[Steve C.]

"...our Pontiacs are intake challenged."

Example. Rocky Rotella and Jim Hand have done flow bench testing on how much a intake manifold reduces the flow to the cylinder head. Jim reports that a stock intake will flow about 91.5 to 92 percent of a ported head at 250 cfm. A Performer RPM will flow about 93.5 percent. I presume these are as cast intakes and not modified. Rocky has tested a RPM intake on a stock 5C head that flowed an average of 208 cfm at .500" lift and stock SD-455 head that flowed an average of 237 cfm at .550" lift. The RPM was a just under 93 percent with the 5C head and just over 92 percent with the SD head. If the percentage stays true as flow goes up, you'll need head flow in the 260 cfm range to get 240 cfm into the engine. Rocky used a as cast RPM intake and not modified. He also reports the RPM intake flows about 2 percent more than the average unmodified iron intake.

In addition to this there is restriction with the carburetor. He measured a QJ with those intakes and found his Holley HP950 and a Cliff-modified QJ each reduced flow by another 5 percent. If the QJ is stock the reduction can be as much as 7 percent.

It's all about the total airflow delivered to the valves.


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