[Bruce H.]

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 #3.

3. 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. Revised March '09. I've been using a method called "scaling" that more accurately estimates engine VE across the rpm band, and this allows more accurate airflow requirements. See the flow calculations and compressor map plotting for my setup in the link in post #69. 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.

 

[Bruce H.]

Page 2.

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Source of both maps: http://www.turbobygarrett.com/turbobygarrett/catelog/Turbochargers/GT28/GT2860RS_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.

 

[Bruce H.]

Page 3.

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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!

 

[Bruce H.]

Page 4.

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

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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.

 

[Bruce H.]

Page 5.

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.

 

[Bruce H.]

Page 6.

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. If you discover any errors, please contact me directly.

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.

The MR2 Owners Guide to the Garrett GT28RS. Details of my set-up, links to other users, dyno results, and lots of other useful information.
http://www.mr2oc.com/showthread.php?t=127363&highlight=Gt28RS+Guide.

Original Turbine Efficiency thread.
http://www.mr2oc.com/showthread.php?t=108366&page=1&pp=30&highlight=Gt28RS+Guide

Special thanks to OLD SMOKEY and SFLMR2 for their assistance and input.

 

[mr2by4]

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.

I have one for sale cheap. Decided I was not that daring. Just went with a more traditional turbo.
http://mr2oc.com/showthread.php?p=1599004
Somebody should make me an offer. :smile:
You could be a trailblazer in turbo tech like Bruce.
Bruce, want to switch from road racing to drag racing?

 

[Bruce H.]

Bruce, want to switch from road racing to drag racing?

Are you kidding? I've just about got the car dialed in for the track, and if I switch to the strip I'll have to go back to square 1, and then learn to drive it all over again!

 

[sab0276]

Nice write-up Bruce! The GT3076R looks very intriguing. It would be interesting to compare that to J&H's GT35R.

-Scott

 

[mr2by4]

You can't fool me, I see you lurking around the gt30 and gt35 threads, asking just a very few sneaky questions. You are dreaming at least a little about what 400 whp would be like.
Don't do it. That is where I killed my first MR2. I was unstoppable at auto-x. I dominated lighter cars and cars with much larger motors with the response of the turbo and the balance of the chassis. I went drag racing one time and wondered why I had no top end (ct26 is dead above 5500 rpms). I put on a bigger turbo and blew the motor on the dyno. The car never spent more than a couple of weeks out of the shop until the day it was burned in a freak grass fire. If only ATS had been around back then. I might still have my stripper black hardtop turbo! (no leather, no ehps, hardtop).

 

[g_man1968]

Thank you for writing this Bruce.

An interesting conclusion can be drawn from your turbo comparisons. It looks like the Garrett engineers had this right from the beginning when the bb turbos that were available were the gt28rs, gt30r, and gt35r. If you are interested in a GT turbo, I would say stick to one of those turbos depending on your horsepower goals and leave the hybrid variations alone.

 

[Old Smokey]

Bruce, I am proud to have helped!

mr2by4, Bruce may think about those 400 horsepower capable turbos, but he made me promise to shoot him if he ever tries!
I do not think it would come to that though. I think he would be far too distraught to actually go through with it when his Supra gets jealous and disappears. I wonder where it would go? :angel:

 

[99_GS-T]

Do you have stock shares in GT28RS turbochargers?

The line on a turbine flow map posted in the Garrett catalog does NOT tell you where the turbine is at the maximum efficiency point. It tells you the peak airflow thru the turbine wheel at that expansion ratio. Truth is, it may only touch that peak efficiency one time on that whole curve (if at all) and the rest of the curve could be 10% lower and you'd never know the difference with the maps posted in the Garrett catalog. The efficiency rating is an isentropic efficiency rating and nothing more. It says the peak efficiency obtained under ALL operating conditions, not just at the choke flow.

