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I have a question regarding the old Joe P. MBC half throttle full boost problem. For those who dont know what that means ever since i got the joe p manual boost controller, when i step halfway down on the gas i get pretty much full boost. It doesnt bother me, but i had a question regarding it. Does it bother my car? Does it put more wear and tear on the engine? I would assume that the ECU when it sees the increased amount of boost will compensate and add fuel so it will even out the mixture, correct? Do i have anything to worry about as long as my boost is 15 psi and under? I was curious if it was a problem because i am only giving it alittle gas and it is sucking in allot of air, therefor theoritically, the mixture is too lean. I have a stock ECU, JOE P, Greddy Cat back, AEM CAI. Thanks for the help.
 

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MBC + CAI + PTFB = very bad IMHO. I'd take that CAI off and get an uppipe before I put an MBC on, but that's just me. You need to learn to drive around PTFB - read everything you can about MBC's and then you can make your own informed decision.
 

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Vishnu's EMI would help you.

I have a TurboXS HPMBC and when set up correctly I am able to eliminate the part throttle full boost condition... even driving in the mountains. Let me clarify 'eliminate'.... I have never seen it happen and I do watch and listen carefully.
 

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Discussion Starter · #5 ·
Actually i failed to mention that i do have a catless uppipe. The bulk of my question was, does the ECU compensate for the half throttle full boost? why should i worry?
 

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Because the ECU restricts boost with the FBC, none of the maps are setup to deal with the PTFB condition. Therefore, with full boost, you will be running very lean which is not good.
 

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"No it doesn't because you have disconnected the hose to the FBC so it has no idea how much boost you are running."

This is incorrect. The ecu no longer has control over the turbo with an mbc, but it definately knows how much boost the car is running. The fbc is not used to monitor boost, the map sensor is. The ecu will make the same adjustments it would normally make if it had control of the fbc, but at the boost level you want. It really shouldn't effect your engine in a way any worse then pushing your foot down all the way without it. Well you car might wear a little quicker if you bump the boost higher then stock levels, and would definately wear much quicker if the levels are raised to high. I don't think that having a mbc set at 15psi is anything to worry about as long as you monitor your boost(I could be wrong though).

peace
 

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Sorry, but this statement is completely wrong.

The ECU knows very well what boost you are running. The MAP (Mass Air Pressure) sensor is what determines manifold pressure, and it is located at the TB. The only thing the FBC does is controll bleed from the wastegate actuator. The problem comes into effect when you have full or high boost at very low throttle position. The ECU operates in closed loop on preset tables, "knowing" what the boost will be. Since you have now taken away it's control over that, you are getting fuel for half of the air you are pushing in. Only past a certain throttle position (changed for 04's used to be somewhere around 60% TPS, and 04s is near 20% TPS) would the ECU go into open loop where it actually took the MAP sensor readings into account and fuel properly.

VV

BlkWRXWag said:
No it doesn't because you have disconnected the hose to the FBC so it has no idea how much boost you are running.
 

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The way I see it is 15PSI of boost at part throttle is 7PSI above what the stock maps are capable of adding fuel for.

That means {[15PSI + 14.7PSI (atmosphere)] - [8PSI + 14.7PSI]} / [15PSI + 14.7PSI] = [7PSI / 29.7PSI] = 23.6% more air than expected... 14:1 a/f ratio (for example) quickly turns into around 11.3:1 and a 12:1 would be 9.2:1!!!
 

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The only question is whether the ECU has switched into open-loop at this point or if it's still trying to use the O2 sensor to keep fuel metered to stoichiometric (= very bad @ full boost). Since the MAF is metering correctly no matter what throttle position you're in, if the ECU is in OPEN-loop, you're fine. If it's closed-loop, your days are numbered.

Typically ECUs switch from closed-loop into open-loop operation based on either throttle position or boost pressure, or both. If it's strictly based on boost pressure, we're fine. If it's throttle position or a combination of the two, again, we have a problem.

