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att:DEF and others re smaller wheel for 2871

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  • att:DEF and others re smaller wheel for 2871

    Hi. I'm new here and found the site via DEF and reading his excellent article over on bimmerforums on reading compressor maps.

    I have a redtop SR20DET in a Peugeot 505 and recently bought a rebuilt GT2871 52 trim/.64. Then I read an article over on boostcruising where the author stated that the 2871 is basically a mismatched turbo and that the 71mm comp wheel is too big for the 53.8mm exhaust wheel and slows the spool of the exh wheel too much.

    He mentions Garrett's recommended comp/exh wheel ratio of between 1.1:1 and 1.25:1 and points out that the 2871 has a wheel ratio of 1.32:1. Going by this formula the max comp wheel size that would suit a 53.8 exh wheel would be a 67.25mm wheel.

    Sorry for being long-winded but I'd like DEF's and other's opinions of the idea of changing the 71mm comp wheel on the 2871 for a 67mm Garrett GT wheel. I tried to source a 69mm wheel as on the HKS2535 but have been told they are not available. I've posted my calculations for the SR20 below using DEF's calculator and drawn them onto the flow charts of the above wheels as well as the GT28RS disco.

    I should say that I don't want to go to cams at the moment and only use the car on the street and am more interested in response than max hp but as the 67mm wheel flows 40lb it should be able to flow 400hp or so. From the maps would the 67mm wheel be a good midway point between the disco's response and the 2871's topend?
    Attached Files

  • #2
    from what i'm told you would need to balance the whole assembly if you did change a wheel or else you experiment you fail pretty quickly. i'm guessing the only way to do that is send it to garrett.

    Most PM answers: F 17X9 +22, R 17X9 +35

    Comment


    • #3
      Yes I would def get it fitted and balanced by a turbo shop.

      Comment


      • #4
        IMO, you're splitting hairs when you start talking about shaving a few mm from a compressor wheel. Does it make a difference? Yes. Is it really noticeable when you're driving the car? Probably not so much.

        It doesn't sound like this is an "all out build", and what you're talking about doing isn't cheap on top if a $$$$ turbo.

        I run a GT2876R .64 because I got it dirt cheap used. Even bigger wheel mismatch than the 2871. You know what? They really don't feel all that different from the driver's seat. It rocks out just fine on track or the street.

        When you get to where your build is "maxxed out," then you can worry about the small details.

        What are you doing for tuning, as that will make a way bigger difference than a few mm difference on the compressor wheel dia.
        '18 Chevrolet Volt - Electric fun hatch for DD duty!


        DefSport Koni Sleeve and Spring Perch Buy!!!
        http://www.nissanroadracing.com/showthread.php?t=5902

        Comment


        • #5
          For tuning I have a Nistune USB chip ready to go in and I'll get an experienced local tuner to dyno tune it.

          For supporting mods I have the usual 3inch exhaust, Z32 AFM, Porsche fuel pump, PWR 500x300x68 front mount etc. Still looking around for injectors and I'll probably go for 550s as this should be enough for the 2871 or '2867'. (I got scammed buying some 740cc HKS injectors from a seller in Europe which never turned up but luckily paypal reimbursed me.)

          My thinking was why not get it right from the start? The HKS2535 is reputed to be an excellent turbo. The 67mm wheel will cost me $140 to buy and another $100 to install and then I can sell the 71mm wheel for around $100 so it wont end up costing me much.

          How does the 67mm's compressor map look to you for the SR? Is it an improvement on the 71mm map as far a midrange torque goes?

          Comment


          • #6
            I would stay with the 71 mm wheel - end of story. You will want more power in the future, give yourself room to grow and don't worry about the gnat's ass sized details.

            There is nothing "right" about the 67 mm wheel other than it supposedly follows some rule of thumb better than the 71 mm wheel by a small amount. Like I said, there are way more important things to concentrate on with the car(suspension, chassis, brakes etc).

            You seem pretty much dead set on spending more money for the 67 mm wheel, so I hope it gives you what you're expecting.
            '18 Chevrolet Volt - Electric fun hatch for DD duty!


            DefSport Koni Sleeve and Spring Perch Buy!!!
            http://www.nissanroadracing.com/showthread.php?t=5902

            Comment


            • #7
              Check out codyace's dynos with .64 2871r...what could you really improve? Spool and max power won't get much better on a pump gas SR.
              ~1992 240SX, SR20/Koni track day car
              ~2016 M3, daily driver

              Comment


              • #8
                Fair enough-thanks for your opinion. Will have to give it some more thought. Yes I've seen codyace's dyno and it looks great but there's nothing to say it coudn't be improved on. From memory he's also had the exh mani flowed. But I asked for opinons and got them-thanks again.

