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Motec M1 on yaris GR - overshoot boost

Boost Control

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Hello guys! I started to map a Yaris GR with motec PNP ECU. For now I would like to keep the boost at 150kpa. What I didn't understand is why we got to the limiter at 7200RPM the boost goes into overshoot. After the gear change, the pressure rises until it reaches the safety target set at 170kpa. how can I stop this rush? thank you all for taking the time!

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Hi Massimo,

This is completely normal. Your turbocharger is still turning and compressing air, and your throttle plate is shut. You can see a spike in pressure as there is a boost pressure sensor near the throttle body. If you compare the boost pressure against the manifold pressure, you will see the spike is registered by the boost pressure sensor, and not the manifold pressure.

The only way you could change this spike is by altering the turbocharger bypass (bow off / bypass valve) threshold value. However, tightening up the threshold on the bypass valve can cause the valve to operate more than required, particularly on spool up or on throttle conditions.

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  • Boost-pressure-vs-manifold-pressure-till-bypass-valve-opens.PNG
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  • Yaris-GR-Turbocharger-bypass-valve.PNG
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Thanks it Was evrithing interesting. I dont understend why the pressure overshoot the value 1.5 bar just with in 5th gear after to give gas in full throttle

Are you referring to the overshoots to the boost aim after the gear change?

The Yaris boost control is exceptionally sensitive, and you need to keep in mind that it is vacuum actuated, using a 300hz proportioning valve instead of a 30hz switching solenoid. These types of valve tend to change behaviour with temperature, which is why Toyota mounted the unit on top of the engine, under an engine cover - to get the solenoid to a normalised operating condition as quick as possible.

If you have reduced the margin, you struggle to get consistently nice control. Also the Yaris combination is highly sensitive to boost control PID numbers, and the base numbers in the MoTeC file are to suit a vehicle with stock intake and exhaust. Any deviation from this will require retuning.

Keep in mind, that in the factory configuration:

- They have had boost control issues, as Toyota released a fix (they increased the overboost failsafe margin, see attached bulletin)

- The factory system uses the throttle servo to assist in controlling boost (the factory system reduces throttle to around 50% approaching peak boost), which is a strategy that was not implemented in the MoTeC.

In testing i have seen the required feed forward number for 1.6 bar of boost change 4-6% over the course of dyno testing (with it increasing as everything heatsoaked) which when you look at how little the numbers change in the feed forward table, what is a drastic change in behaviour based on temp.

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Nathan, You are a gift to this forum! Thanks for the very insightful information.

The best!! In this days we try again with the new solution. Thanks so much from Italy

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