Boost drop at high rpms
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This is a discussion on Boost drop at high rpms within the Engine Modifications forums, part of the Tech & Modifying & General Repairs category; Hey guys quick question, i just noticed today when giving it some gas that my boost was dropping quite significantly ...

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    Boost drop at high rpms

    Hey guys quick question, i just noticed today when giving it some gas that my boost was dropping quite significantly at high rpms. I'm push 18 psi and when almost at redline it drops down to 13 psi if not lower.
    Anyone know whats causing this and a solution? Thanks! =)

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    Master Baiter EJ257's Avatar
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    Re: Boost drop at high rpms

    Stock turbo? What car? What mods?

    Until you get into bigger turbos, they taper.
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    oh sorry for not listing that info xD

    its an 07 wrx with a catback exhaust, intake, and a tune, i bought the car with those mods too
    and yeah stock turbo

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    Master Baiter EJ257's Avatar
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    Quote Originally Posted by Fuzzybunny5
    oh sorry for not listing that info xD

    its an 07 wrx with a catback exhaust, intake, and a tune, i bought the car with those mods too
    and yeah stock turbo
    It's supposed to do that.
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    Registered User nsibanez's Avatar
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    How do you keep it from dropping boost?

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    Registered User Pylonpounder's Avatar
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    Okay, here's the thing that EJ257 just tried to explain. Stock turbo's arent really there for huge power. They don't push enough air for that. Similarly, when the engine is turning higher RPM's and requiring more air, the stock turbo can't possibly keep pushing enough air at full pressure to keep up with the engine without damaging itself, because the faster the engine turns, the faster the cylinders are evacuated and refilled. To keep up with that exponential increase, the turbo has to push a LOT more air to keep full boost, and the turbo simply can't push that much air. So they taper it off at the end, to save your turbo, and your engine.

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    Registered User nsibanez's Avatar
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    Through the BPV or BOV? Depending on what you have?
    And so all you need to do to keep from losing boost is have a bigger turbo?

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    Boost drop at high rpms

    Quote Originally Posted by nsibanez View Post
    Through the BPV or BOV?
    What?No...the whole purpose of the waste gate is to control boost. The BPV/BOV does not control boost.




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    Registered User nsibanez's Avatar
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    True. So what do you need to do? The wategate is letting olut all of your boost you want still

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    Registered User Pylonpounder's Avatar
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    ....You aren't really getting the point. You don't want that boost. That boost will make you overspin your turbo and blow stuff up that you really don't wanna blow up. If you want that boost, It would require a bigger turbo, probably bigger injectors and fuel pump, and most importantly a tune. You can't just throw a big turbo on there and boost out all day. Well, you can, but "all day" will more likely be a couple minutes before you spin a bearing or something. And when you throw bigger turbos on with more power, you have to worry about if the transmission, clutch, driveshafts, axles, differentials, etc can handle that amount of power reliably. Plus, it's alot easier to blow up an engine with a big turbo than it is the stock one. (Relatively, I suppose).

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    Registered User MainFrame's Avatar
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    Quote Originally Posted by nsibanez View Post
    True. So what do you need to do? The wategate is letting olut all of your boost you want still


    Like Pylon is trying to explain, you don't want that boost.. although not really because it will blow the turbo up.


    A turbo is just an air pump, and it can only flow so much air efficiently. The faster the engine turns the more air it ingests at the same pressure. The size, trim, and design of the turbo determines how much air it can push efficiently. The further you get from peak efficiency the hotter the air coming out of the turbo, the hotter the less dense, the less dense the less oxygen, less oxygen less fire, less fire less power. A vf turbo can somewhat efficiently produce around 23psi at 4000 rpm on a 2.5L Subaru engine, but that's about the peak cfm it can flow through without killing it's efficiency. 23psi at 8000rpm would be twice as much air, and you can't physically force that much air through a turbo that size without super heating it. On a small turbo like a vf series you're going to want to run less boost the higher the rpm to keep it from just blowing hot ass air and making no power. That's why you purposely tune the car so that the wastegate opens the right amount to keep the turbo operating within a decent efficiency range. If you didn't you would lose a ton of power and the car would just run like crap.