                Comment


                • #9
                  You couldnt just slap the 67mm wheel in anyway, the cutout in the bearing housing has a machined recess that the wheel sits in, the housing has to match the major diameter of the compressor wheel.

                  That being said, I think you're splitting hairs and probably worrying about nothing anyway, plus the only time I can remember seeing that same statement was on a guys page who didnt seem to know WTF he's talking about, other statements like the S14 turbo only being good for something like 270whp, much larger turbos only being capable of marginally more, etc...

                  Comment


                  • #10
                    Originally posted by Z28ricer View Post
                    You couldnt just slap the 67mm wheel in anyway, the cutout in the bearing housing has a machined recess that the wheel sits in, the housing has to match the major diameter of the compressor wheel.

                    That being said, I think you're splitting hairs and probably worrying about nothing anyway, plus the only time I can remember seeing that same statement was on a guys page who didnt seem to know WTF he's talking about, other statements like the S14 turbo only being good for something like 270whp, much larger turbos only being capable of marginally more, etc...
                    This is also a factor - but I didn't want to get into specifics since there is more to it than just "bolting on a new wheel and balancing it." It'd only work if the housing has the exact right inducer and the same profile as the other wheel(very very doubtful). Any gap created here will absolutely kill compressor efficiency. See cheap eBay turbos for a good example of this, and motorsports stuff like the TR30 for how you can gain efficiency closing this down(at the expense of more life as the CHRA bearings wear).
                    '18 Chevrolet Volt - Electric fun hatch for DD duty!


                    DefSport Koni Sleeve and Spring Perch Buy!!!
                    http://www.nissanroadracing.com/showthread.php?t=5902

                    Comment


                    • #11
                      This is the article and I dont think he mentions the T28 at all so you're thinking about someone else:

