Odd pressures from twin setup.

RonA

RonA

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Feb 26, 2008
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I rode in a truck last week that was having an unusual problem with the air output from it's compound turbo setup. Piping is correct(he has built many sets of twins).
manifold boost ---- primary boost
5psi------just moved of the peg
20psi-----9-10psi
40psi-----34psi
50psi-----40psi
75psi-----51psi

The truck runs well, but seat of the pants says the power is not matching the boost numbers.
He is running a 62/71/.8 secondary and a GT42-02 primary. The boost numbers remain basicly the same with a different primary(75/96/1.32 S400) and several different secondaries(62/65/.7, 64/71/.8, and 66/71/.8). 12valve truck, tuned 215 pump, DDP4's. Drive pressure is only slightly higher than boost (at manifold). Has tried changing boost gauges, egt's are not unusual. Changing the opening point on the secondary chargers wastegate has almost no affect. Gating the primary to 35 makes the truck a dog power wise.
Any ideas would be greatly appreciated.

Ron
 
how do you know it has a 913 pump?
need more info on the rest of the setup.
 
The primary has a built in wastegate. It can be set to hold the primary boost down to 35. But the power goes away. I have a similar setup 75/83/1.1 S400.With my 50mm gate turned off I see 35psi in the primary with full fuel. The oddity here is that the air is not being compounded properly despite trying multiple chargers(both top and bottom) and multiple settings of the wastegate on the secondary.
 
So for us newbies, what pressures would you be seeing if the turbos were compounding properly?
 
I'm guessing around 35ish psi from the primary and around the same or a little higher for the secondary.
 
There is some debate as to where the turbos "should" be.

Most street twins (my setup included) have a primary that is actually too small to run the system 100% compounded.

In a setup similar to the one described, total boost is 60-70 with 30 comming from the primary.

On a truely compound set, you would square the pressure ratio across the primary.

Mathematically: Total boost (gauge presure) = (((PR1)^2)x14.7)-14.7)

if the Pressure ratio across the primary is 3:1 (essentially 30 psig in the cold pipe) then total boost would be ~118 psig

if the top turbo has a wheel that is ~60mm, then the bottom would be close to 85mm (not counting the hub diameter)
 
Is the secondary turbo's wastegate working properly? Is it an external gate on the secondary?
 
There have been 2 different .8 housings and 1 .7 housing. The .7 spools a bit faster but the odd pressure issue still exists. The odds of having 3 bad housings is pretty high.
 
Diesel Freak said:
There is some debate as to where the turbos "should" be.

Most street twins (my setup included) have a primary that is actually too small to run the system 100% compounded.

In a setup similar to the one described, total boost is 60-70 with 30 comming from the primary.

On a truely compound set, you would square the pressure ratio across the primary.

Mathematically: Total boost (gauge presure) = (((PR1)^2)x14.7)-14.7)

if the Pressure ratio across the primary is 3:1 (essentially 30 psig in the cold pipe) then total boost would be ~118 psig

if the top turbo has a wheel that is ~60mm, then the bottom would be close to 85mm (not counting the hub diameter)
Click to expand...

I am impressed, I learn something everyday on here. It amazes me compound turbos can be deduced to a mathematical formula. Sorry I offer absolutly no assistance.
 
RonA said:
There have been 2 different .8 housings and 1 .7 housing. The .7 spools a bit faster but the odd pressure issue still exists. The odds of having 3 bad housings is pretty high.
Click to expand...

That rules out that idea, I was thinking maybe you used the same external wastegate in all different scenarios and it was bad or not adjusted properly.

???
 
Diesel Freak said:
There is some debate as to where the turbos "should" be.

Most street twins (my setup included) have a primary that is actually too small to run the system 100% compounded.

In a setup similar to the one described, total boost is 60-70 with 30 comming from the primary.

On a truely compound set, you would square the pressure ratio across the primary.

Mathematically: Total boost (gauge presure) = (((PR1)^2)x14.7)-14.7)

if the Pressure ratio across the primary is 3:1 (essentially 30 psig in the cold pipe) then total boost would be ~118 psig

if the top turbo has a wheel that is ~60mm, then the bottom would be close to 85mm (not counting the hub diameter)[/QUOTE]

I follow the math to get total boost, but how do you figure out if the top is a 60mm , that the bottom is close to 85mm?
Click to expand...
 
Diesel Freak said:
There is some debate as to where the turbos "should" be.

Most street twins (my setup included) have a primary that is actually too small to run the system 100% compounded.

In a setup similar to the one described, total boost is 60-70 with 30 comming from the primary.

On a truely compound set, you would square the pressure ratio across the primary.

Mathematically: Total boost (gauge presure) = (((PR1)^2)x14.7)-14.7)

if the Pressure ratio across the primary is 3:1 (essentially 30 psig in the cold pipe) then total boost would be ~118 psig

if the top turbo has a wheel that is ~60mm, then the bottom would be close to 85mm (not counting the hub diameter)
Click to expand...

How do you calculate IC inlet temp for compounds?

I am off 100F for IC inlet temp and 30F for IC outlet temperature even though the CE is 10% or more better. This is compared to the calculating compounds as if they were a single compressor.

Jim
 
Last edited:
to get the true inlet temp, just calculate using the second stage inlet at the high temp air from the primary.

It's two compression steps. Each one has to be calculated seperately.
 
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