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Posted by thetallguy [Email] (#1698) [Profile/Gallery] (more from thetallguy) on Wed, 25 Feb 2015 07:07:46 Share Post by Email
In Reply to: Re: Anyone running the GFB #T9301 Diverter Valve?, Simon S, Sun, 22 Feb 2015 10:32:38
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Both control valves (Bosch and GFB) remain closed on boost by using boost pressure to hold the valve shut. Pressure remains equal on both sides of the valve as long as the throttle butterfly is open. Once the throttle snaps shut on a deceleration event or gear shift, pressure in the intake drops below atmosphere and the sprung seal opens; the seal broken by the pressure differential.This is why the Bosch begins to vent before you might suspect based on the boost gauge. The premise that modern turbos spool more quickly and therefore maintaining the pressure in the intake/intercooler is "better" than maintaining turbine speed appears suspect. If one is using a higher performance turbocharger which is generally characterized by a larger compressor wheel (more weight), pumping more air volume (more weight), with more associated "lag" at a elevated pressure (higher than stock),then maintaining turbine speed would appear, in my opinion , to remain a viable concern. Regaining lost rotational speed takes time! This is "probably" best addressed by quick venting. Quick venting allows the turbine to maintain its speed and avoid a "stall". Decelerating a turbo quickly is hard on the turbine shaft and increases lag time. The amount of time to re-spool is based upon the weight of the rotating assembly,bearing frictional losses, the target pressure one needs to achieve the desired acceleration, the exhaust gas heat available at that specific moment in time, exhaust gas pressure, and the initial turbine wheel speed.In the case of the GFB, the compressor wheel would also be fighting the alleged elevated intake pressure although equalization would happen so quickly that it would count for almost nothing. From what I've read, maintaining the compressor spool speed/energy results in the least amount of "lag" between most boosting events. Those using a larger turbo or elevated boost levels on a stock system probably still have the same problems associated with an "older" design. Quick venting is best accomplished (on Bosch) by using the least amount of spring tension possible and the sealing diaphragm's "pull" as the pressure differential drops . The limiting factor, as I recall, being a need for the valve to resist the vacuum on deceleration (hooting). It's possible the GFB design negates a need to resist vacuum; that, however, is not the sales pitch here. So the answer depends: on stock turbo VS higher performance turbo and the actual boosting event in question. By GFB's own admission the turbine's speed on re-spool is lower. This is not a good thing!The risk of deceleration of the turbine could be a problem. Also not good. If the GFB's design allowed less spring tension or no spring tension on deceleration then that, in my opinion, would be "more better". The Bosch unit is very very good in all design aspects to insure fast venting. Just no CNC goodness. IMO.

It would be nice if you could maintain pressure in the intake AND preserve turbine speed...the search continues. Possibly why the newer modern turbo come with the recirculating valve built into the compressor housing.I see this function coming under computer control in the future...possibly already available on Jags; perhaps someone has some info on this.

->Posting last edited on Wed, 25 Feb 2015 07:30:03.
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