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mike01s4's Audi fitment Diverter Valve Testing:
by Mike Shimon (VAST Performance)

All tests were done off the vehicle.

Diverter Valves tested:
1. Stock Audi S4 turbo diverter valve: Bosch 108 pn: 0280142108 (audi pn: 034145710A)
2. Forge diverter valve with aluminum piston, soft green spring, and updated o-rings
3. Bosch 110 pn: 0280142110
4. Bosch 114 pn: 0280142114 (Audi pn: 06A145710N)
Some purposes of test:
1. analyze the piston displacement (travel) versus vacuum pressure
2. analyze if flow rates differed between valves
3. determine the pros and cons of each valve
Some test equipment:
1. Linear transducer accurate down to 0.0001 inches. % FSO error unknown.
2. Rigid clamping fixture for securing DV and measuring piston displacement
3. Vacuum pump with analog mechanical vacuum gauge. Accurate to +/- 0.25 in Hg.
4. stopwatch
5. graduated cylinder (20 fl. oz)
6. varied pressure constant fed water source

To obtain the data for displacement(piston travel) versus vacuum pressure, I applied a vacuum in a slow fashion in ~0.5 in. Hg steps. The vacuum pressure was increased until the piston would not travel any further. I also noted whether or not the valves stuck, or had an inconsistent pull, or leaked. I took two to three trials for each valve, and the measurements were quite repeatable. I obviously could not simulate the ON-vehicle pressure fluctuations and measure displacement.

The forge valve needs the most amount of volume displaced in order for the piston displacement to achieve a full open state. This is a point where the Bosch bellows style diaphragm design is superior. I am making an educated guess that the Bosch will respond quicker to opening due to this fact, albeit, it will probably be only milliseconds faster. Also, it looks as if the 110/710N will take a longer time to open, but, just because it comes open a few in. of Hg later, the typical vacuum pressure developed can be ten times this difference. So there should be no worries with a slow response time with any of these Bosch valves.

One thing that I could not check, is the reaction time for the valve to close after a full open state. But, since the spring rates are now apparent for each valve, all other things equal with the Bosch's, you can see that the 110 and 710N will close the fastest. The forge's closure rate will not only depend on the spring rate, but also the grease used and temperature. With insight from this plot, the forge will close the slowest, then the stock 108 is a bit faster, then the 110 and 710N are the fastest.

Between the 110 and the 710N, you can see that they are exactly alike. Unless Bosch changed the materials used in fabrication for the diaphragm, these valves are totally equal. I did not destroy the valves to inspect the differences since I was using other people's parts and it would cost around ~$50 to do so.

I also checked the ability to open under a cold test of 20 degrees Fahrenheit. The Bosch valves would stick, but only occured once, and only once, on an initial applied vacuum. So, cold runs in the vehicle should never even be a problem. The forge did not stick. I did not know what the relative humidity was when I did this.

As far as diaphragm or piston leakages are concerned. None of the Bosch valves leaked. How they perform after many cycles in an engine's environment, I could not say, but they will not leak unless the diaphragm tears or if the housing cracks. It is very easy to check leakage. All that is needed is an applied vacuum and a little time spent on looking to see if the vacuum drops with time.

On the other hand, the forge will leak if not sealed properly or contaminated by debris. From the opened valve picture, you can see where the Forge can leak. All that is needed is a cleaning and a slight medium to heavy weight grease to seal this. With the Forge, I noticed anything from a near perfect seal, to a low of ~1 psi loss per every 5 seconds and then much worse as a small particle contaminated this seal.

On to the flow rate tests, I found that the bosch and the forges still have the same flow rates while testing with a complete inner fill of water flow. With the valves completely opened, each valve could fill 20 fl. oz in approximately 8 seconds. Of course, even as I put restrictions on each valve, maintaining the same inlet pressure, each valve still achieved the 8 second value, obviously with a greater exit speed.

Bosch and Forge Diverter Valve Plot Chart


  • can maintain it at any time
  • can replace spring with a higher rate
  • virtually a lifetime part
  • smooth/open air flow path when compared to bosch, BUT, still has the same flow rate up to a typical working pressure
  • comes with viton O-rings which have superior temperature and wear characteristics
  • Cons:
  • price
  • fitment, due to larger size, is not as good as bosch(the passenger side vacuum hose can easily kink if not installed carefully)
  • slower response time
  • can have an inconsistent piston travel if not lubed properly or not maintained
  • possible sealing issues if not kept clean
  • Forge Diverter Valve Open

    Forge Diverter Valve Side

    BOSCH 108:
  • price
  • slightly higher spring rate than forge
  • quick response time for opening
  • no fitment issues
  • no warranty issues
  • Cons:
  • cannot inspect or service valve completely
  • possible problems noted with many users for diaphragm tears
  • max piston travel, for its design, is not as large as the 110/710N
  • air flow path not as smooth as forge, but still has the same flow rate up to typical working pressures
  • Bosch Bypass Valve Side

    BOSCH 110/710N:
  • price
  • larger piston travel compared to stock
  • with a small bellows volume: quick response time for piston pull
  • with a higher spring rate: quick response time for piston closure
  • possibly more durable than stock
  • no fitment issues
  • no warranty issues
  • Cons:
  • cannot inspect or service valve completely
  • air flow path not as smooth as forge, but still has the same flow rate up to typical working pressures
  • 2/25/02

    Update: 03/05/2002

    This new graph includes the piston travel for different spring rates with the forge DV. there are 3 springs: green(comes standard) = softest, yellow = medium, and blue = stiffest. for all practical purposes, you can see with the yellow spring that it performs like the stock 108, and with the blue spring, it performs like the 110 or (114)710N valve. Basically, the downside with the stiffer springs in the forges as compared to the bosch is that you are slightly reducing the response time of the valve even further. This is due to the fact that the bosch valve needs a smaller volume of air displaced to pull its piston open or force it closed.

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