Low boost at transition with BNRs
 

Looking for some help. Overall my car drives great, and pulls hard to 8k. But at the transition I can't seem to make more than 7 lbs of boost.

​​​​​​At any primary boost level, I get the same transition, 10-7-15 or 15-7-15.

One other odd thing, I get a quick vibration sound at the transition. Sounds almost like a bull frog croak.

Does anyone else with BNRs have this issue? I'm almost thinking that the larger turbo doesn't pre-spool as much given the manifold design.

- I'm running two manual ball & spring controllers for the WG and PC actuators. WG is controlling boost fine at 15 lbs to redline. PC works as well - reduces primary boost as I open up the actuator.

- I've tapped all turbo control solenoids with a boost gauge while driving around and they all seem to operate as espected. They are all new.

- The CRV and and charge control butterfly check out fine.

- No boost leaks, etc.

​​​​​​- TCA seems to operate fine, maybe a bit slow? Not sure...

Anyone else have this issue with BNRs, or have some new ideas?

I would replace the pre-control manual ball & spring controller with a simple valve controller and adjusting it.

Thanks, I'll give it a try. What's weird is that when I open up the pre-control controller it doesn't seem to have any affect on the transition boost. The primary boost reduces as I'd expect, but the transition boost is exactly the same. Maybe it's just very sensitive? I can see how a valve controller may give me a bit more control to trickle in pressure.

What ECU? Can you post a datalog?

Have you tried adjusting the precontrol actuator rod?

Running a pfc, but I don't have a datalogit.

When I first got the turbos, I adjusted the PC actuator rod to make it a bit harder to open, maybe one turn or so. I tested it with compressed air and it opened.

One more thing, I went to the dual MBCs because I had the same issue with the PC solenoid and PFC controlling boost. I tried all sorts of settings including a tune, and couldn't get a good transition. The MBCs give me much better overall boost control than the profec, but didn't address the PC. I'm thinking of adding the pill back in the PC line with the MBC, and setting the it to a very light setting and seeing how that works.

First, I would loosen the Pre-Control actuator rod back up to how Mazda had it and try that.
Tighten the WG actuator rod instead if you haven't done this already.
Tighter PC rod would limit the exhaust available to spin up the 2ndary turbo in Pre-spool as it will keep the PC exhaust flapper seated longer . Conversely, tighter WG rod will aid in more pre-spool exhaust energy being available by limiting the exhaust flow out the wastegate flapper when under peak desired boost.

Next, if there wasn't the noise and you hadn't tensioned the PC actuator rod I would think it could be the air bleed hole in the MBC could be too large or as you are saying you could try to put the restrictor pill back in the WG/PC vacuum lines to have the same effect of "increasing the gain" of the system
Also if you don't have any air bleed hole in the MBCs it could be too much exhaust out the wastegate flapper before peak boost is achieved limiting PC exhaust available as well.

However, the vibration sound at transition makes me think there may be an issue with the 2ndary turbo air bypass valve (thing that looks like the stock BOV on the Y-pipe). Make sure its the bypass valve and not the bov valve on there too while you are testing it.

Alternately it could be a bad check valve making the sound or the throttle plate in the y-pipe not snapping open fast causing a momentary compressor surge if the bypass valve is cutting the 2ndary turbo pressure relief and the throttle plate isn't opening fast enough (2ndary turbo working against partially closed throttle plate). So check out the boost reservoir and lines to the throttle plate in y-pipe for leaks as well

Did you try raising or lowering the turbo transition point? You need a Datalogit for that.

Thanks. I'll try loosening the PC arm. The MBCs have an air bleed hole. I was thinking of adding the PC line pill into the line that feeds the MBC, but I'll loosen the arm first.

The noise could be secondary compressor surge. This was my first theory... I've triple checked the charge control valve and the lines feeding it. I've tapped the vacuum/boost feed while driving and it seems to be fine. All tests on the CCV pass. Another theory I have is that it's the TCA secondary plate rattling as it opens? It's a very hard noise to explain.

