Hey guys I got a few questions I just installed a compound turbo kit on my 2000 Cummins. My question is why does the small charger need to be set at 25 psi? The lowest I could get it with the Boost elbow is 32 psi when it starts to open, I don't think I can adjust the actuator because it does not have a nut on it. What would the harm be if I just left it at 32 psi? Thanks everybody

@jlbayes would be a good one to ask.

 

Another is @TFaoro he should know as well.

It doesn't need to be set at 25psi. Do you have an interstage boost gauge and a drive pressure gauge? The boost going to the secondaries waste gate is for reference only.

Thank you jlbayes i love when you comment on my stuff lol...so im fine just leaving the 63/68 at the 32?and no i dont have a interstage boost gauge but its on its way

The wastegate is going to control how hard each turbo works.  It also sees total pressure.  So the earlier it opens, the more work the primary will do. ~32 is where mine seems to be happiest.  Good rule of thumb one you get the interstage boost gauge in is to get each turbo to be doing half the work.  

Si if my secondary is say 30 going down the road,i want my primary at approx 15?

Total boost would be 30. But yes, 50/50.  And it'll be as close as you can get it. For me, light throttle cruising I'm at 5 from the primary and 10 total.  Moderate acceleration my secondary makes a few more psi than the primary.  and WOT the two are split perfectly 50/50.

Which is the reason for the other gauges. Need to do some maths to get them close. I prefer the primary to do more work but it makes the setup more laggy.

So what do i do if my secondary is 30psi and the primary is say 25? What is the plan of action to get it to a one-to-one? Sorry so many questions I'm just trying to learn and I really appreciate you explaining stuff

http://atptrucks.com/blog/compound-turbos-and-boost-numbers-the-math/

I found this as I was trying to refresh my own memory just now.  Pretty helpful and a better explanation than what I gave.

Edited March 5, 20187 yr by trreed

(atmospheric pressure) + (overall boost) divided by (atmospheric pressure) = overall P/R
(atmospheric pressure) + (primary boost) divided by (atmospheric pressure) = primary P/R
(Overall P/R) divided by (Primary P/R) = Secondary P/R
(Secondary P/R x (atmospheric pressure) minus (atmospheric pressure) = Secondary Boost

12+60/12=6

12+30/12=3.5

6/3.5=1.71428

1.71428*12-12=8.57136

 

 

These are Trevors numbers. Secondary pr is low. Be nicer to see it closer to 2.

 

Thank you guys i really appreciate it.i will post numbers once my interstage gauge shows up

So i got the bung and gauge done for the interstage boost..can i do the numbers for where the truck will spend its time towing?if so total boost is 30 and primary is at 13-14psi...or does it need to be full throttle run?

So heres the math at utah 14psi ( atmo)

30+14÷14=3.14 (overall)

14+14÷14=2 (primary)

3.14÷2=1.57 ( secondary pr)

1.57×14-14=7.98 (secondary boost)

 

Edited March 13, 20187 yr by Jonny
Did math wrong ha ha

I do my math based on wot as it is typically a constant. Your numbers look ok for part throttle operation. I would tighten the gate to try and get the secondary pr to come up as it will help with response.

Awesome thank you for the reply...so will the secondary usually always work harder until the gate opens to dump to the primary?

Yes as it spools first. The primary is what cools everything off once it is up and moving.