Here is a simplified/model diagram of an external wastegate, without a boost controller on it but both ports hooked up. You shouldn't ever hook up an external wastegate exactly like this, but if you think about the arrangement for a minute it demonstrates how the pressure sources for the wastegate ports affect tuning of the boost controller. Remember that there are basically two forces working against each other, and the purpose of the boost controller is determine how much force goes to the top of the spring.

Which one of these arrangements will result in lower boost, and why? How would each routing affect boost control solenoid valve duty cycle, if it was a NC-normally closed valve (more duty = more air/pressure)? What would happen if you accidentally used the NO (normally open, more duty = less air/pressure) port on this configuration and how would that affect tuning?





Now think about if the top and bottom ports had the same pressure source from the same location, using a Tee and the proper NC port configuration on the solenoid valve. You wouldn't ever actually do that without one of the pressure sources going through the boost control solenoid first (so that air to the top port can be regulated), but it's important to think about when understanding how everything works.

 

So is it better to have different pressure sources or the same. I know the uim will have less pressure then straight off the turbo, but the boost controller regulates the pressure regardless of the source. Would it just change the duty cycle of the controller? Is there an advantage of doing it one way over another? Will one way cause a more stable pattern or increase spool speed? I would think t- ing into it would be best to keep the pressures even but i've never experimented with this.

 

I don't think there's a hard and fast answer to that. It would depend on the control algorithm in the boost controller itself and what range of gain it's designed to use. I've seen different vacuum routing diagrams for different brand boost controllers, even ones that use the same Denso solenoid we are talking about here.

And anyway we've been talking about boost controllers that use a duty cycle controlled solenoid. Some use a stepper motor like the HKS EVC, so that's going to work a little bit differently.

It seems like the best thing to do would be to first follow the instructions in the installation diagram for your particular model. If that doesn't work a lot of people have had good luck using a Tee'd pressure source after the intercooler outlet.