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    Thread: Forge 2.0t Diverter Valve review (long)

    1. 07-13-2007 01:00 PM #1
      I purchased a Forge Diverter valve after my OEM “C” diverter valve failed after 16,000 miles. Instead of opting to purchase another C valve, I decided to try the forge unit due to their claims of quicker spool, OEM response, more peak boost, and better boost holding throughout the mid to upper RPM ranges. The unit itself is very nice. The forge DV is a mechanical piston actuated valve that is controlled by an OEM N75 solenoid. I’m not sure whether or not the solenoid itself is the OEM unit or a one-off for Forge by Pierburg, but the solenoid arrived with all the part numbers ground off (much akin to OEM units that MTM sells). The t-fitting was probably one of my favorite components of the kit and I would like to see it be made available for purchase by itself sometime in the future *hint hint*. After receiving the unit, I noticed that several of the hose fittings on the diverter valve were slightly loose and required my attention with some red loctite. Not a big deal, but with the amount of threads available on the diverter valve unit, I feel as if the hose barbs used should have been provided with more threads than those on the t-fitting unit. The install went fairly well, although I opted to use real hose clamps on the ends of the 10mm hose instead of zip ties. I also found it easier to locate the bracket holding the solenoid on the bottom most attachment hole of the diverter valve, thereby allowing for ease of install as well as enough play in the wiring harness. After trimming all the hoses to leave the least amount of length between the connections, I secured the hose with small zip ties and fired up the engine.
      On initial impression, there were no noticeable differences. Vacuum on my boost gauge stayed the same, boost seemed to rise and fall similar to normal. There was slightly less noise from the forge unit compared to the OEM “B” valve I was running in the interim while waiting for the forge to arrive. Under mid to high throttle driving, boost pull felt the same as stock and as responsive as stock when going on throttle. The only negative I noticed was that shifting seemed to be less smooth as the engine was not as smooth and responsive going out and back into throttle.
      Interested in what was going on, I ran a set of logs using the new “turbo” function in advanced measuring blocks of VAG-COM. “Turbo” allows for much greater sample rate across multiple logging channels. In each of the logs, I was sampling about 6-8 samples per second on each channel. The blocks I specifically looked at were engine speed (RPMs), boost (block 115) as well as throttle plate opening (block 003). The procedure of testing was designed to address each of the claims Forge has made regarding this valve.
      The first test I did on both OEM and the Forge DV was to measure the response and duration of the venting from 20psi to 1070mbar (slightly above atmosphere). I chose 20psi as it was an easy # to hit and hold, and 1070 mbar seemed to be a good number where all the logs leveled out at. Coupled with the throttle plate logs, the very first point on the graphs indicate the throttle plate at full opening. The subsequent data points were when the throttle plate was moving to closed (off throttle completely).

      As shown, with the OEM valve (red), releasing the throttle instantaneously opens the DV allowing it to recirculate the boost back onto the intake side of the turbo, leading to an immediate drop in pressure measured at the MAP sensor. Forge (light blue) on the other hand, was slower in reacting, with a consistent ~.3s delay before boost was vented. During this delay, boost pressure in the system would spike between 2-5psi (observed) as the turbine was still spinning, but the throttle plate was closed and the DV was not venting.

      In the subsequent photo, I removed all the non- corresponding forge plots. The plots left were determined by how close they were to the starting PSI and RPM of the OEM plots. There were more OEM plots that were not graphed due to their overlapping nature. Forge on the other hand was much more RPM (think vacuum) dependent on when it would begin to release pressure. The slopes of both OEM and Forge curves were relatively similar, indicating that the Forge is relatively similar to the OEM valve in terms of flow.

      When measuring going-into-boost performance, I coasted at a given RPM and went WOT. Based on when I went WOT I graphed time vs PSI, showing how quickly it took both the Forge and OEM plots to reach 20psi. The plot I have attached shows the relative differences in rate when starting logs at different RPMs. The two closest plots between Forge and OEM differed actually only by 80RPMs in the logs but the slopes and overall curves are similar enough to indicate no improvement in boost holding capability when spooling-up the turbo.