I also suspected that the CRV may not be snapping shut, so at the transition it remains open for a bit. It doesn't appear to be the case, but I haven't been able to confirm that while driving. I have confirmed the boost/vacuum CRV source is fine. I've also bench tested it and it seems fine.

I don't believe that the tuner tried moving the transition point. He did change the point were it transitions back to sequential operation after both come online to approx 3.8k I think.

I also have dual MBC's and a PFC... I love the smooth boost pattern I get on the stock sequential twins now.

To confirm your setup... you have plugged the lines/nipples from the PC and WG actuators going to the stock boost solenoids and put a MBC in the pressure line to each. Correct?

The TCA can work but act sluggish if it isn't getting either vacuum or pressure.

Have you bench tested the TCA on both the vacuum and pressure ports to ensure both cause it to move?

If you haven't, I would also tee into both the pressure side and vacuum side TCA lines (easy place to tee in is up by the cross over pipe) and observe each while driving. I have permanent tees in mine.

Yep, tee'd both TCA lines and the seem fine while driving around. I benched the TCA with vaccum only and it opened fine.

MBCs are between the compressor pressure source and the Actuators. I capped off the other nipple on the actuators.

Great, at least you know that's getting the right pressure/vac signal.

Might want to check pressure actuation as well just to be sure. Not sure how it could leak on one side of diaphragm and not the other, but it wouldn't surprise me. One more thing to eliminate.

Rather than cap off my nips I threw a couple ball bearings in my PC/WG bleed off lines to confound any poor soul that works on the boost system after me. :lol:

Edit: you mentioned the PC actuator doesn't respond to loosening the MBC spring... did you bench test the PC and WG actuators?

This is the exact noise I hear, but at 4.5 k rpms.

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/stranges-noise-3-5k-1129382/

That just sounded like a resonant vibration to me in those clips.

Like when you have a heat shield that rings at a certain rpm as it resonates with a driveline or exhaust vibration

or when a rotating part is out of balance or accelerating too fast and the rotating components or the parts holding them start to ring at a certain rpm.

If your 2ndary turbo is making that noise it could be that it is being accelerated too quickly through a certain rpm band where there is a secondary resonance.
I remember back when they were developing those high power centrifugal vacuums they had a similar issue. They had to map the vibratory trends and have the speed controller throttle acceleration back through certain rpm ranges while still accelerating quickly through the other rpm ranges so people didn't complain about laggy vacuum cleaners LOL (or worse, exploding vacuum cleaners).

Again, if your Pre-control is tensioned more than stock and you are controlling it via a bleed and ball MBC you are delaying the onset of 2ndary turbo acceleration and then bringing it up to full rpm in a very short window.

The 2ndary turbo would actually be much happier spinning up over a longer window even though it means many more rotations over its lifetime. That is just a very high rpm rotating system dynamic most people don't think about- 2ndary vibrations on perfectly static balanced rotating parts or even dynamcally balanced rotating parts out of their dynamic balanced rpm window.

I don't understand how the MBC would delay onset of the 2ndary turbo. Isn't it the same effect as having a duty cycled pressure signal to the PC actuator? Or does the 2ndary turbo see less exhaust flow pre-transition because both the WG (to exhaust) and PC (to secondary) actuators are opening together?

This question is half for OP's benefit and half for my info. Thinking about switching to PFC control of turbos.

I think you are thinking of delaying the 2ndary turbo transition in terms of engine RPM.

Not what I am saying. Sequential transition happening still at 4,500rpm engine speed.

I am saying it delays the turbo RPM increase relative to time (not engine RPM).
That is the 2ndary turbo goes from not spinning to full RPM over a shorter period of time if the actuator rod is more tensioned or if you have more bleed hole (gain) in your MBC

In the normal sequential system the 2ndary turbo is spooling in pre-spool for a whole lot of the primary turbo only operation. Provides lots of turbo sound track to sequential FD.
The 2ndary turbo is in "stand-by" at elevated rpm waiting to be further accelerated and brought on line. Constant high rpm within the normal operating range of the turbo is not hard or wearing on the turbo. Accelerating the turbo very quickly by delaying the onset of pre-spool nearer the 4,500rpm engine switch-over point is hard on the turbo. High dynamic loading and 2ndary vibrations.

