Hybrid Boost Control - solving the problem of minimizing spool and stabilizing boost.

[BrandonDrums]

Intro

The Subaru factory boost control system is a unique take on controlling the boost levels on a turbocharged engine. When tuning WRX's and STI's using the factory 2 port and even with an upgraded 3 port boost control solenoid, the tuner continuously has to walk a fine line between achieving the quickest spool and providing stable and predictible boost in all conditions.

Essentially, with both a 2 and 3 port boost control solenoid, the ECU applies a correction value via Turbo Dynamics to "Waste Gate Duty Cycle" or WGDC to correct Boost error. WGDC is the percentage of boost signal that the boost control solenoid diverts away from the wastegate actuator on the turbocharger. Turbo Dynamics is a ECU table that checks the raw pressure value difference between target boost and actual boost and applies additional WGDC based on the amount it's off.

What results is a somewhat laggy system of boost control that is difficult to tune for maximizing boost and guaranteeing boost stability in all gears in all conditions.

This issue is primarily with the 02-05 16-bit WRX ECU's and the 06-07 32 bit ECU's. STI ECU's have separate boost control tables for each gear which allows much better control for maximizing spoolup without sacrificing boost stability.

[SIZE=4]A little more explanation
In order to control boost, pressurized air from the turbo itself is applied against a little diaphragm controlled arm that opens a flap that allows exhaust gas to bypass the turbine wheel during an overboost situation and then bleeds pressure away from reaching that diaphragm to keep it closed when trying to get more boost out of the turbo.

In a Stock Subaru WRX, there's a solenoid controlled valve that essentially controls what % of the pressure coming from the turbo reaches that arm. This is called Wastegate Duty Cycle or WGDC. 0% duty cycle means the solenoid is applying all of the turbo pressure to the wastegate actuator, 100% duty cycle means it's bleeding all pressure away from the actuator in an attempt to maximize boost.

The ECU system that controls that solenoid works continuously in a feedback control loop or Closed Loop process which means you can't set how much duty cycle is applied at a given set of criteria. Instead, you only set the boost you'd like to achieve, the maximum amount of WGDC you wish the system to apply at a particular Throttle input and RPM speed and the increment in which the system adds in WGDC per correction cycle as it tries to achieve target boost.

The reason subaru uses this system is because weather and altitude affect a turbo's performance and the car is designed to operate reliably in all conditions. For that reason, the boost control system is pretty good but it sacrifices quite a bit of turbo performance to achieve more day to day reliability.

You can speed up the control system by upgrading to a 3 port boost control system but that is still difficult to tune and only reduces the balance between spool and stability.

The method below allows you to maximize turbocharger spool performance while coming pretty close to providing the same level of all condition boost stability from the factory unit on top of making it

How the Hybrid Boost Control method improves this sytem

Using a Manual boost controller in conjunction with this system, you can essentially 'trick' the ECU to run in "open Loop" where the Manual boost controller provides a mechanical limit to the peak boost you want to apply to your engine and the Electronic system controls partial throttle boost levels. Instead of guess and check, you set the iTurbo Dynamics correction values to 100% for all underboosting error levels (on 16 bit ECU's 02-05 wrx's) and then use the Max WGDC table to set the throttle and rpm dependent WGDC's (if the correction is 100% but the max is 75%, then the applied WGDC will be 75% regardless of the 100% correction).

On 32 bit ecus (06+ wrx's, STI's and 03+ legacies) you instead set the initial WGDC and turbo dynamics becomes irrelevant. Either way, you then have full control over the WGDC being applied in the system at all throttle and RPM levels which remain static.

Tick #2 is to set the partial throttle target boost levels a little bit higher than the WGDC will allow the turbo to achieve i.e. at 75% throttle, you apply 60% WGDC but leave the boost target at 18 psi. A TD04 on a Ej20 using a 3 port BCS would never achieve 18psi with only 60% duty cycle, instead is only reaches 16 psi in the winter and 15 psi in the summer (example figures, not actual data) Many tuners use this method on a 3 port without a manual controller, the issue is setting the max target at WOT higher and only relying on max WGDC to limit boost allows for massive changes in boost pressure depending on ambient temperature and engine load. It's dangerous.

Conclusion

SO Not only does this Hybrid Boost Control method improve performance, but it also makes this process MUCH easier to tune for all weather conditions (temperature affects boost a whole lot), it improves the stability and safety of the system by eliminating boost spikes, boost creep and boost oscillation (overshoot, overcorrect into underboost, overcorrect into overboost etc etc) and leads to a more driveable and more reliable car.

We'll be driving these 818's hard and many will be driving them on the racetrack. Add better power to weight ratios and less grip and you've reduced the amount of time the ECU has to apply corrections during acceleration and de-stabalized the amount of load the engine sees as the tires struggle for grip. You'll need a faster boost control system to keep up.

The hybrid is effective for both internally gated and externally gated wastegate systems and I think every future 818 owner planning on performing any power mods or tuning should consider.

References

NASIOC Sticky on hybrid boost control
http://forums.nasioc.com/forums/showthread.php?t=602674

One of the best dyno result demonstrations of Hybrid Boost Control
http://forums.nasioc.com/forums/show...2165560&page=2

Explanation of the factory boost control system to compare
http://www.cobbforums.com/forums/sho...l-System-Works

I guess this counts as my "Subaru Tidbit of the Day" although it's a bit long.

I'd love to hear comments and questions. This is one of my favorite topics so fire away!