Why is it that our cars go from 19psi to 17 psi at higher RPM... And what is the fix for this?

Thanks

 

There is a boost control solenoid under the air intake box. When the ECU senses the desired RPM range it tells the solenoid to slowly open the waste gate on the turbo. This is just another safety feature designed by Mitsu to keep the average Joe from blowing up his motor. You can remove the two vacuum lines to the solenoid and run a U-joint between them. That will keep your boost at 19 psi but leave the connector plugged into the solenoid or you will get a CEL.

 

Hmmm now is this a bad thing to do, I rev up to my 7g limit often enough, but i mean after i do engine management and the like, i assume i will be running around 22psi anyway. Do you think doing this fix to keep 19psi is a safe modification and what could make it dangerous?

 

Ok after some research, i hear that i should just leave everything plugged in and normal, and just get a boost controller, which supposedly fixes this and holds the boost the whole way.. Yeee haw.. buying one now

 

To answer the question....California's sh*t gas. Mitsu couldn't afford to design a special one for them, and had to dial out the boost to protect the motor from their 91 octane ****.

Think about it...

 

This "boost taper" is a normal thing, its a safety feature programmed into the ECU. Best and safest workaround is a P1/P2 brainflash from Works. Only reflash that holds 19psi boost to redline.

I've had mine for a while and love it.

 

you guys keep talkin bout the cheapest way....what I posted above is the cheapest way and the info was given to me by my very own Service Rep at Mitsubishi where I purchased my Evo! .30 cent for a damn U-joint and it won't cause any problems.

 

well when you get a boost controller, you are going to half to unhook the BCS anyway, because the Boost controller will take its place.

 

Good luck with that. My boost still tapers even with a MBC. Goes from 20 to 18.5ish at redline.

 

There's a thread on here about this very thing. I'm not sure how full proof it is, but supposedly the clamp on the bypass valve isn't tight enough from factory and it leaks pressure through there. Don't know if this answers your question or not. The thread is "Free Horsepower by tightening 1 clamp". Maybe this will help.

 

That's a good one. Where can you buy a U-Joint. Also, one of the vaccum line is easy to remove, this is the one that goes to the intake pipe going to the turbo. The other one looks likes like it has been glued, the one that goes to the waste gate. Should I cut the line or can it be removed. Help.

 

Well, I don't agree with your service rep at all. Maybe he likes having his guys replace blown motors, who knows. But putting a U-joint in those lines is a BAD idea.

Explanation....

Your wastegate operates on pressure to open it. The simplest setup is to run a line from the compressor side of the turbo to the wastegate actuator, that way when the wastegate sees the pressure level it is designed to open at, it opens and your boost holds at the preset level.

On the Evo, mitsubishi included a boost control solenoid as a way to increase boost levels above the wastegate's preset level. The wastegate spring is designed to open at a boost level much lower than 19psi. Mitsubishi put a T-fitting in the line going from the compressor to the wastegate actuator, this line goes to the boost control solenoid. The other line on the BCS (boost control solenoid) goes back to the vehicles intake post-MAF. The BCS is essentially a stepper motor actuating a valve. When the valve is open, it bleeds the pressure from the compressor wheel back into the vehicles intake, essentially making your wategate actuator line act like it's got a leak and is venting back into the intake.

Pressure flows to the path of least resistance, so instead of opening the wastegate, the air flows into the intake and then back through the compressor and just keeps circulating around. When the ECU wants to lower your boost level, it starts closing the valve increasing the resistance of the air flowing back into the intake, so when the air no longer has that path to take, it forces open the wastegate, lowering your boost level.

If you simply bypass the BCS the wastegate will essentially never see any pressure to open. It will only see pressure when the flow gets so restricted through the old BCS lines and U-joint that the air backs up and starts opening the wastegate actuator.

Granted, this "back-up" of air may occur at 19psi, but there is no way to easily know that. Trusting that to occur is a very easy way to overboost your motor. The correct way to control the boost level is with a MBC or an EBC, there is no other easy fix.

HTH

Maybe you got lucky and your setup seems to work, but I would NEVER trust it myself.

- Steve

 

It seems like if that is the case, the BEST way to regulate boost is with an ECU flash and the cheapest is with a MBC. No?

 

Yes, an ECU reflash would be the best. I know the XFlash from Vishnu regulates boost. And when you think about it, you get an ECU tuned for higher boost, it is controlled the way intended it, the timing and fuel maps are revised for more power, and it's the same price or cheaper than a quality EBC. Granted, you can't change the boost yourself, but if you don't want to tune it, but rather just enjoy more power, it's a great way to go.

- Steve

 

is what OnlineAlias said true?? are the evo's in the european and japanese markets not tuned to taper the boost like ours? My last question is if you are running on 93 octane, would it be equally safe to the stock taper to run 19 psi to redline?