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This is a discussion on WOT Engine Problem within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; Or is it the case that the MBC causes the turbo to spool up under low throttle conditions, in the ...

  1. #16
    Registered User Killerkanadian's Avatar
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    Or is it the case that the MBC causes the turbo to spool up under low throttle conditions, in the same way it would spool up under full throttle because until a certain boost is reached, it completely cuts off pressure to the Wastegate Actuator?

    The only thing I wouldn't understand then is how the wastegate opens under low throttle driving when it needs to reach a certain pressure to open the wastegate anyways.

    Sorry if I sound stupid, I'm just trying to understand.

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  3. #17
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    The problem with a MBC is that you can reach full boost at low throttle positions where the ECU is expecting 8 Psi because that's what the stock boost controller is supposed to be holding the boost at. But the MBC isn't holding the boost back at all until it reaches the 17 Psi that you have set it to, thus when you are driving the car and come to a hill suddenly the boost can go up to 17 Psi even tho the throttle is still at 1/3 to 1/4 making the car run dangerously lean... Having said that you CAN get away with it, if you are careful to keep the boost at the 8 Psi level max by backing off on the throttle or giving it 3/4 or more throttle and go full boost... I drove my '02 WRX like that for 7+ years, but the '02 and '03 WRX was more forgiving than the '04 WRX...
    '02 WRX 4EAT, 311,000KMs/194,500Miles, Sold. Best car I ever had. '09 STi stage 1, hope this one will be good too...

  4. #18
    Moderator Donkey's Avatar
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    Quote Originally Posted by Killerkanadian View Post
    Well here's the thing. I don't have the stock resistrictor pill which comes with the car. If anyone could link me to where you can buy it (i believe it only sells with the whole vacuum line assembly), I would appreciate that because I would rather be using that set up. However since I don't have that set up, my low tech MBC is basically doing the job of the missing restrictor pill.
    No, it's not.


    Quote Originally Posted by Killerkanadian View Post
    That's just my logic anyhow. I know it's wrong according to Cobb, but I'm not seeing how it would cause my car to run lean. If I am wrong I would gladly listen to why my I am incorrect.
    It runs lean because of the closedloop/openloop fueling parameters and PTFB. Most tuners remove the CL/OL fueling delays and readjust the OL fueling table to accommodate proper fueling during boost transition. If what I am telling you sounds like greek then that would be why you don't see the issue . It's no secret that OBDII cars number one programming priority is emmisions and fuel economy. Second might be "reliability" with engine performance being maybe being next. You didn't buy a Ferarri or Porsche. You bought an econobox USDM Subaru running an aftermarket OTS conservative map.
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  5. #19
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    Quote Originally Posted by Donkey View Post
    No, it's not.


    It runs lean because of the closedloop/openloop fueling parameters and PTFB. Most tuners remove the CL/OL fueling delays and readjust the OL fueling table to accommodate proper fueling during boost transition. If what I am telling you sounds like greek then that would be why you don't see the issue . It's no secret that OBDII cars number one programming priority is emmisions and fuel economy. Second might be "reliability" with engine performance being maybe being next. You didn't buy a Ferarri or Porsche. You bought an econobox USDM Subaru running an aftermarket OTS conservative map.
    The COBB map does that so he really doesn't need to worry about the Ol/Cl delay, it's the FBPT that if you don't tune for can bite you unless you drive accordingly. (keep boost at 8 Psi at part throttle or go 3/4 to full throttle and full boost)
    '02 WRX 4EAT, 311,000KMs/194,500Miles, Sold. Best car I ever had. '09 STi stage 1, hope this one will be good too...

  6. #20
    Registered User Killerkanadian's Avatar
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    Okay, I'm understanding a little better. I don't really see boosting over 8psi at part throttle unless I was just putting around between 3-4k waiting for a take off, and I really don't think the car is going to boost hard like that anyhow. I've been driving around with my Cobb AP mounted to my dash today and the boost doesn't ever seem to get excessive with out hitting the throttle hard. Again, I am no expert, and am obviously still trying to figure this stuff out so I could probably be wrong.

    But I appreciate the input and explainations. I'm in the market for a new restrictor pill if anyone knows where to order one.

  7. #21
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    Yea, normal driving is not going to be a problem per say, it sneaks up on you... FBPT happens when you just start to press on the throttle to pass someone or when your cruising and have it on cruise control at lets say 70MPH and a slight hill that you don't really think much of comes around, an blammo... You have 17 Psi with only about 1/4 throttle... With a MBC YOU ,are in control of the boost and have to drive accordingly...
    '02 WRX 4EAT, 311,000KMs/194,500Miles, Sold. Best car I ever had. '09 STi stage 1, hope this one will be good too...

  8. #22
    Registered User Killerkanadian's Avatar
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    Filed/Drilled the brass fitting in my vacuum line restrictor so it had a little larger diameter. I'm happy with it for now, and I got to take my MBC back off.

    2nd gear - 14.xx PSI
    3rd gear - 15.xx PSI
    4th gear - 16.xx PSI

    I don't know if that's normal but that's what the car is doing now.

  9. #23
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    Quote Originally Posted by Killerkanadian View Post
    Filed/Drilled the brass fitting in my vacuum line restrictor so it had a little larger diameter. I'm happy with it for now, and I got to take my MBC back off.

    2nd gear - 14.xx PSI
    3rd gear - 15.xx PSI
    4th gear - 16.xx PSI

    I don't know if that's normal but that's what the car is doing now.
    That's much better, at least the ECU now has some control over the boost, Depending on what the MAX boost is supposed to be on that map, you "may/may not" need to go one size larger with the drilling out of the pill/restrictor to lower the boost just a bit more more...


    EDIT; OK, I looked it up and the target boost for your set up seems to be 16.5 Psi so you are there, should be safe ...
    Last edited by wrxdrvr; 06-08-2012 at 07:48 PM.
    '02 WRX 4EAT, 311,000KMs/194,500Miles, Sold. Best car I ever had. '09 STi stage 1, hope this one will be good too...

  10. #24
    OMG WTF MainFrame's Avatar
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    Quote Originally Posted by Killerkanadian View Post
    I'm not seeing how it would cause my car to run lean. If I am wrong I would gladly listen to why my I am incorrect.

    Because the Cobb map is designed to be used with the factor EBCS. Unlike a MBC, an EBCS has the ability to open and close at different boost levels according to what it is being told by the cars ECU. Because the Cobb map you are running was designed for this the map tells the EBCS to bleed off less air at partial throttle (say 85%) than at WOT. This results in lower boost at 85% throttle than you would have at WOT. Since there is less boost at that area of the map the fuel table is also telling the injectors to dump less fuel than if you were at full boost.

    Since you are using a MBC, it can only work at what ever you set it at (17psi in this case). Therefore at 85% throttle you're going to have 17psi, unlike what the map is designed for, which may be more like 14psi. This is called partial throttle full boost. Since the fuel table in the map you are running is only expecting 14psi and it's actually getting 17psi, that's considerably more air than the map is telling the injectors to dump..

    Do you now see how that will cause your car to run dangerously lean in certain load ranges? Lean mixture makes the combustion chamber significantly hotter, as well as potentially creating some rather serious knock. That is not a good thing at all.

  11. #25
    OMG WTF MainFrame's Avatar
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    bleh.. I was stuck on first page, glad to see you got it sorted. I'm tired of surfing on my phone.

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