[Cliff R]

I tested 3 intakes back to back a few years ago on a 428 (440CID), 10 to 1 compression, 260cfm KRE heads and 236/242 custom ground HR cam. I tested a 1970 iron intake, HO (reproduction) and an RPM.

The iron intake and HO were opened up under the carb to the same size/shape as the RPM, all were port matched to a stock intake gasket, no ported of any kind in the runners.

The iron intake made 497hp. The RPM made 491hp and the HO made 487hp, all with no other changes.

IF you are going to use spacers I tested 4 spacers at the track on iron intakes and found that the fully divided and semi-open (open between the secondaries) work best. The 4 hole was horrible and so was the fully open spacer.

I'd also recommend to leave the divider intact on any dual plane intake and at most just remove a small half circle amount between the secondaries or use a 1/4" thick open gasket on it.

Removing the divider or using open spacers can cause distribution and turbulence issues even if they make more power on the dyno. I've tried all of that stuff and experience noticeable hesitation/stumble/bog going quickly to full throttle that will not tune out.

I know that the intakes above were not completely stock but they were all the same under the carb and none of the runners were ported in any of them.

For BIG power levels I would NOT use an RPM intake. I've had several customers experience issues with leaning out one cylinder and spinning rod bearings with them. I'm taking over 700hp and running well into the 9's. For those set-ups you'd be better off with a well made single plane intake and at least a 1" spacer on it......FWIW......Cliff