    If you want to run more boost at high rpm then you need a turbo that is capable of doing it efficiently. From what I have seen, if you want a turbo large enough to not choke off in the top end on a 2.5L then you are looking at something with a 35r sized turbine wheel or larger. That was a large factor in my decision to run a GTX3576r on my car.. it should be capable of continuing to make power past 7000rpm.



    If you want to calculate a particular turbo's efficiency at a certain flow and pressure you need to refer to a compressor map, like this one for my GTX3576r. (although it should be noted that THIS map is with a small .60 A/R turbine housing, not the .82 A/R Tial turbine housing I am running)

    Last edited by MainFrame; 10-29-2012 at 08:05 PM.

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    Registered User MainFrame's Avatar
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    You see the pressure ratio across the left, this is your atmospheric pressure + boost pressure. So if I were to run 30psi, that would be just above the horizontal line labeled "3". Assuming my engine ingested between about 45 to 53Lbs/Min of air (see bottom line), that would mean that this turbo would be operating at about 77% efficiency, which is pretty damn good. As the engine turns faster you move right along that pressure line, and as you can see, efficiency drops. If the engine were ingesting almost 64Lbs/Min at the same pressure ratio then the turbo would only be operating around 65% efficiency, which means hotter air exiting the compressor. As you can see with this map, Garrett and placed the choke line at 65%.. so basically, according to the manufacturer, this is the point at which the turbo can no longer flow any more air, beyond that line you are far enough out of efficiency that you're blowing hot air and no more power can be made.

    From the looks of this map, I would have a decent efficiency island at 38psi boost where the turbo would still be able to operate within 73% efficiency (depending on how much air the engine ingests).




    For comparison sake, here's a compressor map from a Garrett T3 60-Trim, which has near identical flow properties to a VF39 (as far as I can tell).




    Depending on conditions 1Lb/Min = roughly 14.472 CFM.. so the VF39 shows no efficiency at 30psi, and no efficiency at 40Lbs/Min, no matter what pressure ratio.. the amount of air the two turbos are capable of flowing efficiently isn't even really comparable.
    Last edited by MainFrame; 10-29-2012 at 08:25 PM.

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    Registered User MainFrame's Avatar
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    I see someone on nasioc calculated roughly (these are fuzzy numbers due to many factors) 190.61 cfm @ 6000rpm (which = 13.34 lbs/min) for N/A on a stock 2.5L STi motor. Since I plan on running a max of 28psi on my build (pressure ratio of 2.9), that would make my engine ingest 38.75 lbs/min of air @ 6000rpm and 28psi. Which you can see is well to the left of that nice 76% efficiency island, and actually pretty close to the surge line of the compressor. That would be IF I had stock ej257 heads. Luckily for me I have ej205 heads that flow more in the first place, combined with the +1mm raduised valve job, port and polish, and 272 cams my heads should flow a good deal more air than stock STi heads at 6000rpm. Factor in the increased VE from my ported intake manifold, port and polished TGV deletes, and Perrin EL headers the volume of air should increase a good deal. So instead of 38.75 lb/min hopefully my engine will be ingesting closer to 43-44 lb/min at 6000rpm, putting me in perfect position to eat up that 76% island from 5500rpm to 7500rpm. I don't really know how to tell until it's all said and done, and even then the math is fuzzy because there are simply too many factors to take into account.



    Or maybe my logic isn't right here.. will increasing the flow of the heads and VE actually increase the amount of air the engine ingests at a particular rpm? It's been a long day and my brain is running out of juice...

    I guess answering my own question, it has to because otherwise it wouldn't produce such an increase in power. More power because of more air getting into the engine.
    Last edited by MainFrame; 10-29-2012 at 10:07 PM.

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    Registered User MainFrame's Avatar
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    Hope that answers your question.

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