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Boost not consistent.

Sylvain Bourgault

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2.5L engine with 8.75 CR.

VF29 turbo.

40mm external wastegate.

3 ports grimmspeed boost control solenoid.



The tune was completed on a dyno (timing, fuel, boost control).

Target boost 16psi at 4000rpm dropping to 10psi at 7000rpm.

Once on the road, boost was reaching 20psi at 400rom and 14 psi at 7000rpm.

Load seems to be a BIG factor in my issue.



I did try to run wastegate pressure (0 duty cycle) and it is perfectly stable at 9psi.

No leak anywhere in the system.


What could I do?


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My boost control setup is the one illustrated here. I am using the external wastegate setup.


I was able to achieve a good control on the boost by using a restriction pill between port 2 and the top of the wastegate.

Without pill, I was overboosting in higher gears. If I remove the hose between port 2 and the top of wastegate, I was not able to boost above wastegate pressure.

My duty cycle are in the range of 10% to 35%.

I did a 2D map with SPEED as the vertical axis. It is equivalent to a gear compensation but with better resolution. I will test it tonight.

Close loop would be great but I would like to, at least, be able to control it a little in open loop.

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