Not making my target boost

[24OZ]

I'm supposed to be running 14 psi - external wastegate spring is set to this. However I'm not hitting 14psi. I took it to a rolling road and a solenoid was fitted to run closes loop boost via my ecu but still not hitting my boost target.

 

Here's my MAP graph, what do you guys think, does it point to the external wastegate spring not doing its job, or what else could be at play?

 

Cheers.

 

[Tony D]

looks like you get it, and then it has a problem maintaining it. I'd look at the feedback loops on the boost controller, it may have a proportional only offset and once you hit the setpoint of 14psi (0.9bar) it then has that offset.

 

That's what it looks like till 5000rpms, after that it's almost like you are using more air than the turbo can put out. What was the power reading or graph from the rolling road show on that pull?

 

It may be all in the PID control loop of the boost controller, maybe more sampling rate (gain) or some other setting.

[24OZ]

Tony I'm running two old school HT18 38B turbos, so 7psi per turbo.

 

Here is my power graph and for what it's worth my AFR graph too:

 

 

[Tony D]

Each turbo doesn't make 7psi, they both have to make 14psi. I had a plant engineer call me one time because he bought two 150psi compressors at a price of $125,000 each, and expected them to make 300PSI... His logic was '150+150=300' and he was serious!

 

It's flow that is doubled at the same pressure, not pressure.

 

It looks like your spool is way late, smaller A/R on the hot side. What cam is being used? Where is it's power supposed to peak. The power and torque charts coincide with the boost graph...

 

But something isn't right, it shouldn't be dropping pressure at that point. Make sure your wastegate is doing what it should.

[24OZ]

Tony, im using what I believe is the stock turbo cam.

 

I'm wondering whether a leak can be occurring through my bov? Is a leak from the bov possible when at WOT and under boost. It's an old school hks unit.

 

But as you suggest will check the wastegate first. It's a real bugger to remove as it sits facing the block with very little clearance.

 

At what rpm does the stock t3 start to develop boost, earlier then 3,000rpm?

 

With regards to pressure and flow, i got to put my hands up and admit I didn't realise that they each turbo need to be making 14 psi; considering the size of these and the tech (80s tech) no wonder there peaking at just 12 psi, but that doesnt explain the drop does it?

[Tony D]

Did you duct the fitting on the top of the BOV to the intake manifold? That should put spring pressure + boost pressure on the BOV and prevent it from lifting under boost. But if you have a considerable pressure drop across the venturis at WOT that MAY act like a partially lifted throttle and make the BOV lift.

 

The stock T3 starts a 25/2700 or so (with a 0.63A/R, the Euro Versions closer to 2700 as they have a 0.82A/R---using a JDM L20ET hot side with the 0.48A/R you get 12-17psi at 1700rpms when you WOT it--so what the small turbo 'strangles you above 5500, the stock cam peaks there anyway, works for me!!!) This is the continual quandary with guys with big turbos: later spool, which makes for a 'peaky' engine if you don't have the extended rpm range provided by a cam. Since you have a stock turbo cam, expect peak power between 5000 and 5500 tops. So that is about right. You should break 300 once this is sorted and you have full boost and proper jetting for your application.

 

Don't worry on the pressure split---it didn't cost you a quarter-million dollars. Sometimes I wonder where guys get their degrees printed up...and what a salesman tells them!

 

Overall not bad though for a first run! Remember JeffP was making 380ft-lbs at 4500 with his big cam...but that was at 8.39psig!

Edited July 11, 2011 by Tony D

[24OZ]

BOV is Ducted back into the intake manifold.

 

Going back to what you said Tony about flow being doubled not pressure, I'm running twin pipes, each turbo downpipe runs into it's own 55mm outside diameter (2.16") pipe, should each pipe be at least 3" in OD? Concered whether I may be strangling the system at the moment?

 

Thanks.

Edited July 12, 2011 by 24OZ

[Tony D]

That wasn't too clear---by that I meant the vacuum line from the upper portion of the BOV. It adds pressure to help keep it closed. not the big hose, but the small pressure-sensing line. If you lift-throttle manifold vacuum is put to the top chamber of the BOV and it opens it below the spring pressure. I had to hook mine into my manifolded balance tube for a steady vacuum source to mitigate high pulses on the manifold making it go 'POP-POP-POP-POP-POP' at idle, and an interesting WooshWooshWoosh under boost when I lifted!

 

Your pipe size is fine, that's not the issue.

[24OZ]

Yep, the vacuum sensing line on top of the bov leads to my intake manifold.

 

We'll be interesting to see what the map graph looks like once the throttle bodies are fitted.

[skirkland1980]

That wasn't too clear---by that I meant the vacuum line from the upper portion of the BOV. It adds pressure to help keep it closed. not the big hose, but the small pressure-sensing line. If you lift-throttle manifold vacuum is put to the top chamber of the BOV and it opens it below the spring pressure. I had to hook mine into my manifolded balance tube for a steady vacuum source to mitigate high pulses on the manifold making it go 'POP-POP-POP-POP-POP' at idle, and an interesting WooshWooshWoosh under boost when I lifted!

 

Your pipe size is fine, that's not the issue.

I am having a similar problem. Can you explain "manifolded balance tube" please...Thanks

[Tony D]

1/8 NPT X 1/4" Hose-Barbed fitting into each manifold runner, to a 1/4" Hose Barb X 1/2 Hose-Barb "T" linked with 1/2" fuel tubing. Off one end was a 1/2" Hose-Barb X 3/16" Hose-Barb Reducer Fitting to go to the BOV, and on the back was the barb to go to the brake booster.

 

If you did the same thing using a -8 piece of fuel rail and some 1/4" barbed fittings, it would be similar. I just made a balance tube to dampen and otherwise smooth out manifold pulsations inherent in short-runner individual throttle body intake manifolds.

 

Lots of EFI guys do this so they get a consistent MAP signal as well.

Edited July 15, 2011 by Tony D

[HowlerMonkey]

Is the pressure line that leads to the wastegate actuator plumbed to the turbo housing (stock setup)?

 

If so, the slight restriction in your intercooling system is the culprit.

 

The pressure your wastegate is seeing when plumbed to the turbo housing is before the pressure drop through the intercooling system so it's probably operating as normal and the actuator is seeing and reacting to target boost levels but the pressure drop through the intercooler/piping is responsible for the boost you see at the intake manifold.

 

To check this, run a gauge Tee'd in on the wastegate actuator hose and see what the turbo is seeing and compare it to what you are reading off the intake manifold.

 

You could change the "boost reference source" from the turbo housings to after the restriction of the intercooler and piping and you should end up with more consistent boost control.

 

One thing to consider before moving the wastegate pressure source......if your pressure drop across the intercooler/piping is too much and you're already running the turbos near thier limits, you could cause the turbos to overwork because they would be controlled by the pressure after the pressure drop.

 

If you're nowhere near the limits of the turbos, you should be fine.

 

All intercoolers introduce some restriction but some more than than others so this is a good time to see if your issue is caused by too much restriction.

 

Of course....after writing this, I see tony covered it on an earlier thread.

 

http://forums.hybridz.org/index.php/topic/100405-280zx-turbo-boost-issues/

 

When moving the wastegate signal to the intake manifold, keep in mind that an older actuator diaphragm might not be happy being fed near 30 inches or more of vacuum when decelerating since it has lived it's entire life never having seen anywhere near that vacuum level...........but....also discussed in that thread was a poppet type boost controller.

 

These also have the benefit of acting as a one-way valve which should keep the wastegate from being sucked to death.

Edited July 29, 2011 by HowlerMonkey