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Improving boost response during turbo spool

Boost Control

Discussion and questions related to the course Boost Control

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Hi all,

Can someone clear something's up for me please.

Say you have an engine that was fitted with a factory boost control solenoid and the system was running on spring pressure and no electronic boost control enabled.

You do a ramp run on the Dyno and log the boost pressure response vs engine rpm.

Next you remove the factory boost control solenoid and fit an aftermarket 3 port boost control solenoid. Question 1 is would running the car again on the Dyno for a ramp run on spring pressure alone see any change in the response curve of the boost pressure vs the first scenario?

My final question now is concerned with using the 3 port boost control solenoid to improve spool response and bring the turbo on to full boost quicker. Say you want to retain the maximum boost to be waste gate spring pressure, would the approach be to run the boost solenoid at 100% duty cycle during spool up and smoothly bring it back to 0% at the rpm where the turbo comes on to full boost?

Is it even always necessary to run the solenoid at 100% during spool up to achieve this or can this be done with less duty cycle?

There can be a small but worthwhile improvement in boost response with a 3 port solenoid if you apply 100% duty cycle during the spool period as it ensures the wastegate won't begin to crack open. In reality the difference is small though at best. you also need to make sure that you rveert to typical duty cycles before you reach your boost target or you risk over boosting. You're likely to get a similar response with less than 100% duty but testing would be the only way to find out what works best. Ultimately though you're going to be using a higher duty cycle than what you need for your actual boost pressure so in my opinion you may as well just use 100%.

Understood Andre, what about the first part of my question? Would the boost response change from just swapping from the factory 2 port solenoid to an aftermarket 3 port solenoid? All other things equal, ie duty table completely zerod out and engine running on spring pressure. Would the turbo come onto boost earlier?

Is this a general question or do you have a specific engine in mind? Some (although not many) engines come with 3 port boost controllers from the factory (GT-R, SRT-4), or they are more complicated than that and have two 2 port solenoids (Evo for example). Some even come with a factory external wastgate (rare), such as a Porsche 944. Although yes, the majority are coming with 2 port pressure based actuators, a lot of the newest stuff has gone to vacuum or electric (BMW for example).

The biggest difference between a 3 port and a 2 port is that, generally speaking it takes less duty cycle to push the wastegate closed with a 3 port, because the 3 port vents more air from an internal wastegate actuator. Vendors will try to tell you that a 3 port gets better response than a 2 port. I'm not convinced. A 3 port requires a full retune of the boost control system so it's hard to say for sure. You would need to install a string potentiometer on the wastegate to physically measure how much the arm itself moves vs the duty cycle of the soenoid. You also want a pressure transducer tapped into the actuator chamber to map the relationship. This kind of instrumentation is used by companies during development or reverse engineering of boost control systems.

With a 2 port and a 3 port directly compared, 0% duty would be expected to have very similar results unless there is a signifcant difference in the plumbing. A 2 port typically has a restricter orifice and a T fitting installed in line, so now you've got to figure out how much that impacts it. At 50% duty, you can expect the 3 port to make significantly more boost than a 2 port, but again that depends on the size of the restricter orifice used for the 2 port. At 100% duty they SHOULD perform similarly but on a given application it's again going to depend on the relatinship between duty cycle, pressure inside the wastegate canister, spring force, all that stuff.

That's why on the newer more expensive boost control systems it's primarily electric. The stock ECU sends a wastegate position command to the actuator and that's it. Then there's typically a bunch of maps inside the ECU that characterize the relationship between wastegate position (angle), wastegate valve opening area, and actual wastegate flow.

Thanks for the response Raymond. The engine is a Evo x 4b11. Just fitted a grimmspeed 3 port to replace the factory unit. Turbo is also upgraded to gtx3071r

I've used the Grimmspeed before on Subarus. It's just a rebadged 3 way pneumatic valve by MAC. Are you using the stock Evo boost control algorithms? They control based on a target engine load (mass airflow from the sensor basically) rather than a specific pressure.

bring back an old thread instead of creating a new thread.

are the MAC solenoids commonly used fine to be held at 100% duty?

i would assume it would be wise to have 0% duty below say 1500 rpm so to minimise loading of the coils?

also what effect does the pwm frequency generally have on boost control?

most people use around 30hz, but is there any reason to run less or more frequency? my logic is the higher the frequency he finer the control, this would be limited by the solenoid


Typically you don't want to run 100% duty for the "full closed wastegate" functionality. 90 or 95% is fine. If your ECU has some kind of rpm vs manifold pressure or rpm vs throttle/pedal angle map, you can certainly set it up to not run any duty at low speed and low load. I wouldn't be too worried about damaging the solenoid though.