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Discussion Starter #1
Anyone comment on if it is normal for these 67-71mm turbos to drop in boost 1-2 psi by 7000rpm? I have done everything I could think of and am not sure if this is normal. I am trying to push and maintain 30psi.

- No boost leaks found up to 20psi
- Backpressure at 7000rpm is about 20psi
- I've tried two different boost controllers, have not tried the AEM with custom duty cycle at high RPM

From searching, it looks like many others exhibit this. I haven't tried boost above 30psi yet, but these 67mmm turbos can handle 40psi or so, correct?
 

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I know my gt4294r would hold whatever I wanted up to 32psi rock solid. This was a manual boost controller and 1 bar spring on a tial 44mm.

Jason
 

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if you haven't tried ,or already have maybe, but try to move your boost source for bottom of waste gate from intake manifold. if you don't like vacuum part of intake manifold, there is few thing you can do such as using mac valve like you using for the spool up valve.

I have done a lot of spool up valve set up and had same issue on dropping boost at the top end, just run temp hose goes to wastegate and see.

AEM you can jack up the duty cycle where it drop but I think there is reason why boost drop. so you might get more stable boost with physical fix. but yours might not fix it I dont know. lol just a thought


Anyone comment on if it is normal for these 67-71mm turbos to drop in boost 1-2 psi by 7000rpm? I have done everything I could think of and am not sure if this is normal. I am trying to push and maintain 30psi.

- No boost leaks found up to 20psi
- Backpressure at 7000rpm is about 20psi
- I've tried two different boost controllers, have not tried the AEM with custom duty cycle at high RPM

From searching, it looks like many others exhibit this. I haven't tried boost above 30psi yet, but these 67mmm turbos can handle 40psi or so, correct?
 

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Discussion Starter #4
I don't know... I run 25psi WG springs, unless they are somehow being pushed open. I will try to disable the second WG. I'm more curious if I'm topping out this turbo in some way. I have such low back pressure at top end, I may just be flowing too much for the turbo to keep up pressure-wise.
 

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worth a try. you not going to loose anything. unless car like we do with too many an fitting and braided hose that will make your life not easy.
unless you are running on non billet wheel, billet should be flow more than 90lb/min, ( I think they said 94-96lb I heard ) and I doubt that turbo is running out of breath. knowing that compressor map is just a figures on efficiency for engineer stand point, it doesnt mean that it will not flow any more then 90lb/min. I am pretty sure you can push more air into the cylinder.







I don't know... I run 25psi WG springs, unless they are somehow being pushed open. I will try to disable the second WG. I'm more curious if I'm topping out this turbo in some way. I have such low back pressure at top end, I may just be flowing too much for the turbo to keep up pressure-wise.
 

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worth a try. you not going to loose anything. unless car like we do with too many an fitting and braided hose that will make your life not easy.
unless you are running on non billet wheel, billet should be flow more than 90lb/min, ( I think they said 94-96lb I heard ) and I doubt that turbo is running out of breath. knowing that compressor map is just a figures on efficiency for engineer stand point, it doesnt mean that it will not flow any more then 90lb/min. I am pretty sure you can push more air into the cylinder.
dre- just a thought but maybe u should try to overcome the 20psi boost leak test if ur base boost is 25psi, maybe?

REALJDM- and what are you rellay tring to say? tried to make sence of what you typed but its total rubbish if you ask- nut you didnt so anyway- just asking to try to make sence of your reply.
 

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Discussion Starter #7
REAL JDM - No offense but I'm having a hard time understanding what you're writing. It's just rambling on and on with no sentence structure!

I am looking for evidence (boost log?) of folks who can hold 30psi till 8000rpm on 67~71mm turbos. The reason I say this is that my boost gauge isn't obvious. It looks like I hold 30psi till 8000rpm, but my boost log on the AEM shows 1-2psi drop, ever so slightly. If anyone can post a screen shot of a log, it would satisfy my concern that this isn't a limit of the turbo/head/flow.
 

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There is a thread below where a guy held 28psi on a 6265 w/a log manifold. It is the one titled: 6265 w/QSV Log Manifold.