The MBC delays the the increase in 2ndary turbo RPM in pre-spool by keeping any air pressure out of the actuator until the check ball is unseated (against the spring pressure). This prevents the flapper from creeping open (same reason bleed and check ball MBC raises boost response over pressure straight to WG actuator).

Basically, I am saying sharpening the response time of the 2ndary turbo during pre-spool is harder on the turbo from loading and 2ndary vibrations not easier on the turbo from having it spin less over time when not in use as one might think.

There is no performance benefit to sharpening the pre-spool response time like there is on sharpening response time on the primary turbo response time. AKA primary turbo is feeding air into the engine to make more power. Quicker it gets up to full RPM the faster engine makes power.

2ndary turbo is spinning in a vacuum in pre-spool. Quicker it gets to full RPM faster the 2ndary turbo wears out. Plus introduces destructive secondary vibrations (vacuum example).

Thanks for the help so far....

I finally had some time today to dive back in. I lengthened the PC arm (it was tight - gave it 4 turns). That didn't work. I then added the PC pill into the PC line and opened up the MBC. I tapped the exit of the MBC and boost is definitely getting though. No change.

Again, when I open the PC MBC, the primary boost drops, so I know the actuator is opening, but, there's no change in transition boost. Even with the MBC all the way open, I get about 9lbs of primary boost and the same 7 lbs at the transition.

At this point I feel it's something else altogether. Any ideas are welcome!

Here's a vid... you can't hear the noise, but the transition is shown:

boost transition

Is the noise still there?

Whats the issue?

Can you close-up the WG MBC (more boost) and then open up the Pre-spool MBC (more pre-spool-less boost) to get more exhaust to the Prespool so you have less boost drop at transition?

Noise is still there. Issue is still that no matter what I do, I get the same pre-spool at transition. I should see some variation. I've tried closing the WG, but then get higher boost after the transition, and the pre-spool still doesn't change.

The only other things that I can think would affect pre-spool are the CRV (closing slowing) and the TCA (opening slowly). Both seem fine, but who knows at this point.

Does the PFC give control over the exact rpms the sequential solenoids open/close? I'm wondering if they were accidentally set out of sync by a few hundred rpms.
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The PFC switches the turbo control and charge control actuators at the same time, based on rpm and throttle setting in the Datalogit software. There is no visibility to the charge relief; I presume it's adjusted based on sequential switch point. The wastegate and precontrol are also adjusted based on the transition point. When the switching point is reached, the precontrol goes full open (0 duty cycle) and then the wastegate starts controlling boost. Before the transition point, the wastegate is full closed (almost 100% duty) and the precontrol controls boost.

See this thread. https://www.rx7club.com/3rd-generati...tified-841821/

Thanks. That makes perfect sense. So basically, the best way to manually control boost is to have a needle valve on the PC and the ball and spring on the WG, and get things just right so that the primary boost is slightly less than the secondary, and the WG control starts to open above the primary boost max.

I ordered a valve controller (turbosmart), but got an electronic controller by accident... adding a valve controller will have to wait another week. If that doesn't work, I'll just try a full stock boost control setup (when I got the car it had a greddy ebc on the WG with the stock PC solenoid). My last high boost sequential setup (15 years ago!) ran 15-12-15 perfectly with the stock solenoids and Pfc. Reading this explanation, I can see why.

Thanks Mike! I'll reach out. Decided to switch to stock BC and see how that performs. It would be good to log some pulls if there's still issues.

I referred him to this thread about running Power FC boost controller with 3 port solenoids similar to what most aftermarket controllers use:

https://www.rx7club.com/3rd-generati...tegies-874031/