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“On some of our dual-plane manifolds, we will cut down the center divider; it varies from one manifold design to another,” explains Smith. “The amount that the divider is cut down isn’t determined by numbers on a dart board. This is all based on dyno testing and flow testing by our engineers. What this does is help balance the air signal to the carburetor for the air/fuel mixture at higher rpm. This technology goes back as far as Vic Edelbrock, Sr, and the old slingshot manifolds on flatheads."
By this statement, if I understand correctly, cutting down the divider is more beneficial for an engine that sees higher RPMs. (what I am planning to build most likely will be done by 5000 RPM)
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when the GM engineers drastically altered the L-72 intake for the L-88 then ended up stagger jetting the carb, and adding "air flow tabs" to keep all the cylinders balanced. How many guys removed the divider without checking cylinder exhaust temps for lean cylinders? Or modifying the carb to compensate? See my point.
When in doubt, stick with the factory configurations
Good point here, I never knew about the "air tabs."
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As stated, cutting down the plenum divider makes the manifold act more like a single plane manifold than a dual plane manifold. A dual plane (180 degree) takes advantage of some favorable wave dynamics at low revs to enhance low end torque and provides a better fuel metering signal to the carb. The carb will usually require richer jetting to maintain desired fuel-air ratio if the divider is cut or a single plane manifold is substituted for a dual plane. Chevrolet recommends specific "stagger jetting" (different sizes at each corner) for big blocks with cut down center dividers
A single plane manifold or cutting the divider down on a dual plane will increase top end power (assuming the carb calibration is modified to maintain proper fuel-air ratio), but at the expense of low end torque since the favorable low speed wave dynamics are reduced or completely lost.
This quote is directly from Cliff Ruggles, in a discussion about a QJet-specific application. He went on to say how his personal car lost power and ran slower with spacers. (4 hole and open)
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Use extreme caution with any modifications to the divider. We have recorded some significant LOSSES in power taking out the divider or even modifying them. A small notch or divit between the secondary throttle plates is fine, or use an open gasket.
That's it for now, but thought I would share what I found with a quick search.