Steve
 

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Discussion Starter #9
Steve:



This? Boost seems to drop up top...
 

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Steve:



This? Boost seems to drop up top...

Yes, looks like he lost a lb or so. I am in the same boat. That is why I changed over from the Blitz Dual Solenoid over to the HKS EVC6. I hope the EVC 6 holds boost better.

Steve
 

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I would try to add more duty to the Boost WG Base Duty map in the AEM. I would try to add a few percent where the boost is dropping.

Also, what environment is the car operating at when this occurs (street, dyno, dragstrip)?
 

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Try a pressure source for the gates post intercooler. It could be a pressure differential from the intercooler at higher end.
 

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Try a pressure source for the gates post intercooler. It could be a pressure differential from the intercooler at higher end.
totally agreed- try a boost source for the wg from another source- the difference from say pressure side of turbo to intake could possibly have different readings.
 

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Discussion Starter #14
Both of my pressure sources are right at the turbo. I would assume the Profec will adjust WG duty if the pressure starts dropping.
 

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I dont see the problem.. Throw away the dyno sheet and just shift b4 the boost drops and you wont notice. :)
 

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bumping an old thread, but was this ever solved? going thru similar situation but with more boost drop than that.
 

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Discussion Starter #17 (Edited)
On my todo list. I have tried a different intercooler. No impact. I have new beehive springs to install to see if I may have some sort of valve float, as left-field as it may seem.

I've tried a different boost controller that can control up to 42psi. Same issue is observed.

The point of this thread was to find anyone who can maintain the boost they start with all the way to 7500rpm on a 67mm turbo. I've seen a few more logs where no one has been able to. Everyone loses 1 or 2 psi up top.

So, when I quote my dyno figures at xx psi, my peak whp is almost alwasy 2psi lower than my peak torque psi.

ALSO, as for different pressure sources, I do have my ProfecB SpecII reading boost at the intake manifold as a source. I've always thought that is ideal since you want the most stable boost controlled at the intake manifold, not at the turbo side.
 

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This can be solved by adding duty cycle to the wg duty table in whatever ECU you're using. It is pretty typical for a turbo setup that isn't creeping to drop a pound or two between peak torque and peak RPM... Adding duty cycle "forces" the manifold pressure to stay up but the boost curve tells you about the natural breathing characteristics of the engine. If you add duty cycle on the top end to try to maintain boost (like all the subaru/mitsu guys do with turbo dynamics, etc..) and your IAT's bump then you're compressor efficiency is going down. If your manifold backpressure spikes then your turbine or turbine housing is limiting.

I'm pretty sure the Profec b doesn't do a damn thing if it sees pressure dropping on the top end unless you can do some sort of reverse gain. Some of the more sophisticated controllers (the old AVCR, the new EVC6) can target a boost level but the closed loop doesn't work that great on them. You can use the "DRO" function on the HKS controllers to counter it. If you're running a stand alone it's much easier with closed loop or boost wg feedback tables... they'll take care of altitude, air density, and temperature fluctuations.

Ian
 

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Discussion Starter #19
That's odd because you set the determined boost level in kPA with ProfecB, which I thought made the profec automatically modulate the duty cycle to the wastegates to maintain that pressure. Is that not how they work?

If that isn't the case, I'll have to go AEM for boost control and find another PWM output as I'm using the boost PWM to control my QSV. Ugh...
 

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There could be a couple of things going on here actually.

It IS possible to be running off the map. Typically I find very peak flow of a turbo is usually around 2.2 PR. Which means you haven't run OUT of turbo it just means it's not able to give you more boost at that wheel speed due to consumption. If your motor moved more air, you'd probably make more power but wouldn't be able to go past say 25 PSI. Or 20 psi. etc etc.

Eliminate one of the gates from the equation. It's also possible that you don't have enough exhaust energy with that A/R and the effiency of that turbine wheel.
Perhaps this is why precision went out of their way to design a new 66... I'll bet that with a 6766 with a new turbine wheel you'd be able to make more boost. There are sooo many factors here.

One good way to know if you're having exhaust energy issue is to retard timing. If you drop a few degrees and are able to pick up some more boost then there ya go.
 
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