Secondly, all the flow does not go thru the turbine wheel. That's why we have wastegates. That fact alone kind of screws up your theory about the GT28 turbine wheel being able to flow more air then the map indicates. FACT is, that turbine wheel CAN NOT flow more airflow then what is charted out. Notice how the line goes nearly flat at a 3:1 pressure ratio and they don?t bother charting it out higher? It?s because the line remains flat as the turbine setup has reached choke flow.

Third, turbine maps are in CORRECTED airflow. You are not accounting for that anywhere and it makes a very big difference.

So if it can?t flow any more air then that correct max airflow amount, how does it get the rest of the exhaust out and the energy to produce boost? It raises the exhaust pressure so that the corrected airflow remains around the corrected choke flow rate. Once the pressure is high enough to maintain this choke flow rate and get the energy required to spin the assembly, the remaining exhaust is throttled out the wastegate. Throttling is an energy wasting method of controlling exhaust energy, thus the reason for it being called a wastegate.

Turbine to compressor wheel diameter ratios mean very little in the real world. I've seen t31/t67s making well over 600 WHP. 1.30:1 ratio... Must of been like 40% efficiency too since it was an old junker non-GT wheel and spinning too slow...

Don't tell my friend that's making 800 WHP on a .68 A/R T4 turbine housing that it's too small for that GTQ turbine wheel...he has the dyno proof to say otherwise.

I hate to tell you this (mostly because I prefer being able to have math give good solutions), but you can do math all day and be completely out in left field on turbo selection.

 

[sflmr2]

Nice piece of work Bruce, and a good reference for us all. Thanks for taking the time and effort to put it all together.

 

[mr2by4]

I think that what Bruce tried to do was not provide all of the answers, but rather provide some data and a limited analysis. People on this board are very helpful, but are also very skeptical. Just because someone's data is imperfect or inconsistent, does not mean that they are a liar. Frequently they are just providing the best information they have, imperfect or incomplete as it may be. If we become so critical and cynical that we attack anyone who tries to produce a comparison of data, we will end up robbing ourselves of the information which is contained there-in.
This is an excellent talking point for the start of questioning, not the final answer.
We all know that the final answer is the CT27

 

[Bruce H.]

Hi 99 G-ST,

Thank you for your response. I'm aware of your interest in this area and have read many of your comments in other threads. I think we agree more than you might realize and I'll respond to your 3 points, starting with #3.

3. Corrected airflow. I did not discuss the formula at all for calculating estimated airflow, although the formula I used, and the "estimated" flows I used, are for corrected airflow, as are the formulas used in the "Sources" at the end of the article. The correction that '99 refers to pertains to the air intake system pressure losses ahead of the compressor, and adjustment to mass air flow for compressor intake temperatures to make them compatable with Garrett's compressor maps which are at 545 degrees Rankin (85F). This is just one of the many factors that I was refering to when I suggested everyone work through the calculations for themselves. And yes, thank goodness for wastegates!

2. Flow limits of the GT28RS turbine. No where have I said or implied that the turbine would flow more than its map indicated, and the map itself doesn't even say what the max. flow is. The maximum efficiency line is almost flat by a Pr of 2.25, and that only indicates that max. efficiency remains at that 17.5 lb/min turbine flow level no matter how much higher you boost or flow. It is not saying anything about the turbines actual maximum airflow ability.

1. Same as #2. As far as the map showing peak airflow vs peak efficiency goes, if I understand correctly what you are saying, I think you are confusing what this map is showing. There is a curve showing peak airflows at different pressure ratios, and even how that flow varies slightly with pressure ratio at any given shaft speed...but that is not the curve that is shown on this map. This curve is showing the turbine flow that the stated maximum efficiency is attained at across various pressure ratios.

For the record...I don't know everything about turbochargers. This article is intended to cover some of the factors that turbine experts recognize affect performance in a very real and meaningful way, and I hope I have relayed them reasonably accurately. It's a good place for those with knowledge to interpret and discuss those general factors, and for others with little knowledge to learn. Understanding the basics is perhaps enough to help avoid making a mistake, and it seems to go a ways to understanding the results we've seen in the real MR2 world with all of these GT turbos. And it was good enough for me to identify a previously untried model with a certain level of success. Trying to figure all this out on an MR2 has kept me kind of busy, so I don't really know if it applies in the DSM world, or any other

I don't dispute that huge power can be made with a less than ideal wheel ratio. That's not what the ratio is for. They've been suggested by Garrett to identify those turbos that make a good compromise between low end spool/power and top end power. The max. efficiency rating and curve go a step further to determining spool characteristics. I think they both could be used to predict transient response as well.