      Lastly, I performed an overall 4th gear pull from 2040rpms to 64XXrpms. Throttle plate was pegged at WOT for the duration of the run and all logs were performed on the exact same road to ensure consistency. Again, no noticeable change between Forge and OEM. There was slight higher spike in PSI up top, but slightly lower PSI hold in the midrange by Forge. Nevertheless, the differences between the logs were not significant enough to render a clear advantage of one valve over the other.
      After four days of logging and double-checking my findings, I had several questions:
      Where does the delayed response come from?
      What are the effects of slow response?
      How can the response be improved?

      The source of this lag and consequent surge is based in the design of the diverter valve itself. While the Forge is marketed as an electrical solution, the actuation of the valve is still purely mechanical. In order to understand how the mechanical valve functions, I have included a few diagrams I created illustrating how the forge valve in both “open” and “closed” situations.


      As shown from the diagrams, the main component in opening the diverter valve still relies heavily on the vacuum created when the throttle body closes. Actuating the solenoid does not induce vacuum; vacuum is only created when the throttle body closes. The solenoid is simply there to determine whether pressure can or cannot enter the back of the DV piston. However, actuating the solenoid and forcing the back of the piston not to see as much pressure as produced by the compressor outlet does create a slight pressure differential. It is my belief that this slight differential is not enough to open the DV piston given the latency that is still present in the opening of the valve—the valve still requires the increased pressure differential provided by the throttle plate closing and creating vacuum in the system.
      The main component responsible for the valve’s behavior to a pressure differential is the spring in the piston. Removing the green spring from the forge valve reveals a very high rate spring. Because of this, the cracking pressure of the forge DV is subsequently high, thereby requiring a greater pressure differential in order for the valve to open.
      As observed from the logs, there were spikes in boost pressure measured at the MAP sensor after throttle was released. These spikes wouldn’t register on a regular boost gauge since the boost gauge tap is located behind the throttle plate, on the intake manifold side. The spike in boost is in the pre-throttle plate tract including the intercooler and compressor outlet of the turbo.

      Under normal OEM operating conditions, the boost present in this pre-throttle body tract is immediately vented and recirculated back to the intake side of the turbo, in order to avoid backpressure on the turbine which would cause the turbine to slow down. Given that with the Forge, backpressure is not only present but increased, due to the increase in pressure as a result of the latency in valve opening, it can be assumed that the turbine is significantly slowed down during the .3s until the valve opens.
      The term surging is often loosely defined, but I believe I have found some reliable descriptions using Google. Both descrpitions seem to indicate the formation of backpressure, which in turn slows down the turbine. In addition, the second description is more elaborate and specifically mentions throttle plate closure and the buildup of pressure at the discharge as a result of the bypass valve not releasing.
      "Surge is the situation when the compressor "spits out" more air than the engine can swallow, which causes a backup of air at the intake and it actually creates reverse-flowing pressure waves that can be very damaging to the turbo"
      Quote »
      “Typically, surge occurs after the throttle plate is closed while the turbocharger is spinning rapidly and the by-pass valve does not release the sudden increase in pressure due to the backed-up air. During surge, the back-pressure build-up at the discharge opening of the compressor reduces the air flow. If the air flow falls below a certain point, the compressor wheel (the impeller) will loose its "grip" on the air. Consequently, the air in the compressor stops being propelled forward by the impeller and is simply spinning around with the wheel, which is still being rotated by the exhaust gas passing through the turbine section. When this happens, the pressure build-up at the discharge opening forces air back through the impeller causing a reversal of air flow through the compressor. As the back pressure eventually decreases, the impeller again begins to function properly and air flows out of the compressor in the correct direction. This sudden air-flow reversal in the compressor can occur several times and may be heard as a repetitive "WHEw Whew whew" noise if the surge is mild (such as when the by-pass valve is set a little too tight) to a loud banging noise when surge is severe. Surge should be prevented at all costs because it not only slows the turbocharger wheels so that they must be spooled back up again but because it can be very damaging to the bushings or bearings and seals in the center section.“

