Fixed Vane vs Variable Geometry Turbochargers...
10-28-2017, (Subject: Fixed Vane vs Variable Geometry Turbochargers... ) 
Post: #19
RE: Fixed Vane vs Variable Geometry Turbochargers...
Here are some ball-park figures related to the 2350 and turbocharger. Thought I would share them ...

15Litre ISX = roughly 915.4 cubic inches.

It takes roughly 34-36, (38 max is pushing it) psi boost pressure (assuming no egr gas at all) to make 700 HP on the ISX CM2350 to get the oxygen levels correct (give or take a couple psi). This puts the post turbo EGT's roughly at about 950F (1100 pre-turbo) where they should be. More boost pressure would cause detonation, lower than normal EGT's, and risk due to its HCCI + higher than normal compression ratio (18.9:1) combustion chamber design. Being aggressive (on the high side just as an example) and using 38~ish psi puts most of the VE holsets at a VE efficiency of roughly 2.4 from what I have measured in the past. Below 600 HP, it is more like 33-34 (maybe 36 max psi). Add roughly 2-3 psi if EGR is active as a base rule. -- A different size turbo will yield different airflow requirements to achieve the power desired though.

-->> The question is what size turbo???

formula is roughly >> cubic inches * rpm / 3456(4-stroke diesel) * 2.4(desired VE efficiency of turbo at max engine load, 38 psi boost). The value of 3456 is typical of a 4-stroke diesel engine, roughly 90 degrees F at the intake, 85% relative humidity when running hard (courtesy of Donaldson Engine HP and exhaust flow reference guide. Every engine can be slightly different with roughly up to a 5% variance though).

@ 700 HP, 2100 rpm, 1,750 ft/lb-trq. - (absolute maximum airflow at 700HP)...
(915.4 * 2100 / 3456) = 556.2
556.2 * 2.4 = 1668 CFM
1668 cfm = 0.661Kg/s

I.E.> It takes roughly about 0.66Kg/s to get it to 700 HP at the flywheel at 2100 rpm.


@ 700 HP, 1800 rpm, 1870, 2,042 ft/lb-trq. - (airflow at 700HP where it is likely to be driven)...
(915.4 * 1800 / 3456) = 476.8
476.8 * 2.4 = 1145 CFM
1145 cfm = 0.567Kg/s

I.E.> It takes roughly about 0.56Kg/s to get it to 700 HP at 1800 rpm where it is likely to be driven under hard. Even less cfm at 1650 rpm, where the engine has reached its optimal pulling power in road use.

At the desired 2.4:1 BPR/TPR in the chart (left column), the absolute maximum flow requirement @ 2100 rpm falls right on the line between the HE500 series and an HX60 series turbo for the CM2350. At a more likely driven rpm of 1800 or less, it is squarely places the requirement in the 500 series range. An HE561VE would be the most fuel efficient choice while meeting the demand, while the HX60 would meet the demand with a bit better heat transfer. On a truck with high demand, the better heat transfer would be desired at the expense of a percentage of the engine brake strength. On a truck where efficiency and a stronger engine brake is desired, the 561 would be the better choice though at above 1800 rpm, it is in its upper end of range. I.E. > It would depend as much on use, effeciency desired, engine brake desired, as it would be on overall rpm demand. There is no "one size fits all solution" here.


This is just rough estimate ball-park numbers to get someone close to their goals. Every case is different and every engine model is different too. It also does not take into account a realistic 5% error or variance that I would certainly consider myself while making a decision. Some would prefer more flow and better heat transfer out of their turbocharger, and some would rather have the better efficiency. It all comes down to striking a balance between the 2 in the end. throw EGR into the mix and the whole game changes because egr itself reduces the BP ratio significantly, requiring and allowing for slightly higher pressures depending on how much is used. I.E> -- There is no real exacting formula for that matter in the end.

If someone has some corrections to this then they are welcome to post them, but just remember, I have used this method to help many over the past couple years figure out what size turbo to at least "get started with" for achieving their goals and it has always gotten them in the ball-park where things fell into place once everything was optimized and tested. I am certainly no turbocharger engineer but i do know how to read and use proper formulas to get ball-park figures based on actual studies, measurements, etc. and not some hokey mis-leading crap off some other forum somewhere.

Here is some of the sources that I sometimes use as references when figuring things out ...

Although not free, here is some really good research information on this whole subject in general that I have read/followed in the past...

There is a lot of mis-information on this subject in general so ultimately people need to make their own research when it comes to this kind of stuff, I am certainly no expert. I just tend to crunch the numbers that make the most sense and it usually works out when you compare the results with tests and actual dyno results. It is usually pretty easy to see when an engine is starting to run out of air flow and restrictions are starting to set in when achieving higher HP, torque, and rpm outputs. Monitoring things like exhaust manifold pressure to intake pressure ratio, EGT's, requested vs expected vs achieved toque, and the places where it starts to fall off are also very good indications.

I banned the guy who posted the mis-information. Not for continuing to disagree, but rather the fact that he is one of several accounts and I have watched closely due to problems by those same couple of people on here in the past. It is a well known couple of trolls that likes to stalk the forum trying to spread misleading information every chance they get. Trolls like to start arguments that can never be won no matter how correct someone tries to be. A troll does this just for the sake of stirring everyone up unnecessarily so they can sit back and laugh at it while bragging about their fake "superior skillset" with all its partial truths twisted just enough to make people believe what they say. I feel sorry for people who have to resort to such things and leave them to do so on other places instead of here. I try my best to keep this place leaning towards people who genuinely trying to share information and learn through positive discussion rather than plagiarizing and false statements just for the sake of creating arguments.

User's Signature: ->: What I post is just my own thoughts and Opinions! --- I AM Full Of S__T!.
 Thanks given by: Running rough
10-28-2017, (Subject: Fixed Vane vs Variable Geometry Turbochargers... ) 
Post: #20
RE: Fixed Vane vs Variable Geometry Turbochargers...
What your saying makes a lot of sense, take for example the off road version of the Isx15, (qsx15) the tier4 final emissions engine qsx15 has a monster of a turbo compared to the same year Isx15 say a 2016, of coarse the qsx15 has a lot different torque curve then the on highway Isx15, if you compare the top rated qsx15 with 670 hp and 2050 torque to an x15 605 2050 torque the qsx will eat the x15 for lunch providing you keep the rpms above 1600 rpm, the qsx torque curve drops below 1600 rpm, while the 605 2050 torque x15 would just start to churn more torque as the revs come down, of course a lot of this is tuning but by nature more rpms= more airflow requirements if tuned for higher rpm pulling power, where as a truck engine which normally has a lower load factor then a tractor in the field pulling a large emplement ripping up dirt, so it makes sense a truck engine normally turns slower rpm to increase fuel efficiency, less rpm requires less airflow so therefore a smaller turbo also spools quicker making it a better choice in a truck at normal power levels
 Thanks given by: Rawze

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