So keep the feedback and discussion going. There's lots to learn and share.

Bruce

 

[Bruce H.]

Page 6.

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 to 7000 rpm, and a little more, 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 a 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 at 7000 rpm 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.

I've been asked about my conclusion that the GT2871R will make less power at high rpms because of the less ideal wheel ratio and lower max. efficiency. Afterall, both turbos and the GT2876R for that matter all share the same identical turbine...the only difference are the larger compressor/wheels. So why would the GT2871R flow any less power at high rpms and the same 17 psi boost?

It was my understanding, and perhaps wrongly so, that the reduced power that I'd seen in dynos of these models was a result of reduced volumetric efficiency caused by increased turbine backpressure caused by the turbine wheel operating at lower shaft speeds as dictated by the larger compressor wheels.

If my statement as to the cause was not correct I'd like to edit the article accordingly. Can anyone shed some light on this aspect? Thanks to Mike R for the question.

Bruce

 

[sflmr2]

Somewhat related to your general post Bruce, I've seen discussions here and in other forums about balancing pressure between the intake and exhaust manifolds.

It seems there is a mindset among some that pressure in the exhaust manifold should be the same or lower than pressure in the intake manifold. I know there are ways to measure pressure in the ex-mani, but I don't recall if anyone has ever recorded and posted any relative pressure readings with various turbos.

Perhaps someone who has such devices can post their observations and share with us how their VE and power curves were affected as ex-mani pressure rose over the intake pressure.

 

[DougyD19]

It seems there is a mindset among some that pressure in the exhaust manifold should be the same or lower than pressure in the intake manifold. I know there are ways to measure pressure in the ex-mani, but I don't recall if anyone has ever recorded and posted any relative pressure readings with various turbos.

I think it was Kblake that did a test on the CT26 and posted up the increased backpressure. Maybe he can comment.

 

[DougyD19]

Ok, I'm lost in the relation of the turbine wheel to the compressor wheel. I know it is connected by a shaft. You'd think that you could get away with have a honkin big compressor on one side and a small turbine wheel on the other side but we know it doesn't work well. Why? Seems to me if you can spin the small turbine wheel faster, the shaft will spin faster, and make the compressor wheel shove more air in the engine. Now I understand somewhat that once you hit a certain RPM speed your compressor and turbine wheel will lose/gain capability by the flow maps.

Spoolup problems --- So what am I missing, because I know I don't see people with oversized compressors spooling faster than a smaller compressor using the same turbine wheel. Maybe it is something about the mismatched wheel is causing trouble with the spin up. Is the turbine wheel having a harder time trying to get spinning because of the resistance force the bigger compressor wheel creates?

Changing the AR --- I've read posts where people changed out the turbine housing for a smaller AR and got better spoolup and still had the same power....and also posts where they went with a bigger AR and got more power and spooled slower (like going to a supra turbine housing). At what point in turbo selection do you know when to use a different AR housing? How do you account for this change on a compressor map? One of the mods I did to my CT26 turbine housing, I increased the inlet entrance area and bored into the radial area some....does this mean I increased my AR? I know I have more power...but then why is my spool so good at 200tq at 2850 rpm....without a boost controller or TVIS? I only have a 264 intake cam and one cam exhaust cam gear...surely this isn't helping to change VE that much or is it.

I'm sorry guys, I'm still trying to figure out the missing link of what is keeping say a CT27 from getting to 400whp (46 trim compressor limited dyno max I've seen) or the 450whp clipped turbine wheel limit of the Supra CT26 that the supras can push. It must be all be related to the increased AR of the supra housing being bigger.