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Old 10-02-2012, 08:54 AM
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cobalt327 cobalt327 is offline
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The 1" x 3" notched plenum divider of the RPM AG falls in between an intact plenum divider of a dual plane and an open spacer on a dual plane. It allows some of the benefits of a single plane but w/o all of the downside. In my experience using an Edelbrock C3B w/its cut out for the secondary throttle blade of the Holley 3 barrel extended out to a little over 3.5", it performs right in between an open spacer and no spacer or "notched" divider of a dual plane. This also seems to be the trend w/the RPM vs. RPM AG. An unmodified C3B is shown below, left. An RPM AG below, right. Below them is an RPM w/o AG, left and a Vic Jr, right.

An open spacer on a dual plane will increase plenum volume- something that a notched plenum doesn't do, and this is part of the reason why an open spacer/dual plane often falls in between an open plenum and a standard dual plane w/o an open spacer in performance. Also there's no added carb height when using a notched plenum divider like there is using a spacer. The added height of the spacer can help allow the air/fuel to make the turn into the runners w/less deflection of the a/f when it hits the plenum floor at high rpm.

So it's hard to directly compare the Vic/Vic Jr. w/a notched RPM intake or an open spacer on a dual plane (not that anyone is) because of the runner lengths and added overall runner/plenum volume of the Vic that combine to raise its powerband over an RPM w/an open spacer or an RPM AG.

As far as cutting the RPM like the AG, because the carb height and runners are basically the same between the two intakes, that mod will make their top end output equal for all intents and purposes. Not saying that's the thing to do in every case- just saying that if the same powerband as the AG is wanted, it can be had by notching the divider. For the right engine and vehicle combo (low gears, higher stall speed, light weight, more track than street, etc.) notching the plenum (or adding an open spacer) can help more than it hurts. But for the majority of street driven vehicles, I think the non AG w/the plenum left as-is, IS the way to go.

FWIW, I have always had better performance from a divided spacer on a dual plane (not always a 4-hole spacer). The spacer below I made from marine grade plywood for a 455 Pontiac engine running a Performer (this was before there was an RPM Pontiac intake). It picked up the quarter mile ET over a tenth, which was also pointing towards the engine wanting more intake and carburetor. Being wood, it acts as an insulator much better than metal and most other denser materials.

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