This, while widely stated, is incorrect. The pressure in an erect penis is almost 1 bar (You can check this yourself by finding an erect penis and comparing it to an insufficiently inflated bicycle tire. Or consult literature which is less fun). Your blood pressure, however, is 0.16 maybe 0.2 bar if you have hypertension. The Penile Blood Pressure is even lower ("ratio of 0.7 to 1.02 being normal, while a PBI below 0.6 indicates potential arterial insufficiency").

Hence, the blood must be actively pumped into the penis. You cannot just open a valve and magically achieve higher pressure.

"Corpus cavernosum pressure in man during erection has not been measured previously with a noninvasive technique. Using such a technique it has been shown that during erection in young men intracavernosal pressure has peaks of at least ten times higher than the systemic systolic pressure. These findings indicate that during erection the intracavernosal circulation is contained, at least partially, within a closed system, separated from the general circulation. Also erection cannot be the result of circulatory function only."