| Really though, single will do just fine for most applications. The Greddy, HKS, and Apexi units only use a single stepper motor or solenoid. The second solenoid is useful for maintaining more precise boost levels at higher boost (ie >18psi). Unless the OP is planning to go big with his build up, the Spec S model will likely perform enough to meet his needs. I've owned and used them all, and I found no performance difference in the Spec S from the Spec R (I never exceeded 18psi).
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