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Turbo sizing, comparison, etc

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I've been doing a lot of searching on this topic, frankly I still am. This stuff comes up all the time in comparing things even like what size turbo blanket you should get. I figured it might be useful for those who are looking to do the same. I am by no means very knowledgeable regarding this topic. Please feel free to let me know preferable with links and such so that I can add them.


Starting with the very basics of selecting turbo chargers.


Couple assumptions/definitions off the bat.


Pressure ratio:

Is a ratio of pressure (duh). Usually this is in terms of absolute pressure over atmospheric pressure.

To find your pressure ratio you would add the atmospheric pressure to your desired pressure and divid over the atmospheric pressure.


I want 14.7PSI of boost, what is my pressure ratio?

(14.7lbs/in^2(lbs of boost desired)+14.7lbs/in^2(atmospheric pressure)) / (14.7lbs/in^2) = Pressure ratio of 2.


Bar is a convenient measure equating the atmospheric pressure of 14.7psi almost to 1bar.


The formula above would change to:

(1bar (boost desired) + 1 bar (atmospheric pressure))/ 1bar = Pressure ratio of 2

Thus dividing by 1 you don't really need to recall the unit. Simplified the equation for pressure ratio (desired boost in PSI/14.7PSI) + 1.


Corrected mass flow (Lbs/min):

Corrected mass flow is in the units of pounds per minute. This is as you can guess a measure of mass of air moved per minute.

This is how much air the turbo flows, ultimately this is the measurement to determine how much power a turbo can support.

A very basic easy assumption is to multiply Lbs/min by 10 to get the theoretical horsepower the air mass will support.



This is a percentage efficiency of the turbo or a fraction out of 1 in this case.

The value is graphed as islands on the graph (circleish looking things).

Basically as the efficiency number drops the turbo is loosing more work in the form of heat. In laymen's terms that means you are heating up the air as the efficiency percentage goes down. Ideally you want your turbo to stay in the peak efficiency island in your planned use area. 



Rotations per minute of the turbine.

This is the rotation speed of the turbo. Graphed with horizontal lines along the islands.

Causing the turbine to spin faster means it moves more air, at the same time it generates more heat


Here are some flow charts:

Note the Y axis (vertical) is labeled with the pressure ratio

Note the X axis (horizontal) is labeled with Lbs/min

Note the "islands" marked with a percentage number indicating efficiency.



So there is a lot of information you can extract from a compressor map.

For example:

You can say that at a PR of 2.5 or the equivalent ~22lbs of boost this turbo will generate 42lbs/min while at peak efficiency of 75%. Alternatively you can see that this turbo will support 600hp using the assumption of about 10hp/CMF at a pressure ratio of 4, or 45PSI. 

Or you can see that at 15lbs (roughly a PR of 2) this turbo will support ~420hp at its worst efficiency or 310hp at its best.


Now you can tell how to read a compressor map. Looking at compressor maps and plotting points is the best way to shop for a turbo. Plotting points is a matter of determining how much power you want to make then working backwards and figuring out how much boost it would take to generate that power and seeing if the intersection of those two points are in a good efficiency island. 


Additionally you can determine how future proof the turbo is by putting in your theoretical max and seeing if the turbo will support the power. Ideally your planned power points would coincide with more boost so that you travel within an efficiency island up to your theoretical max. Unfortunately reality will set in as high boost levels will require larger ring gaps, potentially multi layered steel head gaskets, o-ringed blocks, ARP hardware, external engine clamps etc. Limiting the ability to travel along that curve unless the block is built for it.


The above is a very good turbo, let us look at some older smaller turbos to see the short comings.






So here we see a common turbo upgrade the T04E 57 trim. You can see at peak efficiency this turbo makes about 32lbs/min at a pressure ratio of ~2.4 spinning the turbo at about 96k RPM. So at about 21PSI you can expect about 320hp. By keeping the wastegate shut longer you can increase the turbine speed. Hot air is not as dense as cold air to start so you might flow more air as is logical, but the air coming out of the turbo will be hot, less dense, and the turbo will in turn have premature wear. Trying to get 450hp out of this turbo would be very unwise. You would be fighting it in some regards.




That is some vary basic info on turbo sizing and reading compressor maps, there are guides out there that are better for sure so please refer to them.



Now it seems like each company has their own nomenclature not all of them consistent VF turbos found on subarus for example are numbered by generation VF22 and VF42 being of a similar size with the VF23 and VF24 for example being much smaller.


Borg Warner uses a single digit body size followed by the inducer value. So S257 would be a S200 series body with a 57mm inducer, an S364 would be a S300 body size with a 64mm inducer. S464 would be a S400 body size with a 64mm inducer, the last example suddenly throws a bit of a wrench in understanding if you are not familiar with body size. 


