Compound turbocharging tuning ?'s

[Todd W]

I have been trying to learn all I can about compound turbocharging with plans to do this to my truck in the next year or so.

My current question deals with the pressure ratio of each stage. I have read that it is destirable to have equal pressure raitos from each stage. After having gone through a few searches I have seen people running there first stage as high as 50 psig. This is around a 4.4 PR at sea level. this would yield like 270 psig manifold pressure if a 4.4 PR second stage was used.(14.7*4.4*4.4)-14.7.

I am assuming that people aren't seeing that kind of pressure so obviously they are running a much lower PR on the second stage. With 50 psig in the first stage I get that a 1.8 PR on the second stage would still give around 100 psig.

Is there any advantage to doing this over using equal PR's in a compound setup.

Also if my math is way off feel free to correct me.

 

[White Knight]

First, most people don't even know what pressure ratios are, they just think half of the total boost should come from the primary :doh:

Second, with twins you have the advantage of compounding pressure, so you don't need to, and shouldn't, run the turbos at such high PR. Try to put each turbo in its peak efficiency if you can. For an all out competition engine pushing the primary to where it makes its max flow works well, then just compound enough to where they work well together. If you need to push the primary harder to get the extra air you should step up to a bigger turbo.


If you were to try to compound both stages at such high ratios the secondary would be uselessly eating up a bunch of energy an unnecessarily heating the charge air. Basically you would be way over working it for more benefit.

 

[Big Blue24]

Garrett seems to have the most readily available compressor maps.

In studying their published compressor maps, I have come to the conclusion that larger turbos in the 75-95mm compressor range are more efficient at 2.5-3.5 pressure ratio range than smaller turbos with 57-66mm compressors.


What this tells me is that in order to achieve maximum efficiency = maximum airflow with minimum amount of added heat/air temperature, a twin setup needs to gain more pressure ratio from bottom larger turbo harder than the smaller second stage top turbo.

 

[kscumminsdriver]

I imagine what is being missed here is that many people wastegate past the secondary charger to drive the primary harder. I know I've read many times that people will setup the wastegate to start opening at 20psi of total boost... I could easily see the primary putting out 50psig if it's already slightly spooled(I typically see 5 psi on my primary with 20 on the system) and being exposed to almost the entire exhaust charge... you could easily have in excess of 60psi total in this instance and only have a PR around 2 on the secondary.