I wanted to post my observations on the stock boost solenoid. Possibly some of you may have more information on exactly what this does and how it works….

It seems that it is a simple open/closed electronically controlled valve that opens to bleed pressure from being seen by the stock waste gate actuator which seems to open “fully” at ~16.5 psi. The solenoid opens briefly to allow boost to increase past the 16.5 psi to ~18.4-19 psi and then closes causing the actuators to immediately open and bleed boost to the regulated ~16.5 psi. If you think about it this is the only way this could be working, as the actuator is a mechanical switch that opens under a preset pressure. In may ways this acts much like a very simplified version of the EBC currently on the market.

In light of this, disconnecting the stock solenoid ensures that you will always run ~16.5 psi and never peak higher.

I am quite not sure what triggers the valve to open; it is closed when the engine is off. I assume it is RPM related or something like that.

 

It is ECU Controlled, it appears to be energized until a pre-set RPM, but it may have other contributing factors.. You basically made the same observations I did.. I do think that if the engine detects bad gas (low octane) and goes into its alternate fuel map, that the engine will not boost above 16.5psi, I'm not inclined to test that theory though..

 

Your assessment is a good one.

Not sure if the wastegate actuator is actually set at a baseline of 16.5 psi, or if the FBC is still bleeding-off a little pressure to achieve 16.5psi.

A quick test would be to pull the vaccum line off of the "T" fitting feeding the FBC. Cap the open fitting on the "T", and plug the open line to the FBC and go for a test drive. At that point you would basically be running static boost.

I did this test on my Syclone last week and was running 8-9psi static boost pressure. VW 1.8T's usually run about 5-6psi static wastegate boost, and my previously owned WRX was similar to my Syclone at around 8-9psi if I remember correctly.

Note: Not capping the "T" will result in immediate overboost as the open fitting would bleed directly to atmosphere, so if anyone tries this test make sure you have everything secured!!

16.5 psi is high for wastegate static boost, but who knows anything is possible. I would do this test myself, but I don't currently have a boost gauge installed yet.

 

I'm suprized a EE hasn't come up with a Radio Shack patch for full boost to red line... or did I miss something?

 

I actually was considering doing that (removing the hose and plugging it) Obviously there is something metering the air in the bleedoff, but logic would dictate that if the solenoid is normally closed when the engine is off, then under full pressure the actuator is calibrated at the lower boost level, and then the bleedoff has a restrictor that maintains a peak boost of 19 (or so) when the solenoid is open.. It makes sense only because as a safety measure it would make more sense that if the solenoid failed and closed, you'd get 16.5psi,

also, nobody has really established if you kept the solenoid open that the boost could spike wildly and the dropoff was actually to keep the boost from ever peaking over 19psi... of course testing that theory would require wiring the solenoid to stay open..

Actually, Another thought is there is a dual restrictor, when the solenoid is closed, it bleeds off boost pressure at a different rate, when its open, it bleeds at another rate.. in any case, your right, the static unregulated boost pressure would probably be around 10psi if that was the case...

I also noticed that the BOV on this car is ALWAYS bleeding off, a little too much, which definitely seems to hamper boost a bit.. especially in part throttle driving

 

I have it disconnected and pugged now... It is much less responsive, but the power is smoother. I don't have a boost guage yet so I can't provide much detail on the levels I am now running.

 

Any info on the static boost without this solenoid? I had an idea about modifying a stock solenoid, but I need its part number... I want to buy a few, take them apart, and see how they really work...

 

Looking on mitsubishiparts.net this may be the part number: MN153089. Their description is "Actuator+t.c Wast." Otherwise you may be able to call them or submit a parts request to see if they can get the exact part number.

Edit: You can also get a circuit diagram of the solenoid circuit on mitsubi****echinfo.com. Just do a search for solenoid under the Evo service manual area.

 

Thanks.. I got the wiring diagram.. its the actual function I'm curious about.. therefore I need the physical part to determine if it does a partial bleed when closed, and then bleeds more when open (to drop the wastegate opening pressure) if that is the case, you can calibrate it for higher boost by altering the ports for more or less air bleed.. (Higher boost, stealth mod.. with a safe bleedoff)

and just so you guys know.. Which I feel this is a little odd because my car seems a bit more powerful than others.. My boost is actually peaking at 20 PSI.. not 18.5 as others are reporting.. I checked this again with a second gauge.. and it falls off to 17 (16.5 or higher depending on the gear) hows that for odd??

 

Ok.. Sorry for the delay, I needed to install a boost guage. With it disconnected the car runs about 11-12 PSI.

 

cdavy- thanks for confirming the static boost pressure, thats good info! If static is around 11-12psi, that means that the stock solenoid opens to a max aperature to provide 19.5psi and then closes back down in size, still bleeding enough air to provide the 16.5 psi.

