Prodigy Performance 3.6 Turbo DIY Install

[UselessPickles]

For us supercharger guys a vacuum bypass works great when you only want boost at wide open throttle one quarter mile at a time. But as Pickles points out his little trouble spot in the mid to upper 2,000 rmp range happens at part throttle. I don't know waste gate theory in turbocharging (despite having several volvos, audis and a 1980 turbo trans am ((worst motor ever!!))).

A supercharger bypass valve is very different from a turbo wastegate. At any given engine speed, the supercharger is always capable for generating full boost for (for that engine speed). The bypass valve allows that air to recirculate back to the supercharger inlet and essentially let the supercharger "free spin". When pressing on the throttle and reducing manifold vacuum, if that vacuum-controlled bypass valve closes too suddenly, you get a sudden harsh transition from no boost to full boost.

A turbo can only produce boost when there's enough engine load to produce enough exhaust to spool the turbo, and the wastegate is there to LIMIT boost (by bypassing exhaust around the turbine), not bypass boost back to the turbo inlet. With the turbo and wastegate, there's no tipping point with manifold vacuum that instantly moves you from no boost to full boost. As you press the throttle, you create more engine load, more exhaust, spool the turbo more, produce more boost, then the wastegate eventually opens as you approach the target peak boost to prevent boost from continuing to rise. The wastegate never causes a harsh transition, aside from noise. If you pay attention, you can hear it open because you can suddenly hear a different "whoosh" sound of exhaust flowing through the waste gate. Just for fun, I have played with feathering the throttle to stay right at that transition point. I can hear the wastegate repeatedly opening and closing, but there's never a sudden transition in power in this situation.

NOTE: wastegate behavior will be different between my install vs Prodigy's official install, because of this: http://jeeplab.com/showthread.php?13...ull=1#post3590

At part throttle, I can get my wastegate to open because it is reaching the target boost in the intake system before the throttle body (where my wastegate boost source is), even though my intake manifold may be a couple PSI below the target. The official Prodigy setup has the wastegate boost source as the intake manifold, so at part throttle, there may be situations where the wastegate stays closed trying to reach the target boost in the intake manifold, but the pressure on the other side of the half-way closed throttle could be above the target boost level, working the turbo harder than desired (hmmmm... I wonder if this could lead to compressor surge in these situations?).

But it's weird that you got it after the tune!

I've always had this issue. It was one of the first things I noticed on my very first test drive after installing stage 1 last June.

[gbaumann]

At part throttle, I can get my wastegate to open because it is reaching the target boost in the intake system before the throttle body (where my wastegate boost source is), even though my intake manifold may be a couple PSI below the target. The official Prodigy setup has the wastegate boost source as the intake manifold, so at part throttle, there may be situations where the wastegate stays closed trying to reach the target boost in the intake manifold, but the pressure on the other side of the half-way closed throttle could be above the target boost level, working the turbo harder than desired (hmmmm... I wonder if this could lead to compressor surge in these situations?).

So are you saying that there may be something in the way pressure is equalized across the throttle body at part throttle? I understand that my system is sucking air through the TB and your system is pushing air through the TB. But I've often thought that my TB was not designed for FI. Too much "play" in the throttle plate.