ISAC Boost Control Overview

[JayC]

Several people have asked for a more detailed overview of the boost control function of the ISAC in regards to setting it up. I wanted to post a more detailed explanation here and sticky it for future reference.


The ISAC boost control is a very full featured algorithm to accomodate lots of different kinds of cars.

Most cars will work best with only a couple of the features activated.

Lets discuss the bleeder/blocker solenoid setup. The thing that distinguishes this type of setup is that there is one solenoid and it stays 'on' when controlling boost. Similar to the stock solenoid (which is a 'bleeder') 50% dutycycle on the solenoid results in xx amount of boost, 75% results in some other amount of boost.

For best results, the software needs to know what dutycycle will cause what amount of boost. this is where the solenoid ratio, base psi, min dutycycle, and max dutycycle are used.

Base psi = the boost you get with the solenoid off, running just with the wastegate spring. A stock GN would be about 12 psi, a high boost actuator is 16 to 18 depending.

Min Dutycycle = most solenoids don't start raising boost until the dutycycle gets to the 50-70 % range. put the dutycycle that gets the boost up from minumum by .5psi or so. we will discuss how to find this in a minute.

Max dutycycle = some solenoids will raise boost too much if the dutycycle goes too high. set this to the max dutycycle you want the ISAC to run the solenoids at.

Solenoid ratio: this one seems to be the most confusing, thats why we are changing it. It is the amount of boost increase created by going from min dutycycle to max. For instance if base psi is 13, and the boost is 19.5 at max dutycycle, then the solenoid ratio is this increase. i.e. 19.5/13 = 1.5

The later software uses max psi, which is the boost you got when the solenoid is at max dutycycle.

The boost control is the same with either, the solenoid ratio just turned out to be confusing.


Now: how to determine these settings.

We want to use the ISAC to run the solenoid to various dutycycles so we can see the boost we get. So we set the Gains to 0, so the unit will not try to change the dutycycle to hit our boost.

Then we set the enables to: TPS enable = 1.0 rpm enable = 0 Start PSI = 0 .

Set the solenoid stuff: base psi = 15 , max psi = 22, min DC = 50, max DC = 95.

And set all the sliders to max so that your target psi is 22 or higher

Set the max psi/sec to 2

And in the config tab, set the solenoid mode to bleeder or blocker, and the trim mode to boost.

Now if you push the throttle (with the engine off) you will hear the solenoid buzz, and you will be able to adjust the dutycycle with the knob.

Now go drive the car and set the knob to different settings and make note of the boost and the dutycycle. In this way we figure out the characteristics of the solenoid, and when properly configured we can set the ISAC for XX boost and it will calculate a solenoid dutycycle that is very close to the proper amount.

Using the data collected, set the base psi, min DC, max DC, and either max psi or solenoid ratio depending on your software version.

Once these are set, whatever you set the knob to for target PSI, the boost should run very close to.

Once the boost closely tracks in open loop (which is how it is configured to this point with the gains set to 0) we can start to add some gain so the ISAC will hit the target boost exactly and follow ramp-ups very closely.

That is the next step.

 

[JayC]

The boost control has some enabling parameters so that the solenoid isn't just buzzing away as you drive down the road.

They are:

enable TPS
enable RPM
Start PSI
WOT TPS

They control when the boost control activates, and a fast spool mode, as well as preventing overshoots if you back out of the throttle and then stand on it again.

First: the enables (TPS and RPM), these must both be exceeded for the boost control to activate.

Set the TPS to the point where the turbo starts to take off. This is about 2 volts on my car. Some cars with part throttle surge will drive better if this is set higher.

Set the RPM to approximately your 0 psi stall speed. 2700 for my car.
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When the boost control first activates, its in fast spool mode until the boost exceeds the Start PSI, or 90% of your target boost. The idea is that by slamming the solenoid full on, the boost will rise faster. This works well, but can cause some boost overshoot on some cars (not a bad thing, really...) Set this about 50% of your base boost to start. If you don't want any fast spool operation, set start Psi to 0. Spoolup will still be pretty good.
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Once the boost it up and the car is pulling, sometimes you may want to back out of the throttle to control the boost. The WOT TPS is this threshold. This really only affects the Int Gain. so we will leave it for now.

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Stage PSI is the target boost when the stage input is activated. If you don't use this feature, then this can be set to 0.

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Max psi / Slider max psi (depending on version) sets the highest boost the sliders can be set to, and the max reading on the boost gauge and graph.


Once you have the solenoid settings figured out from part 1, and the enables set from part 2, the boost control should adjust boost in open loop mode, meaning its just setting the solenoid and ignoring the MAP sensor.

Once the ISAC is operating properly in open loop, we can 'tune' it with the gains.

 

[JayC]

In parts 1 and 2 we configured the unit to control the bleeder or blocker solenoid in open loop. By figuring out the solenoid characteristics, the unit will now set a proper solenoid dutycycle to match the target boost setting.

Now by adding gain to the system, the dutycycle will be adjusted to correct for any deviation in the boost from target.

So we have two numbers: target, and boost. we want 20 psi (the target) but boost is 21 today because its colder out, or whatever. The gains tell the ISAC how much to correct the dutycycle.

Each car will be a bit different, but the basic concepts will be true for everyone.

1: Prop gain (short for "Proportional") makes a change to the dutycycle based on how far off the boost is from the target. This correction is not cumulative, ie. if the target is 20 and the actual is 21, the dutycycle will get set to something smaller to try to correct for this, but if 1 second later the boost is still 21, there are no additional dutycycle changes from the Prop gain. If the solenoid information is pretty close, a small amount of Prop gain should be all thats required to keep the system tracking tightly.

Usually Prop Gain is adjusted first. start small and use only enough to make the system responsive.

2: Int Gain (short for Integral) corrects the dutycycle more and more as time goes on to try to get the boost to match the target. This is usually used to squeeze out the last little bit of correction needed once the Prop gain is set.

Int gain can get 'wound up', i.e. if you back out of the throttle to drop the boost some, the Int gain will keep increasing the dutycycle to try to correct. Then when you step back in you can get an overshoot. This is what the WOT TPS controls. Set the WOT TPS to the lowest TPS that the turbo can create your full boost at (3 - 3.5v in my car) this way, when the TPS is less than WOT TPS, the Int gain is prevented from increasing the dutycycle, so windup is prevented.

3. Deriv gain. (short for Derivative) most people will not need this, but it is there to 'anticipate' when boost changes quickly. If the Boost is rising quickly, the Deriv gain will pull out some dutycycle to try to prevent an overshoot. Most cars will not need this, so it can be left at 0.

With all the gains, start low, and make incremental changes. Keep track of what works, save logs with descriptive filenames, and keep some notes (the same goes for all tuning)

 

[UR50SLO]

Very nice Jay.... I might dust off that new alky kit that's been sitting over on the shelf for the last 2yrs.
I've not wanted to install it since the current controller can not tell when alky is working or back down boost if alky is not spraying.
I love the alky systems and how much you can gain from it with pump gas I just can't get past the dependability part.
This controller solves that.
~Scott

 

[aminga]

I realized this is highly configuration dependent but a library of settings might be useful to those of us starting out. Something of the line of Wastage solenoid type, wastegate type, turbo ... And the settings for that combo

 

[JayC]

If you check the help page, there are recommended settings.

MOST situations you'll run the ISAC in Blocker mode, especially with an actuator setup.

If you run an external gate, you can run it in Blocker mode or in dual mode. Dual mode does require two solenoids.