                      Useful info I thought would be a good new on boost. This is how to choose the Garrett GT turbo that will rock you! The clever turbine engineers at Garrett have shared with us their turbo and performance specifications in the form of measurements, and compressor and turbine maps, and with a little knowledge we can use them to find the right turbo for every application. As we collect more actual results we will be able to even more accurately predict performance on the 3S-GTE. But we already have the tools we need, and you can learn a lot of the nitty-gritty stuff from the section "Turbine Efficiency - Part 2...the missing piece to the turbo selection puzzle". http://www.turbobygarrett.com/turbobygarre...bo_tech101.htmlhttp://www.turbobygarrett.com/turbobygarre...bo_tech102.htmlhttp://www.turbobygarrett.com/turbobygarre...bo_tech103.html I'll just cover the highlights from that thread, and highlight the highlights in bold, to present this information in an easy to understand form, and use it to establish some general guidelines to finding the turbo of your dreams...the one that will provide the realistic best possible spool for the strongest bottom end, fullest mid-range, and extended top end. A quick and dirty review of how a turbo works is essential as it is fundamental to understanding the tools we have to help us choose. A turbo is an air pump that’s powered by the energy contained in the engine's exhaust gas flow by spinning a turbine impeller wheel. That wheel rotates on a shaft that has a compressor wheel mounted to the other end that then also spins and forces more air into the engine's intake. It's the exhaust energy and turbine wheel that powers the compressor wheel to increase intake air pressure, and your boost controller that determines the amount of pressure (with the wastegate redirecting exhaust flow as required to prevent over-boosting). It's important to recognize that it's the compressor wheel that's in charge of reaching the desired boost pressure, and the turbine wheel’s job to spin it accordingly. When the turbine is struggling to do its job effectively the compressors ability to provide boost in a timely manner is compromised and we recognize this effect as turbo lag. When it's completely up to the challenge to power the compressor we recognize it as providing excellent throttle response. In fact, our success in choosing the best turbo for our use rests solely on our ability to understand this relationship between turbine and compressor. And for our purposes of choosing among the GT line that relationship is primarily determined by (a) the relative diameters of those two wheels and (b) the aerodynamics of the turbine housing. The resulting performance is called Turbine Efficiency, and its measure is expressed as a percentage. A turbo whose turbine can efficiently power the compressor to produce quick spool and less restricted top end flow has a higher %, often close to or slightly exceeding 70%, while others are as low as 60%. Here's what we're looking for in the Garrett specs: (a) Garrett recommends a wheel diameter ratio range between 1.1:1 and 1.25:1 (compressor:turbine) to provide the best overall performance. As an example the GT28RS has a ratio of 1.1:1 (60mm/53.8mm) at the quickest spooling end of the range, and the GT3076R has 1.27:1 (76.2mm/60mm)…barely outside the other end of the range. The reason a large compressor wheel mated to a smallish wheel would not be able to spool as quickly is because a largish compressor wheel will need to turn slower to provide any given intake airflow than a smaller wheel would, and this in-turn forces the turbine wheel on the other end of the shaft to turn slower, and at speeds that it can’t operate as efficiently at. This is contrary to those that believe a comparatively small turbine wheel and housing will cause the largish compressor to spool more quickly. Dyno results confirm Garrett’s recommendations every time, while I have never seen evidence of a small turbine/large compressor spooling nearly as quickly. Good examples to see this effect would be the GT28RS, GT2871R (or HKS GTRS), and GT2876R (or HKS GT2540R). All three share the identical turbine housing and wheel, but are mated with 60mm, 71mm, and 76 mm compressor wheels. The latter two compressor wheel diameters push the wheel ratio well outside of the recommended range to 1.32:1 and 1.45:1. Each larger compressor wheel causes a delay in spool of perhaps 750 rpm to ~17 psi and makes less top end power as well. The only way to make these wider spaced wheel combinations make more power is to significantly raise boost pressure. This however will not reduce lag, the restrictively small turbine wheel and housing will limit high rpm power as it reduces the entire engine’s VE, less ignition timing can be run at high rpm causing reduced power from the airflow, exhaust temps will be higher, and you’ll have to deal with all of the risks associated with higher boost levels. The solution is to follow Garrett’s recommendations whenever possible. (b) The turbine housings are designed to maximize turbine efficiency. In some cases though a turbine housing will be made or modified to fit specific user applications like space constraints or the lack of suitable sized exhaust manifold turbine mounting flanges for some popular applications. This has led to small turbines modified to stuff in large wheels, large turbos with small turbines made to fit onto small exhaust manifold flanges, smallish turbos modified to fit onto large flanged manifolds, etc…and all of them have reduced the turbine’s efficiency to spool quickly and produce the strongest powerband. The impact of some twin scroll housings can’t be predicted because of their lack of turbine efficiency ratings by Garrett, but their impact will be seen in dyno results. In some of these cases the wheel ratio will appear to be ideal, but the modification to the turbine housing itself can negatively affect turbine efficiency. This is why it’s important to know the Garrett tested Turbine Efficiency % rating. The various iterations of GT3071R is a good example of these variables. All models use the 71 mm compressor wheel, but some have a 56.5mm turbine wheel stuffed into a machined T28 turbine housing, some have the better matched 60mm turbine wheel fitted to a twin scroll housing of unknown efficiency, and the one that mates it with the 60 mm turbine