I know that in some instances it is possible to use an alternate part number/different solenoid to get more PSI while retaining the ECU driven boost control. The VW 1.8t N75 valve comes to mind, with an obscure highly guarded secret Audi part number, which when plugged into the factory connector would provide an additional 1-2psi over stock!

In the EVO's case I don't think that magic bullet exists as this car seems to run more boost than any previous EVO/diamond star. Hmmmmm....I wonder if the EVO solenoid could be used on other DSM cars to achieve higher boost values? I also wonder what the old 3000 gt and stealth TT's ran for stock boost, anyone know?

Malibu Jack -The only option may be like you mentioned, take a stock solenoid analyze it, and see if it can be modified. If the aperature can be physically modified by minimal drilling you might be able to achieve a stock type boost taper running 20-21 then dropping to 17.5-18.5?
More than likely given the right voltage value, I bet the solenoid can be tricked into opening to a larger bleed aperature. After all we know that ECU tuning will achieve this same goal, so obviously it can be done. Short of having a re-tuned ECU, perhaps building an electronic modifier ckt could get you the voltage to effectively increase the boost relative to the stock ECU commands? I think this is in the same vein as the comments Zeus made earlier.

BTW some stock boost solenoids have adjustment screws(again the VW/Audi N75 comes to mind). I haven't gotten a good look yet does the stock EVO boost solenoid have any adjusters? Some people claim they have gotten boost value changes by FBC solenoid adjuster screws and others claim no affect whatsoever??

I guess some would just say throw a cheap MBC on it, but a stealth modded stock FBC would be really cool and still give some boost taper protection at higher rpms.

 

That was what I was getting at...

...and this was the point in the first place...

"I guess some would just say throw a cheap MBC on it, but a stealth modded stock FBC would be really cool and still give some boost taper protection at higher rpms."

 

Well actually Zeus, the original point was cdavy posting his observations about how the FBC really works. One of the unanswered questions involved understanding if 16.5 psi was "static" actuator boost when you removed the FBC. Thanks to the dialogue in this thread and the test cdavy performed we proved that the static boost is 11-12psi not 16.5psi. I haven't seen that info anywhere else yet, so I think thats good info.

As far as the FBC, variable voltage values provided by the ECU open and shut the FBC aperature by varying degrees. If we can find out what those voltages values are then theoretically a circuit could be built to modify those values.

That won't necessarily provide a straight 19.5 psi to redline, but could provide a way to adjust the boost incrementally across the range.

You know I guess I just need to do you guys a favor at this point and back-off.I apologize for throwing around too many ideas and getting wordy. It sounds like you guys are on the right track ......good luck!

BTW, cdavy....thanks again!

 

Although I havent directly analyzed the voltage off the solenoid while in operation, It looks like the kind of part that is either on or off.. Solenoids are generally used either on or off.. In fact, they work alot like fuel injectors (considering they are essentially solenoids too)

Now.. Here's the tricky part.. Does the solenoid work by either being on or off.. In that case the orifice size is calibrated to bleed a certain amount of air when off (16.5psi), and additional FIXED amount when on (19.5psi), you technically can establish that by disconnecting the wiring harness to the solenoid, and running the car. If the boost is 16.5, then we can assume this is at least partially true.

At that point it would require reading the voltage, and the frequency of the signal (if it remains fairly constant) although on an analog multimeter, a lower frequency may read as a lower voltage) because you would be reading the duty cycle of the solenoid.

Another thought is this works somewhere in between.. however if you listen to the solenoid closely, you may hear it buzzing or ticking.. How you would do that at different boost levels I have no idea.. hehe..

Any of this make sense?

 

Oh.. and before I forget.. The boost solenoid from another car may be a good idea.. Keeping in mind that the boost levels are irrelavant between the cars, but the RATE at which the actuators pressure line is bled off, and the size of the vacuum lines, since all of that controls the rate of pressure rise, volume of pressure (since volume the volume of the actuator diaphragm, and its surface area also govern the pressure required to govern its rate)
all of these factors contribute to max boost, min boost, overboost and spike prevention, etc..

Before I get slammed for providing incorrect information, I'm not saying that the different solenoid will or, or that boost levels are truely unimportant for the part.. I just mean that these solenoids work by bleeding off a set amount of air to offset what is already there.. So the function of the solenoid is partly a factor of the properties it has to manage, in theory that means that if a 2G DSM allows more air through the solenoid while closed and open (assuming it functions the same way, and has the same size VACUUM lines) It should mean your boost levels should be higher.. The important thing to also point out is since its an unknown.. Just trying it could either lead to serious overboost, spiking, low boost, or whatnot depending on its characteristics.

This obviously demonstrates why electronic (third party) boost controllers are so expensive, and why they generally are preferred by many over manual ones. Personally once I have a few extra dollars, I will swap out the Hallman for an Apexi S-AVC.