Offy "Dual-Port" intake manifold

[MagicRat]

Hi.
I have two aftermarket aluminum intake manifolds I can run on my stock '69 GTO (400 cid)
One is a conventional dual plane Weiand intake from the 1980's.
The other is a strange Offenhauser "Dual Port" intake, probably from the 1970's
The Offy had two intake runners for each intake port, one large diameter and one small.
The small runners all hook up to a plenum for the primary barrels on the carburetor.
There is a second plenum hooked up to the secondary side of the carb which feeds the larger diameter runners.

I can understand the theory of this design, at light throttle use, the engine uses only the small runners, so intake velocity is kept high fro sharper throttle response. At wider throttle openings, both are used.
(Some more modern DOHC 4 valve engines used a similar theory, the SHO Taurus and ZR-1 Corvette come to mind)

Obviously this design requires a spread bore carb, with larger secondary barrels. I am using a spread bore Holley 650, although I have a bucket load of other carbs available to me.

So, Which one should I use on an otherwise stock engine?
Is this Offy manifold rare? Have you heard of it before?
Finally, if this Offy manifold is any good why does no one else use the design?

[MrPbody]

Forget them! The Offy is a doorstop, at best. While the idea was good, execution was not. The Wieand isn't a whole lot better, and won't fit with HEI. Your best bet is to use a factory iron intake made from '68 through '72. These will need the "spread-bore" or Q-jet as well. But the factory iron intake will outflow and outperform EVERY offering from Offy or Holley (Wieand is owned by Holley), of the single 4-bbl. variety, including the Street Dominator. The only aftermarket stuff really worthwhile is Edelbrock. But that's just my opinion. I could be wrong...

[cody2dog]

I have another opinion on the Offy dual port. I think it is true that it is not an excellent performance manifold, but may be better than others if your goal is better gas mileage. I had one on a Pontiac 400 years ago, and it got 25mpg on the hiway, but I don't know how much of that was due to the manifold. I did find that it choked the 455 I installed it on later, so I sold mine. Just not enough air flow.

[MrPbody]

Cody,

You probably had small valve heads and a stock cam. A 400 is known to knock down excellent mileage when at a mild level of tune and a good tune-up. The Q-Jet would have more to do with the efficiency than the intake, but the high velocity of the primary runners COULD be responsible. But as you learned, it's a POS when it comes to making any power. The factory intake is still among the better choices.

Jim

[cody2dog]

Sorry, wrong on both counts. Large valve heads and a Comp Cams camshaft with much more than stock duration, though not a real big cam. I will agree that the q-jet had more bearing than the intake. Anyhow, don't make ass-sumptions, right?

[MrPbody]

Okay then. You're using high output cam and large carb with a small orifice to push (or pull, as the case may be) it through. You will find, if you install a "Performer" or factory Q-Jet intake, your low-end will still be good, but high-end will improve significantly.

Believe me, we've tested these intakes TO DEATH. Today, Performer is the popular replacement for a mild engine. Performer RPM is for 400s and smaller, while Torker II works best on the larger engines (all for street use, not "race only") that are modified for high-end performance. Still hard to beat the factory iron intake for most mild-to-medium level builds.

Jim