I have looked into buying an Offenhauser intake manifold for my 68 240. JC Whitney sells one with an "open plenum" for $199.99. Jegs sells one that is discribed as a "dual plain" (I believe). It is also $199.99 The dual plane shows four round holes where the carb attaches. The open plenum shows one big square hole instead. I am planning on installing a 390 4-barrel Holley carb. Which is best for me? Or, are the they the same? Thanks, John

 

MANIFOLD DESIGN

Heat impacts these intake pipes. Production engines have heat risers that warm the manifold to improve cold start conditions.

Once an engine warms up, however, this hurts high performance. What happens, is the manifold gets hot enough to actually warm the air as it comes through the intake runners. This is why you see many performance manifolds with an airspace between the lift er valley and the bottom of the manifold.

For this same reason, many performance and racing heads do not have a heat riser from the exhaust ports.

Another factor of manifold design affecting performance is the plenum. This is the chamber where the intake runners collect under the carburetor or throttle body.

An open plenum design has a single junction box connecting all of the intake runners. A divided plenum has half of the runners coming into one box and the other half coming into an adjacent box.

With divided plenum manifolds, each cylinder can only draw from half of the carburetor. If you are using a 2-barrel carburetor with this manifold, it means each cylinder would pull air from a single barrel.

With an open plenum manifold, each cylinder can draw air through both sides of the carburetor. If you use a 4-barrel carburetor with this design, then each cylinder can draw air through four barrels of the carburetor.

CARB SIZE
In the case of a 2-barrel carburetor and a divided plenum, it is very likely that the single barrel is the most restrictive part of the intake system leading to the cylinder.

If you race in a 2-barrel class and wish to modify a stock engine, you are likely to gain more performance with an open plenum manifold than you would by going to larger intake valves.

At the other end of the spectrum, if you have a monster 4-barrel on a stock engine, the carburetor is likely to be oversized in comparison to the intake ports and valves. This can also hurt the performance.

For a carburetor to work its best, it should be balanced with the intake runners and intake valves.

If you have a carburetor that is way too big, a divided plenum manifold may actually improve you engine's performance in comparison to using an open plenum manifold.

Cheers
Colonel Flashman
Red '58 Mercury M-100
Blue '58 Mercury M-100 Panel

 

The "dual port" version will probably work best for most people wanting to run a 4bbl carb. It's a true divided runner intake so the primary and secondary sides have their own flow path. The primary runners have a smaller cross section to give the best intake velocity at low/mid RPM where the 300-6 spends most of its time. The secondary runners are larger to provide sufficient flow when the 4bbl opens up. Offy claims this design provides a 15%-30% power increase at all rpm ranges.

The single plane version might be better if you're doing a serious build where upper RPM performance is most important, or if you want to use a single 2bbl carb. You could use an adapter plate to make a 2bbl work on the dual port version, but me thinks that would completely defeat its purpose.

I'm using the dual port version w/ the Holley 390 4bbl carb and couldn't be happier.

 

I also have the dual plane w/ the Holley 390CFM on my 300, and previously had the single plane from JC Whitney (both are available from Summit Racing, as well). I see slightly better mileage with the dual plane, but I haven't done any serious towing with it yet either.

Also, on the dual plane, the carb mounts *sideways*, primaries toward the engine and the throttle linkage toward the rear, and I'm having a slight clearance problem with my throttle cable mount hitting up against the heater box. Had to get a little creative to get it to work. My truck doesn't have A/C, so I don't know if that's another concern.

I hear that a lot of people use the EFI headers with this setup, I wish I did. I'm using Pacesetter headers (from JC Whitney), what garbage. The fifth exhaust header must be slightly thinner than it should, as it leaks like crazy, and I've been playing with it quite a lot to seal it up. That's how I broke the first Offy intake, by overtightening the manifold bolts. Also, header collector gaskets seem to burn out, though not a problem I can blame on Pacesetter. I recently switched to these *high perfomance* soft aluminum collector gaskets, so far so good. With the EFI headers, you can run a stock exhaust system, or a performance system for an EFI.