exhaust backpressure data they would like to share?
We just dyno tuned an ex-Ruf 3.4L that had been in storage for many, many years. Another shop converted it to EFI and "600hp" -- not with a K29-7006#13, buddy! At 17.2psi boost in the manifold, the exhaust backpressure was just a hair under 30psi. About a 1.75:1 pressure ratio (backpressure:boost) despite the larger #13 turbine housing. Car is choking on the backpressure, spooling up slowly, keeping the Group B cams from doing their work.
One of the most responsive EFI 1994 Turbo 3.6s we have built had 2 different large ball bearing turbos. The first had a 1.4:1 backpressure ratio and drove great. The next turbo we used had a lower 1.2:1 ratio from a significantly larger turbine wheel -- and it spooled up even sooner than the smaller turbo! ...
Old 10-13-2010, 10:24 AM
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