Engine/Transmission (1998.5 - 2002) compound turbos/turbo maps

[TLayton]

I have a question concerning compound turbos and turbo maps. When figuring the lbs. /min air usage for the turbo map do you divide the number in half since you have two turbos? I have read that you do but that was on a system running parallel turbos not two turbos in series. Any ideas? Thanks



Tadd

 

[Diesel Freak]

In our application, the Mass flow rate is constant. Both turbo's flow the same, however the density ratio goes up as the air gets compressed.



both turbos running at a Pressure ratio of 2. 54 should net 60 PSI of boost.



Assuming 68% effeciency for both turbos



Inlet air temp= 80F



Ambient pressure is 14. 7 PSIA



Air out of the large turbo is 319F, 22. 6 PSIG, and has a density ratio of 1. 76



Air out of the small charger is 678F, 60 PSIG, and has a density ratio of 1. 79





Overall Density ratio is 3. 15



Assuming an 80% efficient Intercooler, Charge air temps should be 219F and aftercooled Density ratio should be 3. 51 (assuming 10 psi head loss through all piping and the intercooler. )



If an interstage cooler was used..... well, you know!





Also assuming total mass flow of 120 LBM/min

 

[TLayton]

Diesel Freak

Thanks for the reply.



Ok, I am a little lost.



I seem to understand the pressure ratio side of things. But since the turbos each flow the same and you put total airflow would be around 120 then how would I pick turbos. Most turbo charts dont go anywhere near 120 lb/min. Say I wanted 550-600 hp out a 5. 9L. Maybe you could help me by suggesting what air flow requirement per turbo so I could find the right efficiency on the map. Also on compound systems how do you determine what airflow the small turbo should do and what the big turbo should do? Any help will be greatly appreciated.



Thanks

Tadd

 

[Diesel Freak]

Originally posted by TLayton

Diesel Freak

Thanks for the reply.



Ok, I am a little lost.



I seem to understand the pressure ratio side of things. But since the turbos each flow the same and you put total airflow would be around 120 then how would I pick turbos. Most turbo charts dont go anywhere near 120 lb/min. Say I wanted 550-600 hp out a 5. 9L. Maybe you could help me by suggesting what air flow requirement per turbo so I could find the right efficiency on the map. Also on compound systems how do you determine what airflow the small turbo should do and what the big turbo should do? Any help will be greatly appreciated.



Thanks

Tadd

There are turbos that flow 120 LBM/min... just do a little digging.



On a compound system, both turbo's flow the same ammount of air

 

[TLayton]

You're gonna make me work on this aren't you.



Thanks



Tadd

 

[Diesel Freak]

Originally posted by TLayton

You're gonna make me work on this aren't you.



Thanks



Tadd

LOL, yep!

 

[Hohn]

Yes, in a twins setup the two turbos will have the same mass flow.



But this mass flow is determined by the LARGE turbo. The purpose of the small turbo in a twins setup is to complement the large turbo in this way:



1) it spools up faster than the large turbo, which helps the large turbo spool more quickly. In a twins setup, the combo will always spool slower than the small turbo alone, but faster than the large turbo alone. Thus, you are dividing the lag between two turbos, which optimizes the compromise between spoolup and flow.



2) The small turbo reduces the required pressure ratio that the large one would otherwise have to produce. For example, you can have two turbos each running 3:1 PR, instead of having the one larger turbo have to wrestle with a 9:1 PR!! Most larger turbos are designed to flow a high volume of air, but cannot handle an astronomical PR without exploding. Plus, dividing the PR between two turbos reduces the total amount of temp increase that the intake air will see.



It's interesting to note that as you add more compound turbos in series, the benefits taper off. For example, going from a single to twins give you a 50% improvement. But twins to triples gives another 34% improvement. Going from triples to quads nets another 18% improvement.



As you can see, the benefits taper off fast.



Justin