      Given these descriptions and the data from the forge DV, I would carefully say that the Forge valve is inducing surge. Several other users have reported hearing “surge” after installing the forge DV, and it is possible that it is actually occurring. Most of these users have intakes which are essentially “filters on a stick” that amplify the sound. Unfortunately in this case, I have a carbonio which utilizes the stock airbox that mutes any intake noise. In addition, my car is also fitted with a full set of VF mounts as well as a downpipe, so any audible surge would be muted out by engine and exhaust noise.
      Well what could be done in order to improve the response of the forge DV?
      The two ideas I’ve thought of are: reducing the spring rate of the valve and/or installing a vacuum reservoir.
      A reduction in spring rate would improve response by lowering the cracking pressure required to open the DV. On the same note, if the spring rate is lowered too much, then the Forge would “flutter / crack open” as boost was ramping up before the pressure is able to equalize on both sides of the piston. Unlike the OEM valve, the forge piston is completely flat and sealed, thereby relying on the pressure from the compressor outlet, traveling through the solenoid and to the opposite side of the piston in order to equalize the pressures. On the other hand, the OEM valve has holes in its “piston” face, allowing pressure to flow to the back of the valve, instantly equalizing the pressures between the front and the back of the piston, which is also why it can get away with a much lower spring rate (~1lb/inch). I would recommend forge owners to try to keep the silicone lines running from the valve to the solenoid as short as possible in order to reduce the overall volume and distance the pressure needs to travel for maximum response and performance.
      The second option would be utilizing a vacuum reservoir, much akin to those found on the 1.8ts. A vacuum reservoir would make DV function more independent from RPMs and throttle plate closure as it would provide a more instantaneous source of vacuum to create a pressure differential and thereby retract the piston. In addition, the vacuum reservoir would improve overall consistency of the latency at which the forge valve opens, as it was shown that with greater RPMs at which the throttle plate was closed, the quicker the forge valve responded as vacuum was created sooner in the intake manifold.
      In conclusion:
      - Forge vs OEM showed no significant improvement in boost holding capability
      - Forge vs OEM showed no significant change in boost ramp-up
      - Forge exhibited no improvement in mid-range and upper RPM boost over OEM
      - Forge showed a ~.3s decrease in response over OEM when recirculating boost
      - Forge exhibited comparable rate of releasing boost to OEM
      - Forge increased backpressure on turbine compared to OEM, as observed by a 3-5psi pre-throttle body spike.
      - The Forge valve is still a mechanical diverter valve and still relies on vacuum from the throttle plate for recirculating boost, whereas the OEM DV relies on electricity to retract the piston and recirculate boost.
      Ultimately, I ended up removing the forge DV from my vehicle after performing and analyzing the logs. All lines to and from the forge valve were checked to ensure there were no obstructions or pinching off of the line which would cause the valve to malfunction. Whether or not .3s latency in boost release is pertinent or not, it certainly exists with this valve. Being a 6mt owner, I mainly opted to remove it simply because between each shift, load drops to 0 and the throttle plate closes (thereby subjecting me to the possible surge condition more). Owners with DSG may not find this to be an issue as between shifts, the engine stays in boost as there is still load being placed on the engine as a result of the wet clutches slipping to disengage and engage. The forge valve certainly offers a more durable, rebuildable DV option, however the response simply cannot match that of OEM. Since it took approximately 16k miles for my “C” diverter valve to “blow” I’ve opted to chance it again with another OEM valve. It is also likely that in the future, VW/Audi will offer yet another DV revision with a different diaphragm material which may be more resilient to bursting. Unfortunately, the forge valve didn’t meet my expectations as a more durable replacement for the OEM valve in terms of performance.
      Dave

    2. Member c1rcausa's Avatar
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      07-13-2007 01:27 PM #2
      Great write up [IMG]http://*****************.com/smile/emthup.gif[/IMG]
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    3. Global Moderator iThread's Avatar
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      07-13-2007 03:16 PM #3
      Got Cliff's notes?