In one of my previous cars, I worked around this by building a small circuit that incorporated an adjustable pressure switch, and spliced it into the TPS (throttle postition sensor) wiring. Whenever manifold pressure exceeded a certain level, it switched a relay and provided the ECU a "false" reading of full throttle (about 5 V) rather than whatever throttle position the sensor was actually reporting. This forced the ECU into open-loop and all was well.

One way to tell whether the ECU is in open or closed loop operation is with one of those otherwise worthless A/F meters with the LEDs. If the LEDs are scrolling back and forth, rich-lean-rich-lean-rich-lean, you're in closed loop. If they stay in one place for any length of time, you're in open loop.

If anyone thinks this might be a useful mod for PTFB issues, I'd be happy to help anyone out with trying this little mod.
 

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Your math is wrong. 9.2:1 is insanely rich, not the lean that it would actually be. It is air:fuel ratio...so it would be over 14 parts air to 1 part fuel...

VV

rexxer said:
The way I see it is 15PSI of boost at part throttle is 7PSI above what the stock maps are capable of adding fuel for.

That means {[15PSI + 14.7PSI (atmosphere)] - [8PSI + 14.7PSI]} / [15PSI + 14.7PSI] = [7PSI / 29.7PSI] = 23.6% more air than expected... 14:1 a/f ratio (for example) quickly turns into around 11.3:1 and a 12:1 would be 9.2:1!!!
 

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Hippy....You are theorizing about something that doesn't exist. The Subaru ECU is in closed loop until a preset throttle position...do you understand what closed loop means? There is a problem here, but only in closed loop....which can be at less than 60% TPS for pre '04 cars.

VV

Originally posted by hippy78
The ecu changes the fuel and timing based on more things then throttle position. It also takes into consideration things like maf coltage, map pressure, a/f ratio, rpm and what not. The ecu will compensate for the extra air and pressure! If the engine knocks, the ecu will compensate even more. I don't see the problem....

peace
 

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does throttle position really matter so much? I mean the maf tells the ecu how much air is going into the engine, the map tells it what pressure it's going in at, and the rpm let's the ecu know what part of it's tables to be using... I mean it would be a bad thing if you were pushing full boost at 14.6/1 a/f ratio. The ecu switches over from closed to open loop at 63%tps and or 5500 rpm, but I also figured that at a specific pressure, it would also switch over just as some type of safety feature.... Maybe I was wrong?
 

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hippy78 said:
does throttle position really matter so much? I mean the maf tells the ecu how much air is going into the engine, the map tells it what pressure it's going in at, and the rpm let's the ecu know what part of it's tables to be using... I mean it would be a bad thing if you were pushing full boost at 14.6/1 a/f ratio, but I'm pretty sure that the open loop fueling kicks in at a specific pressure also doesn't it? Shouldn't it, at least for safety?
In closed-loop, the O2 sensor dominates over the MAF - it meters the incoming air, which provides BASELINE fuelling, then the O2 sensor says "hey this is too rich for part-throttle" so the ECU pulls fuel as much as it can (usually the O2 sensor can shift the map like +/- 15-20% in my experience). So it WILL try to run lean if it's in closed-loop. If VV is correct and it's 60% throttle pos. that switches to open-loop, then my suggestion above would be helpful in dealing with this stuff.
 

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If it DOES have a MAP-based switch to open-loop, it would be fine, but sometimes they don't do that. Someone more familiar with the ECU would be helpful in here.
 

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well, it's obvious to me that certain cars will get more boost at a certain tps then other cars. This could be be based on many things, most contributing to load(like weather). It would be weird if all wrx's ecus did not have a pressure bases closed to open loop change as well as the others. That's just common sense though, and might not actually be the case(I dunno)......

peace
 

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hippy78 said:
well, it's obvious to me that certain cars will get more boost at a certain tps then other cars. This could be be based on many things most contributing to load(like weather).
But remember, even if car "A" spools faster than car "B" due to weather or magical mystery juice or whatever, the factory ECU uses the stock boost control to keep you from reaching full boost under partial throttle, so it's possible that the ECU has no provisions for the possibility that you could hugely exceed the mapped boost pressure.
 
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