Garrett has sizes in GT. GT30 and GT35 used to be fairly standard nomenclature usually taken off diesel engines referring to an overall size. With more companies expanding into the performance turbo world now we have more choices for example GT30 has a variety of choices like the GT3076 and so on and so on. The first two numbers generally refer to a body size, the later two refer to the exducer on the compressor side


Precision likes to use inducer size so 6266 would refer to a 62mm inducer on the compressor and a 66mm inducer on the turbine side.


Holset is another company that supplies turbos to diesel engines they also like Garrett and Borg Warner use body sizes HX30 and HX35 and so on.


Then you have the T-series which are made by a variety of manufacturers without necessarily a correlation. Usually a body size with a trim designation. T04E 57 trim for example





There are 4 main measurements that will help determine turbo size. First is the inducer/exducer of the compressor side, next is the inducer/exducer of the exhaust side. For the compressor, the inducer is where the air goes in and the exducer is where the intake charge is compressed with the larger diameter. For the turbine the inducer is the bigger diameter where the exhaust flow spins the wheel and exits via the exducer. So looking at a cut away it would go:


Intake -> Compressor inducer -> Compressor exducer -> Turbine inducer -> Turbine exducer -> Exhaust


I would be remiss if I did not discuss trim. The term trim is used to compare the two values. Trim is calculated by (inducer/exducer)^2 * 100 for the compressor and (exducer/inducer)^2*100 for the turbine. This is useful information if you are say talking about the same compressor or turbine housing. The trim would tell you the ratio between the inducer and the exducer. The larger the number correlates to a larger wheel given the square comparison. The downfall is that theoretically the same trim value can be used for the same ratio wheel, and frankly is a slightly harder concept to decipher and grasp, although in the chart below I will include the calculated values.


So the next easy comparison is compressor exducer to turbine inducer comparison with the compressor wheel coming first. This tells the diameter of the two wheels in the turbo.


The only caveat is that older turbos can have small compressor inducers, that means the blade on the wheel is going to be very skinny then flare out wide at the bottom. So a holset HX30 which we would call a 7370, will not match a BW S257SX-e which is a 7670 as the holset has an inducer of 44mm vs the BW 57mm

Edited by seattlejester
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Reserved for chart info


Please feel free to recommend turbos interested in

Name:                              Nomenclature      Compressor Inducer     Compressor Exducer    Comp Trim     Turbine Inducer     Turbine Exducer     Turbine Trim



GT2860R                                   6054                             47                                        60                            62                        54                           47                             76

GT2860RS                                6054                            47                                         60                            62                       54                           47                             76

GT2871R                                   7154                             53                                       71                              56                        54                           47                             76                 

GT3071R                                   7160                             53                                        71                             56                         60                           54                             84 

GT3076R                                   7670                             57                                         76                            56                       60                            55                            84

GT3582R                                   8268                             61                                        82                             56                        68                             62                          84 

GTX2860R Gen II                     6054                             46                                        60                             58                        54                            47                            76 

GTX 2867R Gen II                    6754                            50                                        67                             55                        54                            47                            76

GTX 2967R                               6757                            50                                         67                              55                         57                         52                            84  

GTX 2971R                               7157       54    71    58    57    52    84

GTX 2976R                               7657    58    76    58    57     54    90   

GTX 3076R Gen II                   7660     58    76     58     60    55    74

GTX 3576R Gen II                   7668     58    76     58    68    62     84

GTX 3582R Gen II                   8268      66    82     64     68     62    84  


Borg Warner:

BW S252SX-E                        7070                                52                                         70                         56                               70                           61                     76

BW S257SX-E                        7670    57   76   56   70  61   76

BW S362SX-E                       8376      61     83     54     76     68     80 

BW S363SX-E                        8776      63      68      52     76     68     80

BW S364SX-E                        8780     64      87      54     80

BW S366SX-E                        9180      66      91     52    80    73     83

BW S369SX-E                        9180      69       91      57      80 

BW EFR 6258                        6258      49      62     62     58

BW EFR 6758                        6758      54     67     65     58

BW EFR 7163                        7163     57     71    64     63

BW EFR 7064                        7064     52     70     55      64

BW EFR 7670                        7670      57      76      56     70     61      76

BW EFR 8374                        8374      62     83     56     74

BW EFR 9174                        9174      68      91     55     74

BW EFR 9180                        9180      68     91     56      80     73     83


T3/T4 57trim                  7666     51   76            66     56

HX30                               7370      44    73           70     60

HX35                               8370      58      83         70      64

P6266                              8666      62       84          66      82      76   

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From the reading the EFR is basically an S-series on crack. Stainless steel divorced and profiled turbine housing, titanium turbine wheels, ceramic ball bearing, integrated waste gate and pop off valve. Basically the only reason to run an S over an EFR is price, although if you wanted a larger waste gate or a blow off valve in a different location you could argue the EFR might be a tad wasteful in that regard


The S252SX-E would be the closest comparison I think being an almost 70mm compressor exducer and a 61mm turbine exducer.