wheel and T3 single scroll housing. The latter is surely the best of the bunch using Garrett’s specs and recommendations, and it’s very high efficiency rating of 72% and ideal wheel ratio of 1.18:1means that for this size of turbo you are unlikely to find anything that will out-spool or out-flow it. It also means that the similarly flow rated GT2871R models with less than ideal wheel ratio and as little as 60% turbine efficiency will not perform as well as the GT3071R at 72%. Some feel that the GT3071R versions that have been used on the 3S-GTE have been less than stellar performers, but these results I'd suggest are consistent with Garrett’s specs, ratings, and the recommendations presented here. Let’s choose the best model and set some powerband records! I’d recommend that you first use the Garrett compressor maps to identify the compressor that can flow your requirements (see Garrett’s Turbo Tech section for these calculations), and then to consider the wheel ratios and the turbine efficiency ratings found on their turbine maps as a guide to matching that compressor with a suitable turbine wheel and housing. While efficiency actually varies with flow rate, pressure ratio (think boost level), and turbine wheel rotation speed, the stated maximum efficiency rating is going to be quite comparable among all models within the GT line. Now you need to choose the turbine housing AR. You’ll notice on the turbine maps that the efficiency curves are different for the various available turbine housing AR. These shows that the lower AR housings are more efficient at lower flow rates generated at lower engine rpm, while higher AR housings are more efficient at higher rpm flow rates. These housing options will allow you to choose between maximum low rpm spool and power at the cost of a little high rpm peak power, or maximum peak power at the cost of a little lower rpm performance, or something in-between if there’s a 3rd option. Valendia and RickyB provided a good dyno comparison of this AR housing trade-off using a .64 and .82 AR on the GT28RS. The lower AR made for a significantly stronger powerband overall on this setup, and I believe we will see this trend with each turbo model and engine setup…if peak power is your goal the higher AR will likely provide that every time. I hope this will help you better choose from the GT turbo options that are available. Bruce Hadfield Garrett specs, compressor and turbine maps can be found at http://www.turbobygarrett.com/turbobygarre...bochargers.html , and Compressor selection guidelines at http://www.turbobygarrett.com/turbobygarre...ech_center.html Turbine Efficiency - Part 2...the missing piece to the turbo selection puzzle Let’s quickly review the resources we’ve been using to choose a turbo so we can better appreciate our current needs: 1. Testimonials. While it may be entertaining to hear about how somebody smoked another car at a stop light, or how they got pushed back into the seat when the turbo kicked in, the general lack of useful information and the subjective nature of the comments leads us directly to #2. 2. Dyno Graphs. Dynos measure power at the wheels (or hubs) and that power is affected by many non-turbo factors. While dyno results have been widely regarded as the best tool we have to measure the difference certain modifications make, they are not a perfect tool. General engine condition, supporting mods, boost levels that can change during a run, aggressiveness of the air/fuel ratio and ignition timing, octane, 3rd vs. 4th gear, and a wide range of dyno equipment and testing factors and conditions can make it difficult to clearly see the impact of only the turbo, or compare one turbo to another. Throw in some mods that can greatly differ from car to car, such as the state of tune of an EMS, or mods that affect volumetric efficiency like a set of cams and cam gears, custom intake, and maybe a little head-work, and it becomes almost impossible to determine how much of the dyno results are the result of the turbo alone. At best you can see what is possible on a given set-up. If you want to research a turbo not yet dyno’d, or learn more about the ones you see in the dynos, you proceed to the dreaded step #4. 3. Turbo “Power Ratings/Estimates” Often around as useless as Testimonials and with all the limitations of Dyno plots (so many other things that effect power other than turbo by itself) 4. Compressor Maps. These are the turbo manufacturer’s graphic representations of the compressor’s ability to flow air across a range of pressure ratios. Compressor efficiency and shaft speeds are shown. We then need to “estimate” our engine’s airflow requirements throughout our desired powerband using a complicated formula designed by the devil him self, and then learn all about a compressor map so we can check to see which compressor “might” be able to provide the required amount of airflow. You really should struggle through the formula of estimating your engine’s airflow requirements to truly appreciate all the factors that affect it. While you may have been led to believe that finding a suitable compressor map will identify a suitable turbo, this isn’t necessarily true, and many members have discovered this the hard way. That’s because the ability of the compressor to deliver its indicated airflow is dependant on the turbos turbine section, and something called turbine efficiency…the subject of this article. Turbine Efficiency So what is turbine efficiency and why should we care? The compressor relies on the turbine to use the exhaust gas energy to power the shaft that spins the compressor wheel that pushes the air through the engine to create ungodly amounts of torque when mixed with fuel and a well-timed spark. And if the turbine goes about it’s job in a sloppy and inefficient way then the compressor won’t be able to do its job well, and performance will suffer. A turbine operating at high efficiency will be able to more quickly spool a compressor when called upon to make good low-end power, and/or will provide less back-pressure at high rpm to enable the turbo to make more top-end power by actually improving the engine’s volumetric efficiency. Turbine efficiency is the ratio of useful exhaust energy to total energy supplied, the flow at which it’s efficiency is the highest at all pressure ratios is plotted on it's “turbine map”. and it's maximum efficiency is stated as a