It still seems to be unclear to some about the boost control restrictor. There is a brass restrictor in the boost controller hose that goes to the wastegate. It is approximately 1" or so from where the hose connects to the controller.
Below is a pic of the restrictor, given it is so small it is difficult to photograph. The result was a spike to 21.5 (maybe 22) and it tapers down to 18 and holds at 18 all the way to 7000 RPM. This is with a 3" DP and stock exhaust. As soon as I get the exhaust on it may go higher.
Next plan is to enlarge the restrictor. It currently fits a 1/8 drill bit perfectly. I am going to use a 5/32 bit to open up the hole and test it on the car. Looks like 5/32 is the only upsize that will work.

 

Maybe I'll go yank mine out...

 

I know higher boost is supposed to translate into more power. So does it feel any stronger with the restrictor out ?

What's your impressions ?

 

Well I plan on putting the restrictor back in after modification but I will try and get a good road dyno run on it without the restrictor.
It's hard to feel a big difference but I did not drive that much and the weather sucks right now. Hot and Humid.
I've seen it elsewhere on this site that it is about 10hp or so gain so it's hard to feel that. I will know more when I drive it more.

 

I wouldnt even be messing with it. Just get an $80 MBC and hold 20 all the way to redline.

The only reason the stock boost solenoid hits 19 is because it spikes there from 16...

 

Correct me if I'm wrong, but if the restrictor is between the solenoid and the wastegate actuator would'nt you want to make the hole in the restrictor smaller rather than larger? Less pressure to the actuator keeps the wastegate closed longer equals more boost.
Cheers,

 

wtz, you're correct. More restriction means slower wastegate opening means more boost. Basically the concept behind a manual boost controller...

 

I don't agree.
The simple explanation is, the removal causes the boost to hit 22 and taper down to 18. So how is making the restrictor smaller going to help?

 

Removing the restrictor pill works because there is a three way split, a vacum line goes from the turbo oulet to the waste gate actuator and then splits off to the selonoid. The selonoid increases boost by venting some of the pressure going to the waste gate actuator, if you remove the restrictor it vents more pressure away from the waste gate actuator giving you higher boost. The other selonoid vacum line vents back into the intake. The MBC uses a slightly different configuration so...

 

What happens to fueling at the increased PSI's?
Are you now running leaner or is the ECU compensating and adding more fuel (so you still run factory rich)?

What is the max pressure that the EVO ECU is able to compensate for and what about the limits of the injectors/fuel pump?

 

I don't think so, Until a point. Remember that you have a rising rate FPR< so for every PSI of bost, you get another PSI of fuel pressure. You should be fine, but I beileve that these gains would be more realized after a A/F tuning is done.

 

I took it out on my EVO. I can feel the gains out of it, not extreme, but enough to actually feel. I think that I will end up putting it back in and getting an MBC though.

 

timzcat, why are you planning to enlarge the restrictor instead of just leaving it out?

 

CT9A, I can feel the difference, too but it's not huge. Although I had a nice 4 wheel drift going tonight in the rain around a corner.

I am enlarging it because with it removed it is peaking at 22PSI which is kind of high. I don't want the ECU to start pulling timing, not to mention it is summer time. I already did the enlargement tonight to 5/32 which is the only choice unless you can get a 9/64 drill bit (smaller). I will put it in tomorrow and see what happens.
I am hoping it will knock it back to like 20-21 peak and maybe hold 17 or 17.5 up to 7K.

 

Talking about restrictors, I feel like dropping a word. Feel free to add comments as I am not an extreme expert on this, and will never be. But, this is all from my experience on my stg4 wrx, and recently on my evo as well.

If you are bone stock and not running any type of boost controllers, enlarging the the restrictor will actually reduce the boost your car can make. Worst case, replace the line with the restrictor inside with a regular vacuum hose will only give you ~10psi which is what the stock actuator can make. This is because the stock solenoid cannot work over the larger flow. This happened to me two times - one on each car - when I took my ebc out and return it to stock with my own vacuum lines - I didn't know why my car is not boosting until I remember the restrictor.

Some boost controllers actually work better with a restrictor still intact - especially the ebc ones with gain/start boost controls like the SBC-ID2, ID3 and the Profec B spec 2. In these units, without the restrictor will like cause the unit to be "out of tune". The ability of these units to force boost to come a little earlier would be interrupted and easier to cause spikes. I found out about this when my ID3 and spec2 throw me warnings but my car is boosting fine. But with the restrictor in, no more warnings and steadier boost. This is from my very own experience on these units along on a dyno, and some street tunes.

I cannot comment on a mbc with or without a restrictor, since I have never tried it myself.

I know it is a common practice for DSM and Subies as a "free mod" about the restrictor. Like the all famous 3/16 mod on the WRX. My assumption is that it works great with a MBC... but in my case, having and leaving the restrictor alone is better off all the time...

-Kenneth