boost spike on primary

7

72chevy

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Jun 17, 2008
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Got my twins built & have been experimenting with the wastegates a little. I noticed tonight that when I let up on the fuel that the boost on the primary spikes 5-6 psi, is this normal? The setup is a 62-65-13ss over gt4202r with a 1.15 t4 housing, I have a 45mm gate in the hot pipe to go around the 4202.
 
My guess, is that when you lift your foot off the pedal, your not sending any more hot exhaust to your secondary, and it slows down...less speed, less compounding.
At the same time though, your primary has more enertia, and can't slow down as fast as the secondary...the pressure out of your primary comes up against the slowing down secondary...which has become a bigger restriction, and pressure in the pipe goes up.
Just a guess though.
 
Extended Power said:
My guess, is that when you lift your foot off the pedal, your not sending any more hot exhaust to your secondary, and it slows down...less speed, less compounding.
At the same time though, your primary has more enertia, and can't slow down as fast as the secondary...the pressure out of your primary comes up against the slowing down secondary...which has become a bigger restriction, and pressure in the pipe goes up.
Just a guess though.
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exactly, the smaller turbo is swallowing all that air and then it slows WAY down really quickly when you let off and it quits accepting the air so easily. restriction = boost increase
 
Extended Power said:
Can I ask why you are gating around the primary?
Curious is all...
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Try to control drive pressure on the primary, the 1.15 t4 housing is smaller than what is usualy used so I thought it would be a good idea. I was trying to build a setup that would spoolup as quickly as possible & still support decent power. The boost comes on earlier & faster than the single 62 did, I also added a 181-210 cam & a little more fuel. Not sure what made the most difference but I am very happy with the way the setup responds.
 
Forrest Nearing said:
exactly, the smaller turbo is swallowing all that air and then it slows WAY down really quickly when you let off and it quits accepting the air so easily. restriction = boost increase
Click to expand...

That makes sense & wondered if that was it. I was also wondering if it was possible the 62 was stalling & causing the spike.
 
that is one sweeeeeeeeeeet turbo!!!

I ran with the 1.01 housing for a while...friggin thing ripped!!!

wanted more power on top, so slapped on the 1.15
 
I have a 50mm Wastgate on it...can set it anywhere from 18 PSI to 35 PSI with either housing
 
Diesel Freak said:
I have a 50mm Wastgate on it...can set it anywhere from 18 PSI to 35 PSI with either housing
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The little bit I have adjusted the gates I have been between 20-35. What kind of overall boost can you get & where do you make the most power?
 
72chevy said:
The little bit I have adjusted the gates I have been between 20-35. What kind of overall boost can you get & where do you make the most power?
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overall boost is 55-60...with 30 from the primary.

Made 635 HP on my last pull...

But, I have a ported head, Helix 2 cam, and a BD intercooler
 
Do you make max power at max boost? If I adjust the gate on the secondary to make 35 psi on the primary overall boost is around 55, If I adjust the gate on the secondary to make 25 on the primary I see 60+ overall. I cant say I can feel any difference either way.
 
your system is different than mine.

on my truck the Primary wastgate affects both turbos...primary boost signal is piped to the top of the dome on the seccondary gate...so as primary boost pressure rises, the seccondary wastgate setpoint rises accordingly. Once the primary gate opens to limit boost of that turbo, then the seccondary gate opens fully



...and the seccondary wastgate only affects the seccondary...has no influence on the primary other than making the primary gate open further to dump excess gas flow.
 
You got me a little lost here, lol. I dont see how the secondary wg can not affect the primary unless you are dumping it to atmosphere. So here is how mine are set up at the moment, my 362 has the internal wg that dumps into the hot pipe the signal for this gate comes from the piping between stages, the 4202 wastegate is in the hot pipe & dumps to the down pipe, I have boost from the 362 discharge going to the top of the gate & regulated boost from the 362 discharge going to the bottom of the gate. How is your set up different?
 
running two external gates.

Top turbo has a 38mm gate, 2.2 bar springset. Dumps to the hotpipe.

Bottom turbo has a 50mm gate, 1.0 bar springset, dumps to atmo.

Bottom gate is large enough so the top turbo and gate will not affect the peak boost of the bottom turbo...bottom turbo is controlled by a simple ball and spring boost controler.

There is no boost controller on the top turbo, only spring pressure and boost pressure. total boost opens the top gate,(goes to the bottom port of the wastgate dome) Primary boost holds the top gate closed (goes to the top port of the wastgate dome)

I can reduce the primary boost almost to 18 psi regardless of what the top turbo and gate do...

as I raise primary boost...secondary boost also raises accordingly since primary boost helps hold the top gate closed. Once the primary boost hits it's "setpoint", so does the secondary, and both gates open sequentially, and boost pressure stops rising.


QED!!!
 
It's normal when you let off to see a spike. That's why gas rigs have blow-off valves. The primary will spike because the secondary has the intercooler piping and engine to take up the airflow slack (lots of volume) versus the primary just has one short pipe. When we had my 95 on the dyno, I saw a 15psi drive pressure spike when I would let off.
 
JQmile said:
It's normal when you let off to see a spike. That's why gas rigs have blow-off valves. The primary will spike because the secondary has the intercooler piping and engine to take up the airflow slack (lots of volume) versus the primary just has one short pipe. When we had my 95 on the dyno, I saw a 15psi drive pressure spike when I would let off.
Click to expand...

Most turbo gas engine are fueled my throttle angle and a few other sensors, so when the throttle blade closes, the fueling cuts off and the blow off releases the boost so it dont run lean.
 
Diesel Freak said:
running two external gates.

Top turbo has a 38mm gate, 2.2 bar springset. Dumps to the hotpipe.

Bottom turbo has a 50mm gate, 1.0 bar springset, dumps to atmo.

Bottom gate is large enough so the top turbo and gate will not affect the peak boost of the bottom turbo...bottom turbo is controlled by a simple ball and spring boost controler.

There is no boost controller on the top turbo, only spring pressure and boost pressure. total boost opens the top gate,(goes to the bottom port of the wastgate dome) Primary boost holds the top gate closed (goes to the top port of the wastgate dome)

I can reduce the primary boost almost to 18 psi regardless of what the top turbo and gate do...

as I raise primary boost...secondary boost also raises accordingly since primary boost helps hold the top gate closed. Once the primary boost hits it's "setpoint", so does the secondary, and both gates open sequentially, and boost pressure stops rising.


QED!!!
Click to expand...

Got it now, I had to draw a picture from your discription lol.
 
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