percentage. Turbine efficiency and maps are closely related to the compressor, and further discussion would be easier if related to an actual turbo. Only Garrett publishes turbine maps to my knowledge, and since I was able to use them to select my turbo and then acquire actual 3S-GTE results, let’s use my GT28RS for our example. It will then be interesting to see how we can carefully navigate through a staggering choice of 17 GT turbos models and predict their performance. I’ll make this clear by keeping it fairly simple…I promise! Example - Crunching the numbers for a stock Gen 2 3S-GTE Calculating Engine Airflow Requirements The turbo analysis starts with finding a compressor that might meet our engine’s air flow needs. As mentioned in step #3 above, we calculate those estimated requirements with a formula, and then try to plot them on the compressor map. The stock 3S-GTE will flow ~15 lbs/min of air @ 3000 rpm up to ~30 lbs/min @ ~6000 rpm at a pressure ratio (Pr) of 2.25, which is approximately a boost level of 17 psi at sea level when accounting for normal intake system pressure losses, and making various educated guesses including a volumetric efficiency of ~95% at 6000 rpm. This estimate is telling us how much air the engine is capable of ingesting at 17 psi, and we calculate it over the range of rpm that we are hoping it will have full boost. I focused on 3000-7000 rpm as being the most important area for the widest range of driving needs, and you might choose something else for your needs. Power at any given rpm is dependant on the amount of air (and fuel) that the engine is consuming, and this is why we study airflow. Garrett very generally uses 9.5-10.5 flywheel hp per lb/min of flow for power estimates…so let’s think massive flow! Compressor Map See the compressor map below where I have plotted these airflow requirements at 3k, 4k, 5k, 6k and 7000 rpm on the map at the 2.25 Pr. Note that airflow is shown on the x axis and Pr on the y axis, and that all of these points fit nicely on the map suggesting that the compressor should be able to provide the required amount of air to maintain 17 psi from around 3000-7000 rpm. Plotting all of these points was not possible on other compressor maps that I had found back in the summer of 2003. The various concentric lines and numbers are noting the changing compressor efficiencies as airflow increases. Lower compressor efficiency at each side indicates that more heat will be added to the intake air than when it’s providing the airflow shown in the middle of the map where it’s more efficient. Temperature has an impact on air density, which is one determinant of airflow. Lower intake air temperature generated by a more efficient compressor, or improved inter-cooling does make more power…and so does higher turbine efficiency! Please note that a stock VE motor will reach it's maximum airflow around 6000 rpm, and after that the VE drops off and less air is consumed. The 7000 rpm plot would therefore only apply to a motor that had improved VE, and it could be more or less than what I've shown, and I'd suggest that both 264 and 274 cams could cause the motor to use considerably more air at 7000rpm. Source of both maps: http://www.turbobygarrett.com/turbo...RS_739548_1.htm Compressor maps show only the flow of air that may be possible under ideal conditions, but to predict how it will perform when attached to any given turbine we must get into the turbine maps. Turbine Map This map indicates the turbine’s ability to convert the exhaust gases kinetic and thermodynamic energy into mechanical power to turn the shaft (and the compressor wheel) through the use of a turbine wheel. This “ability” is expressed as an efficiency rating. The manufacturer’s detailed map contains a lot of information related to exhaust flow, pressure ratio, shaft speeds, and efficiency. Garrett publishes a simplified version which can be found on their website, and I have shown the GT2860RS map below. Using this info can be a very good way to choose between two turbos with seemingly suitable compressor maps, and between two turbines with different area radius (AR) housings for the same turbo. It’s the missing piece to the turbo selection puzzle that turbo-machinery engineers have used for years. This map shows a range of Pressure Ratios (Pr) on the x-axis and Turbine Flow on the y-axis. It also states that the turbo has a maximum turbine efficiency of 72%. There is a blue line showing the flows and pressures using the .64 AR housing that I have, and a red line for the .86 housing. Without knowing the exact method of calculating the turbine flow or pressure ratio, I think it suffices for the purpose of this discussion to assume that the turbine flow and pressure ratio is about the same as the compressor flow and pressure ratio that we would be looking at when using a compressor map, and we can jump between both maps and use the same measures inter-changeably. Carrying on with our example at 17 psi (2.25 Pr), we can see that when this turbo with .64 AR housing is operated at a pressure ratio of 2.25, that its best efficiency will be achieved when flowing about 17.5 lbs/min as read from the y axis. I have highlighted that point in red. It also means that at any flow other than 17.5 lbs/min that the efficiency will be lower. If we now plot 17.5 lbs/min @ 2.25 Pr on the compressor map we find that it is half way between our 3000 and 4000 rpm points, or at about 3500 rpm. You will see later that 72% is an extremely high efficiency rating, which should indicate that the turbine will be able to meet the demands of the compressor to spool, and in fact my dyno plot and boost gage both indicate that the turbo actually spools slightly sooner to 17 psi @ 3250 rpm (where airflow would be close to 16.5lbs/min), but not by 3000 rpm that was plotted on the compressor map! This indicates that the turbine was not quite efficient enough at the 3000 rpm flow rate of 15 lbs/min to extract enough energy from the exhaust to power the shaft to spin the compressor to provide the airflow to reach 17 psi by 3000 rpm, but by 3250 rpm and 16.5 lbs/min it was! This is important because it shows how high turbine efficiency has to be to reach our spool goals, and at 15 lb/min flow, the rating of 72% @ 17.5 lb/min was not quite enough. Go back and make sure you grasp that last paragraph and the rest will just fall into place…I promise! Now what happens