    4. 07-13-2007 03:16 PM #4
      Thanks for putting in the time and effort on that review!

    5. Member TypeR #126's Avatar
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      07-13-2007 03:37 PM #5
      Wow, very thorough and interesting. Food for thought.

    6. Member f_399's Avatar
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      07-13-2007 03:38 PM #6
      damn nice write up
      i took mine out less than a week after i got it
      i realized i was happy boosting 20+ with the stock dv
      if the stock dv breaks ill buy another stock one
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      07-13-2007 03:55 PM #7
      Quote, originally posted by iThread »
      Got Cliff's notes?

      he went back to stock.

    8. Member prodigymb's Avatar
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      07-13-2007 04:31 PM #8
      Quote, originally posted by chris86vw »
      he went back to stock.

      great write up though
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    9. 07-13-2007 04:38 PM #9
      Excellent...I am so glad you shared this information.

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      07-13-2007 04:57 PM #10
      This is one of the most thorough and informative write ups on this piece to date. I would love to see what Forge themselves have to say.

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      07-13-2007 05:03 PM #11
      goood job.
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    12. 07-13-2007 05:59 PM #12
      Sweet Review! You saved me some $$$ I'll keep my stock DV and let them keep replacing it under warranty (Had mine done at roughly 28Kmi)!

    13. 07-13-2007 06:04 PM #13
      You are the man a big [IMG]http://*****************.com/smile/emthup.gif[/IMG] to you . keep it up

    14. 07-13-2007 06:09 PM #14
      Well the conclusion I am grabbing out of this is that performance is basically the same when using a stock sized turbo and the OEM valves still fail.
      I'm still getting forge unit and never worrying about my DV again.
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    15. Member magilson's Avatar
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      07-13-2007 06:13 PM #15
      Quote, originally posted by ShadowGLI »
      Well the conclusion I am grabbing out of this is that performance is basically the same when using a stock sized turbo and the OEM valves still fail.
      I'm still getting forge unit and never worrying about my DV again.

      Surge.
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      07-13-2007 06:14 PM #16
      Quote, originally posted by ShadowGLI »
      Well the conclusion I am grabbing out of this is that performance is basically the same when using a stock sized turbo and the OEM valves still fail.
      I'm still getting forge unit and never worrying about my DV again.

      I am sure Forge will provide some data to address the comp. surge. If it is truly surging, that would be a very bad thing.

    17. Member TypeR #126's Avatar
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      07-13-2007 07:08 PM #17
      After thinking about this for a bit I'm wondering if the noise people with tubular intakes have reported could really be surge. According to the data posted the potential area of surge only occurs for .3 seconds. Would that be long enough to be detectable to the human ear in a moving vehicle?

    18. 07-13-2007 07:17 PM #18
      Quote, originally posted by ShadowGLI »
      Well the conclusion I am grabbing out of this is that performance is basically the same when using a stock sized turbo and the OEM valves still fail.
      I'm still getting forge unit and never worrying about my DV again.

      Your right... You will be worrying about your turbo. When I first noticed this I did about 3-4 consecutive WOT and then off the gas in my DSG and I could have sworn I smelled something cooking.
      Looking forward to some friendly debate on this:

    19. 07-13-2007 07:23 PM #19
      Quote, originally posted by TypeR #126 »
      After thinking about this for a bit I'm wondering if the noise people with tubular intakes have reported could really be surge. According to the data posted the potential area of surge only occurs for .3 seconds. Would that be long enough to be detectable to the human ear in a moving vehicle?

