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Tips for Turbo Sizing....Compressor,Trim,A/R,Turbine

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    Tips for Turbo Sizing....Compressor,Trim,A/R,Turbine

    Im posting this to try and help explain the whole turbo sizing deal....Info is everywhere on this but its here now.....Hope this helps out............ Compressor sizing
    Choosing the right compressor is arguably the most important part of turbo sizing and perhaps the most often bungled. Assuming a basic level of understanding as to what the turbocharger's chilly side actually does, we'll forgo the turbo anatomy lesson and jump right into sizing details.

    Ideally, you want the most efficient compressor; not the biggest or shiniest, but the one that can pump the most air into the cylinders without raising temperatures more than complicated thermodynamics' laws say it should. For this, most turbocharger manufacturers supply us with compressor maps, graphs that assist us in compressor sizing by providing efficiency, surge limit, boost potential and shaft speed data. But before we look at these maps or arrive at an ideal efficiency zone, we first need to arm ourselves with two figures: the proposed boost pressure ratio and the given engine's airflow rate.

    Determining an engine's boost pressure ratio is the easy part, but it requires a reality check. First, the calculation: The pressure ratio is simply the proposed absolute outlet pressure divided by the absolute inlet pressure. Restraining yourself to a realistic boost figure may prove more difficult than the calculation itself. Start with realistic numbers, say 7-10psi for street cars on pump gas and upwards of 20psi for race cars on more potent petrol. The key is being a realist. Sure, you know you want to show your friends the 40psi-capable pressure cooker under your hood, but the chances of a turbo like this ever seeing that much boost on the street are slim. Stick with reasonable boost figures and your setup will be better for it.

    Airflow is a bit more complex and, unlike boost pressure, you can't pull this number out of your ass; it is what it is. Airflow measures how much air enters the engine for a given period of time and can be quantified at a given RPM if you know the engine's displacement and volumetric efficiency (don't feel bad if you don't; see the attached calculations). With honest pressure ratios, calculated airflow figures and a compressor map, you'll have to try real hard to not end up with the right compressor.

    Each compressor has a boost pressure and airflow point at which it works best and often times, more than one will seem to work. The key is matching a given compressor's maximum efficiency point to the most useful part of the engine's RPM range, typically its peak torque point. Plotting based on the maximum horsepower point will do little for daily-driver performance. For the best possible turbo response, mid-range and top-end power, compare compressor efficiency at multiple RPM points. It's best to plot a map at redline and another at, say, 70 percent of redline, near where the compressor may potentially hit full boost. Fortunately, we have those nifty aforementioned compressor maps for this. These charts aren't too far off from something you'd see in your high school math class; they display compressor efficiency by expressing the boost pressure ratio along the map's Y axis and airflow ratings along the X axis. A number of oval-shaped islands within the graph represent different efficiency zones. Any given boost/flow point plotted on an island will yield an efficiency point, ideally as close to the center island as possible with efficiency decreasing as points move outward. Where the two points intersect on the map represents the maximum amount the compressor can flow in the proposed situation. Compressor efficiency is a percentage, with most peaking in the 70 percent range. Stay above 60 percent and you're in good shape.

    It's important to note here that, contrary to popular belief, compressor size affects turbo lag little. Lag is mostly associated with the speed at which the shaft spins, in other words, the turbine wheel. But that doesn't mean there aren't consequences from an overzealous compressor selection. As compressor size increases, efficiency drops and heat rises, never a good thing for performance. Sure, 10psi is 10psi no matter the size of the turbo, but while air quantity may be equal, air quality differs between varying turbos, as will power. As a turbo loses efficiency, it produces less dense air which, in turn, yields less volume of incoming air for the engine to inhale.

    There are two places you don't want to end up on a compressor map: the choke point and the surge point. Points to the right of the choke line represent the lowest efficiency points, excessive shaft speeds and mean a larger compressor is in order; pretty straightforward. Points to the left of the surge line are bad news and require further explanation.

    You'll recall we mentioned that compressor maps may reveal numerous candidates fit for the job. Selecting the most efficient one will also be the one with the lowest surge airflow limit. In case you didn't know, surge is bad and its consequences range from a slight power loss to severe bucking and jerking. This occurs when the engine is unable to inhale what the compressor wants to feed it. As a result, air backs up in the intake tract and inside the compressor, in turn, wielding itself furiously against the compressor wheel - not a good thing for those turbo thrust washers. Surge may sometimes be identified by a chattering sound mistaken by many a newbie for a blow-off valve. As far as compressor maps are concerned, be sure and select a compressor that stays to the right of the map's surge line.

    Trim is the relationship shared between the minor and major diameter of a compressor or turbine wheel. On the compressor side, these are referred to as the inducer and exducer, respectively and vice versa for the turbine. Its calculation is simple and its results influence a turbo's flow characteristics. For the most part, larger trims equal more flow, assuming all else remains constant. As expected, there are tradeoffs as far as trim selection is concerned. When dealing with compressor wheels, larger wheels are generally less efficient. The solution is to increase trim size without increasing the overall diameter of the wheel. As for turbine wheels, bigger trims will reduce ackpressure at the expense of increased spool time.