when airflows goes much higher than 17.5 lbs/min, and all the way up to 30 lbs/min at 6000 rpm? That’s a long ways from our best efficiency flow. The engine already finished spooling to 17 psi at 3250 rpm so why do we even care? Because turbine efficiency also affects turbine backpressure at high flow rates, and that reduces the engines volumetric efficiency and limits power. Although we do not know what the efficiency is at a flow of 30lb/min, we can assume that the turbine isn’t too small and restrictive, and still has a high enough efficiency, because the 200 ft-lb @ 7000 rpm result on the dyno is typical of those using even larger and less restrictive turbos. This turbos compressor and turbine is therefore not limited in airflow capacity or efficiency within the requirements of the stock 3S-GTE and 17 psi. My dyno using the GT2860RS .64AR turbine If you go back to the turbine map on page 3, you can see that the .86 AR also has a 72% efficiency but the whole curve is raised up 3-4 lbs/min to 21 lbs/min at the same Pr of 2.25. This means that at the lower rpm and flow rates that the engine consumes during spool up, that the turbine isn’t capable of working as hard, and it will not spool as quickly as the .64 AR. While we don’t know what the exact efficiency would be at 6000 rpm, we can assume that the .86 should be more efficient than the .64 AR, and that it may provide slightly more power...at those higher rpms. The 21 lbs/min peak efficiency would be a flow of about 4200 rpm, and it might also reach 17 psi a couple of hundred rpm sooner if our experience with the .64 AR is any indication. Conclusions We can see from our example that the GT28RS compressor map is a great match for the airflow requirements of the stock 3S-GTE VE engine because we were able to plot all of our calculated flow requirements from 3000 to its flow peak at 6000 rpm, and then out to redline where it actually consumes less airflow. The .64 AR, 72% @ 17.5 lb/min efficiency turbine is a great match for the compressor because it was able to power the compressor to reach each of those requirements, only barely missing the 3000 rpm point by 2 psi, and because it also provided the required flow to reach 200 ft-lbs @ 7000 rpm that is the standard set by the larger turbos. We can tell that the .86AR turbine will not be able to spool quickly enough to reach the flow requirements until closer to 4000 rpm, but its higher efficiency above 4000 may help it to make a little more power at 6000 rpm. Garrett’s estimates 10-11 flywheel horsepower per lb/min of flow, and Ray Hall figures 10.86 per lbs/min of flow. So at a flow of 30 lbs/min @ 6000 rpm I should be making about 326 flywh.hp (10.86 X 30). Factoring in 15% driveline losses would mean ~277 whp @6000 rpm, and this is about what you'll see on the dyno. This perfect match is quite a coincidence given the number of variables involved, but it should at least show that making good educated guesses can get you awfully close. You can analyze other models in the same way. While it’s an imperfect science, it can help you find the best match for your needs, and it can definitely identify a poor match. Even if a turbine map is not available for the turbo you are analyzing, there are things to consider. Factors that will improve spool There are a variety of ways that you can improve your spool, and while buying a different turbo, or perhaps a different turbine housing, is the big one because it directly addresses turbine efficiency, there are others also: -Turbine Efficiency 1. Compressor wheel to turbine wheel ratio. That’s the compressor wheel exducer diameter divided by the turbine wheel inducer diameter. A large compressor wheel will not be as efficiently powered by a small turbine wheel. Garrett claims this is because the larger compressor wheel will need to turn at a slower speed to provide any given airflow, and that will force the smaller turbine wheel to work at shaft speeds that it is not as happy with. And a small turbine wheel will eventually cause a restriction with increasing exhaust flow, thereby limiting high rpm power. Garrett recommends a ratio in the range of 1.1:1 to 1.25:1 to provide the best compromise between spool and high rpm power. Considering wheel ratio alone can be misleading if the turbine wheel isn’t well matched to the turbine housing. 2. Turbine wheel to housing match. A larger turbine wheel adapted to fit into a smaller turbine housing will restrict exhaust flow and decrease turbine efficiency. Note the GT3071R with 56.5mm turbine wheel below, where turbine efficiency drops due to the housing matching. 3. Wheel and housing design. The most modern turbines outperform the older ones with increased turbine efficiency. They spool quicker and make more power. Choice of housing AR will shift the efficiency from faster spool to higher flow capability with each higher AR selection. -Exhaust Flow Energy: Increase flow at lower rpms by improving volumetric efficiency with headwork, increased displacement, etc. -Exhaust Temperature Energy Increase the exhaust temperatures at lower rpm with ignition tuning, exhaust cam timing, heat coatings, etc. -Expansion Ratio Reducing turbine housing and exhaust system flow backpressure improves expansion ratio. And since reducing backpressure also improves the engines volumetric efficiency, this improvement will also increase your power as more air is consumed. TURBO ANALYSIS Let’s look at several Garrett models, and compare their compressor map flow ratings, compressor/turbine wheel diameters, wheel ratio, and the maximum turbine efficiency and flow rate that it occurs. For ease of comparison I will record this information in a chart instead of trying to post all the maps, and I’ll use a Pressure Ratio (Pr) of 2.25, which is roughly the equivalent of 17 psi with a normal intake. I’ll list them from the smallest to largest turbo. I will then try to interpret the data using our knowledge of turbine efficiency, information from Garrett, and observations from a very limited number of dynos that I’ve seen on our member’s cars. Note that airflow range listed is for 2.25 Pr (~17 psi) where our engine airflow requirements are estimated to range from approx. 