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JakobM JakobM is offline
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Join Date: Sep 2010
Posts: 307
My advise - go with a 4 port MAC valve and you get a better solution - you get to add your boost pressure as your "amo" to fight against your back-pressure. A 3 port valve can not do this. A 4 port valve will provide you a 2:1 force ratio (double force compared to a 3 port valve) to fight back-pressure.

This is how the 4 port solenoid valve work:
Port 1 - Inlet
Port 2 - When building boost ALL pressure is 100% routed to the top of the WG only (above membrane) - NO pressure under membrane. I.e. the membrane has both spring and boost pressure to hold wg valve closed.
Port 3 - When target boost is reached, the solinoid shifts pressure from top to below membrane to open WG valve
Port 4 - To maintain boost level the solenoid bleed pressure out of this port/filter to maintain stable boost.

This is how the 3 port solenoid valve work:
Port 1 - Inlet
Port 2 - When building boost, pressure is shared to both the top of WG / membrane by solenoid port 2 - AND shared stationary below membrane by a separate "stand-alone" hose and stays pressurized here below membrane at all time! So the membrane only gets equal pressure below and above until target boost is achieved - I.e. Only WG spring holds wg valve closed.
Port 3 - When target boost is reached, the solenoid bleed pressure out of this port/filter to maintain stable boost.

Our 930's (if tuned) normally have a good length headers, and good sized turbines, so back-pressure is normally equal to - or if running very big turbine - slightly below boost pressure through out entire rpm. If a 930 is close to stock then back-pressure is higher than boost pressure and WG setup gets even more important. However, as we are running "only" 1 - 1,5 bar of boost when tuning these effective large displacement engines, many owners and tuners only setup their WG(s) with let say 0.8 bar springs and bleeds air for the rest to reach the typical 1 or 1.5 bar of boost. As the gab increases between spring rate and boost target, the risc that your WG will start open way before you reached boost target is considerable higher. The 4 port setup will - in this example - help allot as it provides a 2:1 force ratio with both 0.8 spring load + 0.8 boost on top of membrane = capable of handling 1.6 bar of back pressure running 0.8 bar boost. The 3 port valve setup can only offer the spring rate alone, and manage to cope only 0.8 bar of back-pressure for a 0.8 boost pressure (i.e. only a 1:1 force vs. back-pressure ratio).

Real life example:
We worked on a car (none porsche) recently to find reason for slightly late boost. It was setup with a "so called" 1 bar spring in a tial 38mm wg running a 3 port boost solenoid valve setup in closed loop like the drawing above. When we did our test and applied pressure underneath the wg on the bench, the WG valve started to smile-open at 0.3 bar boost, half way opened at 0.6 bar, and fully opened at 0.8 bar. I.e. the spring advertised as 1 bar was in real life opening this 38mm WG between 0.3 - 0.8 bar sitting on the bench. So, if still using the 3 port solenoid in closed loop in this application, the WG valve would start open when back-pressure reached only 0.3 bar which equals appr. 0.3 bar of boost - i.e. way before the intended target of 1 bar boost. The spring setup alone was one part of corrections needed on this car. The WG will not tell you it slightly opens when installed unless you have a digital input sensor installed on top of the WG measuring movement ($$). The difference can be 2-500rpm spool-up from "smile-opening" in difference, 300rpm in this example, and you will not hear it from WG. You normally get to hear the WG when appr. >30% open if running no WG muffler. Solution was in this example to keep the "so called" 1 bar spring and install another smaller diameter, a "so called" 0.8 bar advertised spring. Then with the two springs installed the WG started "smile-open" at 0.8 bar of boost which was good enough for this application together with a 4 port boost solenoid valve to increase the force holding WG valve closed. Now with the 4 port valve we had 0.8 + 0.8 = 1.6 bar force to hold wg closed for 1 bar of boost. With the 3 valve solenoid, and same double spring, we would only have had a 1:1 force vs. back-pressure ratio to fight with and hence still a WG opening before our target boost at 1 bar. With the 4 port valve we got 2:1 force ratio in our advance

Always pressure test your wg before installing, and always run a 4 port valve in closed loop is my advise
Old 07-19-2019, 05:31 PM
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