      .3 seconds is a fairly long time. I also would think that perhaps GTI owners would hear it more if they still have the noisemaker pipe assembly installed. Nothing of the sort comes on the A3. Either way, .32s was the average I found during my testing. Sometimes it was longer, sometimes it was shorter. All the connections were trimmed to prevent any excess since less volume to fill = better response. Another member has contacted me regarding doing the same testing and I believe he has an EVOms so perhaps he would be better suited to comment on it.
      Dave

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      07-13-2007 07:46 PM #20
      Good write up Dave. I like seeing empirical evidence in this forum for a change. I am wondering if a change in spring rate would help as well. I have the trusty old 1.8t valve spring kit so I will wait till forge responds before i try switching them. If i recall correctly the Green is the toughest spring that forge utilizes. Maybe the red or the blue would be better.
      Did you get any of the needle shaking issues that many others have had? what would you attribute that to?
      Edit: also do you think that temperature can have some effect on either valves effectiveness?



      Modified by Rub-ISH at 7:49 PM 7-13-2007
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    21. 07-13-2007 08:05 PM #21
      Quote, originally posted by Rub-ISH »
      Good write up Dave. I like seeing empirical evidence in this forum for a change. I am wondering if a change in spring rate would help as well. I have the trusty old 1.8t valve spring kit so I will wait till forge responds before i try switching them. If i recall correctly the Green is the toughest spring that forge utilizes. Maybe the red or the blue would be better.
      Did you get any of the needle shaking issues that many others have had? what would you attribute that to?
      Edit: also do you think that temperature can have some effect on either valves effectiveness?

      IIRC, Forge mentioned that the 007 springs would not fit on their valve. Nevertheless, I believe they would have selected the lowest spring rate that would hold boost for our application anyways in order to have the best response.
      The needle shaking is attributed to the diameter of opening in the boost gauge tap. Mike@forge mentioned that their machine could not make a smaller hole, therefore many have opted to hack old no-buzz fittings or simply live with it. I found a slightly smaller vacuum restrictor at advance auto parts and used that in-line with the boost gauge tap.
      Temp shouldn't have much of an effect considering once the engine is warmed up, the temperature stays relatively constant around that region.
      Dave

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      07-13-2007 08:25 PM #22
      This is a very good review, I must say, nice write up...
      To me, maybe it's just me, but this looks like a review a company did, and instead of bashing it themselves. They help write a review to bash a product, for their own valve which is coming out soon? These logs are a little advanced for the normal vag com user. But hey, he could just be damn good.
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    23. 07-13-2007 08:32 PM #23
      Quote, originally posted by 97jazzgti »
      To me, maybe it's just me, but this looks like a review a company did, and instead of bashing it themselves. They help write a review to bash a product, for their own valve which is coming out soon? These logs are a little advanced for the normal vag com user. But hey, he could just be damn good.

      Hardly, ask any grad student in the science field and this stuff comes natural when submitting papers for publication. Either way, the only people who had insight into this project were PeteA3, iThread and magilson. I suppose it'd be a big shame if people didn't notice that VAG-COM had the capability to perform these functions with the "advanced measuring blocks" and "turbo" function
      Dave

    24. 07-13-2007 09:02 PM #24
      Quote, originally posted by crew219 »
      Hardly, ask any grad student in the science field and this stuff comes natural when submitting papers for publication. Either way, the only people who had insight into this project were PeteA3, iThread and magilson. I suppose it'd be a big shame if people didn't notice that VAG-COM had the capability to perform these functions with the "advanced measuring blocks" and "turbo" function
      Dave

      Wow Good write up! Im glad I went with a different solution.. I still have no surging, no codes, no cels or anything with it!

    25. 07-13-2007 09:14 PM #25
      Nice write up Dave. I really appreciate the effort and time this review took. My "butt dyno" has never been very accurate, so I'm really digging the growing availability of data-logging and chassis dynos. I need to get more familiar with my Vag-com

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