15 lbs/min @ 3000 to 30 @ 6k, and then less to redline on a stock VE Gen 2 3S-GTE. Stock VE refers to the motors internals, and flow requirements include bolt on VE mods like high flowing downpipes and exhausts. Readers should calculate their own airflow requirements, as there are various factors that can affect it. Many of these turbos will be used on modified engines that will actually consume more air at each rpm level and have extended rev limits. GT2860RS 12-35 lbs/min, 60/53.8mm, 1.1:1 wheel ratio, 72% eff. @ 17.5 lbs/min. This turbos is a great match, it can efficiently provide the required 30lb/min flow I needed for 17 psi, and has achieved terrific results on the stock VE 3S-GTE. See the “Conclusions” in the example above for details. GT2871R 13-38 lbs/min, 71/53.8mm, 1.32:1 whl. ratio, 66% eff. @ 17.5 lbs/min. This turbos compressor would have provided the airflow required from below 3000 rpm if the turbine efficiency was higher. The lower turbine efficiency of 66% however suggests that it will not spool as quickly as the GT28RS, nor make as much top end power regardless of housing AR choice. The 1.32:1 wheel ratio is less than ideal and would be the factor in the lower max. efficiency. The one dyno I’ve seen with unknown turbine AR appeared to spool approx. 750 rpm later and made no more power than the GT28RS despite 4 psi more boost. This is the comparison that best illustrates to me the significance of a lower efficiency rating since the turbos are identical except for the GT2871R having a larger compressor wheel which throws off the wheel ratio and efficiency. While variations in engines and dynos can be misleading, I think this does confirm Garrett’s own claim that on this size of turbine, a difference of 8-15% efficiency can cause a spool change of roughly 1500 rpm. GT2876R 16-48 lbs/min, 76/53.8mm, 1.41:1 whl. ratio, 62% eff. @17.5 lbs/min. The compressor has a much higher flow, and the 3000 rpm requirement of 15 lbs/min can not be plotted. It has a very low turbine efficiency caused entirely by the mismatched wheels according to Garrett, and the 1.41:1 wheel ratio results in efficiency of only 62%. Even Garrett does not recommend this turbo for general use, and a number of our members with the identical GT25R would readily agree…enough said. GT3071R 15-47 lbs/min, 71/56.5mm, 1.26:1 whl. ratio, 64% eff. @18.5 lbs/min. This compressor has a very appealing flow rating, and the 3000 rpm requirement can just barely be plotted. But the turbine has a poor efficiency rating, despite the acceptable wheel ratio. They used a larger GT30 turbine wheel that has been modified to fit into the smaller T25 housing according to Garrett, and this would explain the poor efficiency. The one dyno I’ve seen unfortunately seems to confirm that it will spool slowly in keeping with its turbine efficiency rating. GT3071R, T3 turbine, single scroll 14.5-45 lbs/min, 71/60mm, 1.18:1 whl ratio, 72% eff. @19 lbs/min. Great new turbo for those with a mild build that can consume more airflow that the stock VE motor...see post #46 below for details. GT3076R 18-52 lbs/min, 76.2/60mm, 1.27:1 whl. ratio, 72% eff. @ 21 lbs/min. This compressor can flow even more air, but again misses the 3000 rpm point. The lowest flow requirement would plot ~3600 rpm. Those with a build that will flow this kind of air at the top will likely not mind the lag in exchange for the huge power. The turbine efficiency rating is excellent, reaching its peak at 21 lbs/min of flow. That means this turbo will spool like crazy reaching 17 psi around 4000 rpm with the .64 AR turbine, and will flow to beyond redline with any of the three turbine ARs. This could be the next GT champ for those with a power goal up to 500hp if they have the build to flow this kind of air. That could mean whp exceeding 400 whp with modest boost. This turbo is the best reason I’ve seen to buy an EMS, large cams, cam gears, intake manifold, a full head job, possible stroker, and professional tuning. Somebody try this one quick and post your results! GT3271 15-38 lbs/min, 71/64 mm, 1.11:1 whl. ratio, 64% eff. @ 19.5 lbs/min. This journal bearing GT has an appealing compressor map, but a low efficiency turbine. Looks like it could flow a bit more top end than the GT28RS until you realize the turbine just won’t support it with only 64% efficiency. The only dyno I’ve seen did produce slightly more power at 7000 rpm with an extra 4 psi of boost, and spooled what appeared to be ~750 rpm later. GT3571 14-38 lbs/min, 71/68 mm, 1.04:1 whl. ratio, 70% eff. @ 29 lbs/min. This journal bearing GT has a compressor map that fits our requirements, gives a little extra flow at the top, and it also has a high efficiency rating. I’m a bit puzzled by the max. efficiency being reached at a high 29 lb/min flow. That means it will spool hard around 6000 rpm to reach 17 psi. It has the compressor flow to support another 400 rpm, and the turbine should go along with the plan. Let me know if anyone has tried it. GT3082R ? lbs/min, 82/60 mm, 1.37:1 whl. ratio, ? eff. Also known as the GT3040, I’ve included this model as it’s a turbo that a few members have used and it is available in a kit. There are no maps shown for it on the Garrett site, but I’ve listed the only details pertaining to turbine efficiency that I can find. What we’ve discussed about the wheel ratio affecting turbine efficiency perhaps doesn’t even apply in the same way at this power level. The 60mm turbine looks undersized for the top end power that a 82mm compressor might flow. See the dyno results in the Racing Records section to see the impressive results. GT3582R 22-59 lbs/min, 82/68 mm, 1.21:1 whl. ratio, 70% eff. @ 22 lbs/min. Also known as the GT35R, this compressor has huge flow capabilities and high turbine efficiency intended for the seriously modified engine (and perfect for the 3.0L Supra!). I'd refer you to the Racing Records section to see various impressive results. I hope you’ve found this information of interest and will be able to apply it to your search or understanding of turbo performance. Bruce Hadfield Sources “Turbo Matching” by Mike Kojima, SCC June 2003. Formula for calculating engine airflow, and plotting a compressor map. “Performance Dictionary” by Jason Kavanagh, SCC July 2002. The Garrett engineer uses a detailed turbine map to discuss turbine efficiency. Garrett website http://www.turbobygarrett.com/turbobygarrett/index.html. Compressor and turbine specifications and maps. Some useful information for thought if you are interested in some maths behind turbos and what turbo is right for you. Copied with permission from mr2oc. mark


