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Thierry25 Thierry25 is offline
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Join Date: Feb 2009
Location: FRANCE (Besançon)
Posts: 310
K27 HYBRID some question to expert

My best wishes to everyone for this 2010 new years !

As some know, I own a 1991 C2T with a setup which is described in my thread

ELECTRONIC CIS fueling & BOOST control

At present time , it seems that I hit the limit of my K27-7200 turbo. Actually, using my boost controller which is RPM programable, I was able to test a lot of boost setting. Today, these numerous test lead me to think that the compressor side of the 7200 is able to maintain high pressure (1.2b) until the red line ( 6500 rpm). However, in the mean time, the turbine side ( hot side) is not able to " follow" and the back pressure which is created kills the engine efficiency. In fact , I strongly believe that the exhaust flow is too much restricted by the hot side. So the back pressure is too much increased and the exhaust flow can not get out correctly ( I think too much burned gas are not evacuated of the barrels) .

This is not a secret , as many expert think, I also suspect the back pressure to be the key point either.

In order to improve my setup, I am thinking to upgrade my turbo. My purpose is not to extend the efficiency to lower RPM . I want to keep the on/off behaviour of my car ( I reach 1 bar at 3400 rpm and get 60 mkg of torque @ 4200 rpm with 1.25 bar of boost) .... I don't want to change this behavior.... I want to keep it ! But I would like to improve the top end ( from 5300 rpm to 6200 under 1.2 bar).

I know that some will suggest to use GT35 turbo. But by philosphy, simplicity ( no water cooling and it goodies) and reliability , I don't want to go in this direction. I do prefer a turbo which could be directly bolted on.

I know that some others will suggest to use a K27 HF. This turbo is quite interesting. However, as it keep the hot side than the usual K27-7200, I suspect the K27-HF turbo could get some limits due to the back pressure at high RPM. I do believe the bigger wheel allow the intake the get more pressure & more flow at lower rpm. However, I suspect the back pressure to be nearly the same at high RPM under the same boost pressure & flow. So finally , the power gain at high RPM can be obtained only from the intake & cams improvment in order to keep the boost pressure near 1 bar.... yes that's true that a minor improvment can also come from the it cold side side which could provide colder air. But it is not such huge I think.

In the mean time, I must say that I don't want to change my cams (SC/X33) or modify my stock intake setup. So, somewhere, the power increase should come from a boost increase.... ( that's why I am concern with the back pressure) Thank to my AIC fueler & pump setup, I have much enough fuel capability to support a new turbo.....

I made a lot of research on this forum and others. I read all the opinion and thought from everyone.

At present time, it seems the K27-7006 is able to carry more boost pressure & flow until red line because it hot side is a bit bigger than the K27-7200 and although it compresssor side is a bit smaller ... !

If the informations I read and my understanding are correct, the K27-7006 hot side casting as well as the turbine are a bit bigger.
If the informations I read and my understanding are correct, the K27-7200 cold side casting as well as the compressor wheel are a bit bigger.

So question for the experts, would it be a possible to build an hybrid turbo using the hot side of the K27-7006 coupled to the cold side of the K27-7200 ? Are the parts compatible ? Is it mechanically possible ? Would it be possible to get this described turbo from a turbo tuner or spécialist ? Additionnal question, would it be possible to get a titanium turbine wheel in order to reduce the inertia ?

All idea and thought are welcome !!!
965 C2T / 1991 3.3 TURBO
Old 01-01-2010, 06:11 AM
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