Compound Turbos and Boost Numbers - The Math


Compound Turbos Explained
There is a lot of misleading information out about compound turbo boost numbers.  Many think the high pressure turbo (secondary) boost number can be determined by just subtracting primary turbo boost from overall boost.  This is NOT the case.

To figure out how much work each charger is doing, you need to convert to pressure ratios (P/R) first.  The correct pressure ratio formula to use is:

•    (atmospheric pressure + boost from gauge pressure) / atmospheric pressure = P/R.

After you find the secondary turbo P/R then convert to psi using the steps below.

Here is an example of a customer’s truck with a set of non-VVT turbos that I’m helping dial in:

•    75psi overall boost (gauge pressure)
•    37psi primary boost (gauge pressure)

We take the baro (atmospheric pressure) reading from the truck at the time of when these boost numbers were recorded (in this case 14) and add to the boost psi, and then divide by the same baro number:

•    75psi overall boost (14.0 + 75) / 14 = 6.35 P/R
•    37psi primary boost (14.0 + 37) / 14 = 3.64 P/R

We now divide overall P/R by primary P/R:

•    6.35/3.64 = 1.74

Secondary P/R = ~1.74.

Using the above formula, just in reverse, now convert back to psi:

•    1.74 x 14.0 - 14.0 = 10.4 psi (secondary boost number)

In this scenario, currently the primary charger (37psi) is doing far more work than the secondary (10.4psi). This customer has pressure tested his set up with shop air so we know we are not dealing with any boost leaks, so I recommended a couple things.

First, adjust the waste gate to stay closed until the P/R comes in line.

If this doesn’t work, then the turbine housing will need to be reduced.  Hopefully the compound set-up was designed for the application and not just a stab in the dark, and making these changes will fine tune the set-up.

The best case scenario is the pressure ratios match between the turbos.  This sounds good but in reality is not easy to do.  On a race application, you’d like the P/Rs to be close to 1:1 under 100% throttle; on a street truck the P/Rs should be close during the load range where the truck is most used.

Correctly designing a compound set-up for the goals of the truck is the first step.  Fine tuning the setup using the above information will optimize each turbo for the best HP, lowest drive pressure, and coolest EGTs possible on the truck.

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