                      His ideas about the 3071 being a better matched turbo than the 2871 seem to be born out by the current popularity of the T3 3071 over the 2871 (at least here in Oz) so I don't think he's just some crackpot.

                      I never planned to just 'slap on' a comp wheel of any description. Please don't assume I'm a total noob to turbos, cars or the SR-just new here. I bought the 2871 without a comp cover and so I bought a std S15 T28 comp cover cheap which has to be machined for the 71mm wheel anyway (got quoted $250 for a new 2871 comp cover over here in Australia) so it won't be an added expense to machine it for the 67.

                      I guess I hoped to have a discussion based on the comp maps and maybe learn a bit more about them.
                      Last edited by nisspug; 07-22-2010, 02:44 AM.

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                      • #12
                        Garrett doesn't release real turbine maps, do the compressor maps are what they are.

                        Not sure what you really wanted to talk about. Yes, the smaller comp wheel will not make as much power as the 71 mm wheel. Will it reach lower massflows at a given PR with the same RPM? Probably. It will also require more RPMs for the same power potential. Does it match up better to the 53.8 mm NS111 turbine wheel? Maybe a little bit, but no one knows for sure.

                        The important question is will that few percentage points of extra turbine efficiency and little more compressor efficiency really make a tangible difference in how the car feels? No, it really won't.

                        People get "target fixation" om certain numbers or efficiency percentages without realizing there is a "big picture" that it all had to fit into. It's just not likely to do much more than limit total power capacity on the engine and give you an infinitesimal spool gain and weaker top end.

                        See the GT2860R for a more extreme example of this.
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                        • #13
                          I thought compressor maps were a more useful and exact tool than you seem to be saying they are. Sounds like its alot more of a suck-it-and-see situation. The 2871 map shows a peak efficiency at around 6000rpm at 22psi whereas the 3267's peak is around 5000rpm. I would have thought this would be quite a big effect on how the car makes power and drives but maybe I'm wrong.

                          Anyway not sure which way I'll end up going but if I decide to use the 67 I'll post up a dyno. Thanks for your input.

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                          • #14
                            A few percentage points don't make much difference - so the math. Yes, 76% sounds way better than 72%, but in the end you only increased efficiency 5%. You also have a turbine that affects things MORE than the compressor, and Garrett doesn't give out detailed turbine maps. Which is why I say there's no way to do all the math... because well, there's no way to do all the math when not everything is given.

                            Wheel inertia plays a larger role than a few points of compressor efficiency, but it's still not that huge. You're paying attention to compressor maps too much.
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                            • #15
                              Is the car on the road and running ?

                              Im reading this and thinking that you are doing all the math before fitting etc ?

                              I have the larger of the 2871's installed and i have a fully built and flowed engine so my opinion of the turbo is going to be no use to you at all unfortunately!

                              It sounds like you are worrying about nothing to be honest, if you are looking to get the most out of it then you need to be looking more towards the engine and increasing gas volume and flow.

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