Comparing the two compressor maps you can see that even with a similarly sized wheel it is drastically different.




You can see from the shape the EFR turbo is much more linear in that more boost nets you more compressor flow while still being efficient while the S series does eventually cap out and efficiency drops as you attempt to extract more flow from it. At basically any given point the EFR makes about 10 more lb/min within the same efficiency island at the same pressure ratio under about 50lbs/min over that and the difference seems more negligible given that you are maxing out the turbos capabilities. 


The counter argument would be you could upsize the turbo to match the flow like the S257SX-E which is a 7670 in EFR terminology would flow similarly to the EFR 7163. 


Flow isn't the end game though, given the titanium wheel and the ball bearing, the EFR will spool much sooner and recover faster from throttle off moments. Depending on your activity that might make or break your choice.


So basically if you want the ultimate response and power for a certain package going for an EFR turbo would be a good idea. I was at formula D and it seemed almost all the drivers with 2JZ's had EFR turbos under the hood 5/7, with only one driver having a traditional S series, and another I think having a turbonetics turbo. The S series shouldn't be taken lightly though, the S series is in my humble opinion one of if not the best option for a journal bearing turbo considering factors of flow, price, and size.

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Without looking up any information and looking at just the chart, I would assume it would be a 7654 by my nomenclature? The 76 is kind of a nice sweet spot for pretty darn good power numbers, but the smaller turbine inducer makes me think the hot side would be really really small, most likely to help spool smaller engines I'm think SR20, EJ22, K20. If you are putting this on an L28 with higher compression numbers you might be spinning it quite fast and build a lot of boost early, but have it run out of steam higher up. Granted given the bigger compressor size it would move lots of air very quickly before it ran out of puff.

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It might be 7154 by my nomenclature, I looked up an ad for it, if I still remember the formula looks like this turbo will support 400 or so hp at a whopping over 2 bar or 30lbs of boost according to the compressor map while still being pretty efficient at .75. The listing says it supports up to 500hp, while the compressor map shows that it can go to high 40's CFM that would be at the lowest efficiency on the chart of about .6.


It reminds me of like the old school hybrid T3/T4 turbos, just with a more modern wheel and ball bearings. Probably a good fit to push about 400hp, in theory with the right hot side it should still ramp up pretty quick, but maybe sacrifice the top end probably being at or under 400hp. If you went with a bigger hotside you might get over 400hp, but would have a later threshold. 


This is all in theory and looking at charts, no first hand experience of course.

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Yeah, that was the model recommended by a tech over at Jim Wolf for my l28et. 


This being my first turbo build. I was looking for something that had a large window for low boost for cruising and a peak of 550hp once in get the forged pistons installed. So I bought a used Holset hx35 twin scroll of of a 4 cyl diesel school bus lol. Readily available, economical, and local support for parts was my thought. I was told it was going to come on way to late, and needed to fab a equal length header. Thought abouy saving that project for next winter in an effort to getting it on the road for spring...so started looking at alternatives. 


First turbo project, and only a loose grasp on the technical side of them.

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With the low compression L28ET, a gtx3067r seems like it would be a pretty good fit for a fast 350-400hp. I'm not sure the compressor map would support a claim of 500hp or more though.


Generally for cruising you aren't in boost. If you wanted variability in low to high you are going to have a pretty large wastegate to manage the exhaust and an electronic boost controller to differentiate from cruising to full power.


HX35 seems like it may support the power just looking at the size of the compressor exducer wheel, but I believe their hotside choices are designed for large displacement diesels. For spool if we take the fact that diesels run at a higher compression ratio basically and add that to the large displacement, then reduce that by the revs you can get some semblance of an idea for how late the threshold would be. Anecdotally on a 2.5Lish motor people are reporting boost doesn't come on till 4k or more. Depending on your rev limit that could be something as small as 2.5k rpm window of boost.


By comparison my turbo a BW S257sxe a 7670 with a small hot side overboosts to over 20lbs before it hits 4k rpm in a 3L motor. 


It used to be to have a turbo that can support 500+hp while having a low spool meant you had to do some fancier tech like compound turbos, QSV's, or twin scroll. While that definitely helps, with things like hybrids and better tech you can spool a large compressor wheel to a small hotside, you will reach a point where the hotside is too much of an exhaust restriction though.



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