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the Performer AND the Performer RPM are BOTH dual plane manifolds. 
The Performer saves you some weight over the factory iron piece, but very little as far as performance increase.
The Performer RPM is a VERY good manifold, just too tall for a lot of applications. Makes a lot of lowend grunt along w/ good upper rpm power.
The Holley SD has almost all the same characteristics of the Performer RPM (good lowend torque and drivability as well as good upper rpm power) even though it is a single plane and is a shorter manifold.
I totally agree. The SD is very short & works fairly well on the low end for being a single plane. No longer available new for an RB iirc but they are around. I hear there are better absolute choices but it works well for what it is & if you need the clearance...
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