    A/R ratios are ways of further sub-classifying compressors as well as turbine housings by offering differing flow characteristics within equally sized housings. The A/R is calculated by dividing the inlet (compressor) or discharge (turbine) diameter's cross-sectional area by the distance between the center of the wheel's shaft and the center of the measured inlet or discharge area. When calculated correctly, the A/R ratio will remain constant throughout the housing. Varying the A/R affects compressor performance little but the same cannot be said of turbine housings.

    sizing While the compressor brings the air in, it's the turbine that actually powers the compressor wheel through a shared shaft, but you knew that so let's get back to sizing. Because of this relationship, smaller turbine wheels spin the compressor wheel faster and vice versa. While keeping that compressor wheel spinning fast can be a good thing, small turbine wheels/housings can prohibit exhaust gas flow, causing buildup between the combustion chamber and the turbine, also known as backpressure. Despite all this, there's some leeway here on turbine sizing, unlike the finicky nature of compressors.

    There are two things to consider when selecting a turbine: size and A/R. In most cases, turbine sizing is dependent upon its exducer bore size; a larger bore will potentially yield more power. The key is keeping the turbine wheel diameter within 15 percent of the compressor wheel's. This 15-percent relationship is clearly illustrated in many of the popular T3/T4 combinations that have proven time and again to work well on small-displacement 4-cylinders.

    On turbines, the A/R is just as important as turbine size. You should care about the area part of the equation because it dictates how well exhaust gases are evacuated by maintaining their velocity. Erring on the small side might get you a faster spool-up but can lead to reversion back into the chambers. Upsizing will yield some more ponies on the top-end but at the risk of a late spool-up or worse, no spool-up. The radius portion of the equation is important in that it influences turbine speed. Increasing the radius supplies the turbine shaft with additional torque for turning the compressor wheel. While this may seem like a sneaky way to stuff a larger compressor wheel in place, most turbine radiuses are fixed and experimentation lies mainly with turbine area. Perhaps the best way to select the proper A/R ratio is from experience. There really is no magic calculation here. Too many factors are involved including exhaust gas pressure, turbine inlet pressure and boost pressure.

    The bottom line: There's no longer any reason to consider turbo sizing black magic. With a few compressor maps, a pencil and a calculator, there's just no excuse for hooking up that oversized, boost-lagging, beast of a turbo and feeling good about it. Despite what they say, size does matter.

    How to plot points on a compressor map:

    Calculate airflow for the X axis: measured in lb/min, airflow = (HP target) x (air/fuel ratio) x (BSFC/60) where BSFC is lbs fuel / (hp x hr) If you don't have access to BSFC data, you can plug in an estimated value between .50-.60.

    Calculate volumetric efficiency = (actual CFM / theoretical CFM) x 100 where theoretical CFM = (RPM x CID) / 3456 and actual CFM = 90%, a good average number if you don't have access to measuring this. Multiply by .0610237 to convert cubic centimeter displacement into cubic inches.

    Calculate manifold absolute pressure: map = [(airflow) x (639.6) x (460 + intake temperature)] / (VE) x (RPM/2) x (CID) Upwards of 100 degrees F is a good estimate for intercooled intake temps if you don't have access to measuring this. Plan on adding about 1psi to take into account pressure drops related to the intake and/or intercooler.

    Calculate pressure ratio for the Y axis = (14.7 + map) / 14.7 If you're at sea level, 14.7psi will work. As elevation varies, this figure will need to be adjusted.

    Note: Don't worry about the constants like 3456, 60 or 639.6, they're simply there to tidy things up and convert units.

    VE = volumetric efficiency
    BSFC = brake specific fuel consumption
    HP = projected flywheel horsepower
    CFM = cubic feet per minute
    CID = cubic inch displacement

    Other useful calculations:

    Trim = (minor/major)squared x 100

    Trim describes the relationship the minor diameter and major diameter of a compressor or turbine wheel shares with each another. The inducer, or minor end when referring to compressor wheels, is where the air enters. Air exits through the exducer portion.
    Only give advice when I know or have done it and it worked

    Thank you! Sticky this yo!


      20 psi is more than attainable on pump gas.
      spin city


        If stickied needs info on;

        -exhaust housings, pro's vs con's

        -types of bearings

        -maybe a bit of info on why compressed air gets hot rather then blaming the turbo, then emphasize why quality intercoolers do matter.

        You know, "cold air is denser, compressed air gains temperature, cooler charge temps=better atomization, quality intercoolers matter" Something like that.

        -Flanges; types, sizes and pro's vs con's

        -water+oil cooled vs oiled only

        -Quality manufacturers vs ebay china crap

        -A bit more of an emphasis that quality matters?
        Originally posted by wed3k
        im a douchebag to people and i don't even own a lambo. whats your point? we, douchbags, come in all sorts of shapes and colours.


          actually, warmer charge temps= better atomization. That is why your computer enriches the fuel air mix when your engine is cold.
          spin city


            Very good read


              Originally posted by dbsharp View Post
              actually, warmer charge temps= better atomization. That is why your computer enriches the fuel air mix when your engine is cold.
              I think we are saying the same here.

              When I say cooler, I mean cooler then heat soaked. Not cold enough to effect performance, just cooling well enough to avoid heat soak issues.

              And on a side note cooler temps are denser and do allow more fuel to enter the cylinder.there's obviously a point of no return in either end of the spectrum that will hinder performance.

              I was more making mention of the topic because its become all to common to run a $75 intercooler off eBay, and then people don't understand why they have heat soke issues.
              Originally posted by wed3k
              im a douchebag to people and i don't even own a lambo. whats your point? we, douchbags, come in all sorts of shapes and colours.


                Where's the Like button? Great Thread...
                Last edited by zmos86; 03-10-2012, 09:45 PM.
                Originally posted by deevergote
                Nobody says "Ebonics" anymore... It's thoroughly racist to assume that only black people talk like idiots. This site is proof that EVERYONE, of every cultural background, is capable of talking (or rather, typing) like a total fucking moron.


                  Very well written.
                  "Self Renewed"