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drs_install <-- search key word, most posts are on the NG900 BB
I made an adapter to allow for a 25mm blowoff valve to be installed between the throttle body and the idle air control valve. (I feel that it is important for it to be installed on the highside of the IAC valve to avoid putting negative pressure on it.) This was done a couple of weeks ago.
I made the BOV into a pop-off by shimming the spring with the 6 washers that came with the valve, as well as 3 nickels which happened to fit perfectly. This was serviceable, but hardly adjustable. Yes, most purpose built pop-off valves can have their spring tensions adjusted via an ajustment screw.... but keep reading.
The characteristic of a pop-off valve is a lift off pressure, a pressure where flow gets serious and a increase of flow after that that is proportional to the pressure as determined by the spring rate.
A longer spring and spring chamber will allow for a lower spring rate which will make the unit more sensitive and offer better pressure control. I put a small pressure regulator into the mix. It gets the boost pressure and regulates it then applies the regulated pressure to the BOV's hose barb. So with all the shims and nickels removed from the spring chamber of the BOV the valve will start blowing at around 10PSI. By applying a regulated pressure to BOV's hose barb and spring chamber, the pop-off characteristics of the valve are easily adjustable via the knob of the pressure regulator. This makes for easy adjustment. It also makes the valve closing pressure equal to the spring preload pressure, plus the (quazi static)air pressure from the regulator. The spring can be weaker and have a lower spring rate and the regulator applies a pressure that is independant of the piston motion. The pop off valve is more sensitive VS a spring only arrangement.
I have a MBC pressurized from the throttle body. So the tendency for boost pressure overshoots is increased. I an using the BOV to eat the transient overhoots to allow for MBC setting closer to the ECU's fuel cut pressure. It seems like the recently installed Viggen IC does reduce the boost pressure overshoots. But the real test of things will be with the return of cold weather!
To dial in the initial setting of the pop-off contraption, I made a 'christmass tree' which has a air chuck fitting for my shop air. It has a 1/8" tube to allow my boost guage to be removed from the vehicle to get pressure references that relate to what my boost guage shows. Who knows what these guages really read? Use the same guage and it tells you alot. So I can take the pop-off from the vehicle and attach it to this test rig. The pressure regular take pressure from the same test fitting which has a 1/4" barb for it. I can adjust the pop-off valve regulator then adjust the regulator of the shop air and watch the boost guage readings and watch where the valve lifts of its seat, when the piston gets to the blow off ports and how the valve blows as the shop air pressure increases from the there. I am quite happy with the way that unit works. I did some intial vehicle runs, but it is really too hot outside to bother with more of that. What I did find, in hindsight was that the shimmed valve was leaking. It could have been set just right or leaking. But my power is greater so it was leaking. I did not get any fuel cuts with the prior arrangement, but was able to get fuel cuts with the new arrangement with the same MBC setting. I had increased MBC settings a bit earlier trying to find out what the shim only arrangement was doing. So it is clear that it was blowing some flow before. I was not able to hear any leakage from the unit.
A MBC presurized from the throttle body will have much better boost at high RPMs than from the turbo compressor's scroll casing hose barb. The downside is time lag induced pressure overshoots. All of these efforts are aimed at refining the use of an MBC. Some will feel that at this level of effort and incidental expenses that a modified ECU would be more practical. I can't totally disagree. It is all an interesting learning curve. The final evaluation will be in the winter. Many MBC arragements can be very prone to fuel cuts in cold weather as the turbo spool up very much faster. The lag in the boost controls response leads to this.
Another speculation has been that the base setting for the wastegate is too firm for a ball-spring MBC. A softer mechanical setting might increase the slew rate of the wastegate in reponse to the MBCs signal. Again mucking with the mechanical adjustment would be a pain in the butt. So a pressure regulator could be put in to play to create an initial pneumatic preload on the wastegate actuator. Then the wastegate could be opened faster when the MBC produces flow. This might seem counter intuative. But the wastegate preloads which are set for use with the BPV bleeder valve charactgeristics are not suitable for use with a MBC which avoids all premature pressure on the actuator. This arragement would be a highbred MBC with a threashold pressure plus a preload instead of a threashold pressure only. So there is my next and perhaps final project. But then I will have to determine if the highbred MBC or pop-off arrangements are adequate by themselves... I guess there is more to be done.
If the change to the wastegate arrangement has merit, the presure to crack open the wastegate can be measured, then a one time effort could be made to adjust to this mechanically and remove the pressure regulator etc. If there is enough mechancial adjustment range in the actuator linkage, then this would be great nil-cost mode for folks with MBCs.
95SET 151000 miles
TurboXS HP MBC (Dawes works as well I expect, $35 VS $129)
TurboXS BPV
TurboXS 25mm BOV with hose adapter converted to pop-off service
Saab Sports exhaust
severely 'ported' throttle body transistion casting TBTC
Viggen IC
turbo compressor discharge silencer was removed, but with Vikggen IC the disinction is meaningless
VRK
SAS rear bar
Koni's
K&N with airbox funnel mode, need to get a 3" tube mod done now
posted by 65.68.100...
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