Dual Plain VS Single Plain. Pro's and Con's of both.

Dual plane; good throttle response,best for pulling skiers and cruising,economy and the reliability of stock parts. Single plane;used for maximum airflow for top end horsepower.

Very seldom does a single plane work good for a relatively low rpm marine engine (an engine that operates below 5900 rpm). Single planes are best for car racing where only one carburetor is allowed and engine rpm's are at 6000 to 7500 RPM.

The single plane intake manifold was the best manifold in the early 1970's Super Stock drag race cars. These cars where limited to stock carburetors but could use any intake manifold. These flow limited engines required a huge plenum chamber to make power at high speeds. Low speed torque was not required at all.

On an engine that is not limited to stock carburetors, flow can be increased with a larger carburetor.

The most important aspect of an intake manifold is to separate cylinders 5 and 7 (port side) and cylinders 4 and 8 (starboard side). These cylinders will rob each other of fuel and create lean cylinders at low speeds (as in a single plane manifold). Dividing these cylinders is the main function of a dual plane manifold.

A modern dual plane manifold (with a large carburetor) will make as much horsepower as a single plane manifold but have more low speed torque (when operating in the rpm range of most boat engines).

Dennis Moore

I'm running RPM Air Gap's right now, with Holly Marine 850"s started with 84 front and 90 in the rear. Now im running 86 front and 92 in the rear. 77 to 78 mph @ 5200 with labbed 28's. as always looking for alittle more. and wondering if I made the right choice of intake.

+ 2 on airgap. I am running about the same jetting.
Dennis Moore is one of the best respected people. If you can get a copy of his book, its good reading.

This thread, and thus my answer is for a 502cid BBC.

Depends on cam and heads.

If running a modern cyl head and a modern HR cam (even low duration) a single plane will win. I spent many hrs on a dyno with wet exhaust testing different intakes, spacers, etc,etc with a short duration, relatively high lift cam.

Also proved it on the water with that engine and others.

500+ cids, good heads, and good cam = old rules change.

454, OEM Rectangulars - 95% of time I'd side with the AirGap RPM.

This is nothing new, but some still want to make or believe a marine low compression 502 look like a tow truck engine. It's not.

So , after what I wrote above above, please share the rest of your build and it should make it an easy answer.

I would think that installing single plane intake manifolds would be a step backwards.

5200 rpm is just not fast enough to feel a gain from single plane manifolds.

For experimental reasons I suggest a taller open spacer under the carburetor on your dual plane manifold.

Dennis Moore

You made the right choice. One of the best dual planes out there.

On a stock 502 you can't do any better. I run a single plane on a 700hp 524 at 6500 rpm. anything less would be the RPM Air-Gap.