hello im playing around with boost control and run in to a problem it hits peak boost around 3600 rpm and about 12 psi then by around 6500 rpm its at 7.5 psi the fall off is linear.
the more duty you give the boost solenoid the bigger the problem for instance if i up the duty and hit 23 psi it will peak just under 4000 rpm but again by 6500 rpm it will have dropped to around 14 psi i've leak tested the boost system but doesn't seem to be any leaks its a 4x4 t3 turbo on a zetec engine.

 

They will alwys drop a bit, but if its excessive could be dodgy actuator?

 

Its not the actuator as ive tried two others

 

There are many variables. Whether engine airflow/efficiencies, turbo or actuator abilities, control methods etc etc or simply a fault somwehere

Some will easily hold rock solid, some wont.

Presumably you're only using a very crude method of boost control ?

 

Ecu controled on a mac valve

 

Then why does it sound like you're entering a single static value for duty to control boost ?

You will almost always need to vary duty with load to achieve a steady boost figure throughout the rpm range, especially if there are inefficiencies within the setup.
Which at a minimum if it isnt closed loop control, will need to be boost vs rpm

Map the boost properly and you will achieve a far steadier figure.

But as said, there are many many variables from the engine itself, turbo, actuator, control methods, possible faults etc etc that can affect the boost you see.

 

What im doing is I'll entre a duty value of say 20 percent do a pull from 2000 to the limiter log it then in a graph mark down what boost it achieves at what rpm then i will do the same for 30 then 40 and so on till i get to 100 percent duty.
I will then have a table with all the info to make a boost duty table.
But with the boost valve turned off its dropping boost after peaking with duty at 100 percent it peaks around 30 psi and then drops under 20.
Obviously with setting the duty i could hit 20 psi and hold it to red line but it should do more and 20 psi at say 6000rpm would be 100 percent duty.
100 percent duty is pretty much like pulling the pipe of the actuator so there must be a problem

 

As load will vary with boost, that isnt an ideal way to try and populate a duty curve for boost, as dong a pull at a higher boost with engine under different load, the turbo will just behave differently than you trying to assume a graph logged at a different boost level/duty will behave the same when trying to marry it up with different figures.

You need to populate the duty table with sensible figures to achieve a flat boost level ( if that's what you want to achieve ) and then do a pull to see if you've achieved it.

As said, there are many variables.

There are even many variables as to control valves, how you plumb them, wastegate actuator strength.

100% duty should be akin to pulling the pipe off the actuator, but you cannot assume it should make more than 20psi.

If the actuator is weak or not adjusted properly, or there is a leak internally, or a boost leak or turbo simply not capable etc etc...agan there can be many reasons for boost not being maintained.

I've tuned cars where you could pull the actuator pipe off, and it's never overboosted or had a problem and been left that way. The turbo was just a poor choice for the engine.
Or there are others that require very little duty to achieve loads of boost.

 

i do the boost table as a starting point to put realistic duty to rpm then work from there and its running a -34 actuator and with boost control turned off its not achieving the actuators base psi so there is a fundamental problem its just highlighted the more boost you try and give it.

 

this one has no duty


this one has duty added but not sure how much

 

IMO You are making boost very late, that is not a big turbo for a 2.0 so peak at 4000rpm is awful, I would be pressure testing your intake system to test for leaks and checking the actuator pre-load and strength before doing anything else.

 

Or maybe you have a big restriction somewhere, manifolds, TB, boost hoses, etc?
Unlikely on a ZT but could also be worn cams not opening the valves fully.

As Stevieturbo has said, many variables.

 

thats what im saying there seems to be a big problem ive pressure tested the intake system before and couldn't find leaks but will try again with more pressure.
its pointing to a massive air leak its just i cant find it

 

I'd hardly say 1.6bar at 4000rpm is terrible for such an old turbocharger, especially with such little information.

ie road conditions, gear used and again how boost control is configured, not to mention exhaust, tuning, cams etc

And can you not datalog valve duty ?

And whilst you say it is a -34 actuator, is it installed correctly ? any pre-load ? is the swing valve in the turbo closed and sealing ? etc etc


Really, given your main question was about boost dropping...map the thing properly and boost probably wont drop.

 

what is the point of mapping the boost around a fundamental problem the actuator has 3 mm preload its not hitting actuator pressure sure i can add duty to make it hit and hold a low boost figure but thats not getting to the obvious problem

 

You havent proven there is a fundamental problem.

Have you tested the actuator ?

 

tried 3 different actuators the original that was a alloy forge one a brand new -31 and a brand new -34 all do the same thing. ive connected them to air and pumped them up to there opening pressures and all of them seem fine. so there all holding the arm shut and it does make boost just not hold it very good

 

IF the actuator behaves as expected under test, then you either have a boost leak, a huge restriction, huge EGBP, the swing valve is not sealing, there is a turbo fault..etc etc

The list goes on really.

But the turbo has shown ability to make more boost whether you feel it is perfect or not, so with proper boost mapping you will be able to achieve steady boost.

 

Boost "Pressure" is only really a measure of the restriction in your engine. What power is this engine making?

As an example of what I mean, if its making 350bhp, you havent a chance in hell of that tiny turbo holding 20psi at 6000rpm.

 

correct the turbo does have the ability to hold boost and i am quite sure i could make and hit 20 psi to rev limit and also don't expect it to hit and hold over 30 psi to the limit as i understand its a small turbo and is unable to hold that higher boost that high up the rev range.
but at the same time i'm confident it should not drop around 6psi just on actuator pressure.
i will test the intake system again tomorrow and report back as for power no idea its not been on the rollers yet trying to sort theses niggles first

 

4 leaks on the inlet manifold so will seal them and try again.