[DeTomaso] Offenhauser Intakes

[Daniel C Jones]

> What does the collective know about the Dual Plane Offenhauser intake with
> the intake runners split all the way out to the ends of the runners.

Dual port and dual plane designs are fundamentally different.  Dual plane
intakes have the plenum separated left to right.  One side of the plenum is
on a higher level (or "plane", that's were the name comes from) than the
other side.  On a V8, each side of the dual plane pelnum feeds two cylinders
on both sides of the engine.  The factory 351C intakes were dual planes.
The dual port design has the runners split into top and bottom sections,
withthe plenum split fore-and-aft (rather than the usual side-to-side).
It's essentially two single plane intakes, stacked on top of each other.
Refer to the pictures here:

 http://351cleveland.wetpaint.com/page/Offy+Dual+Port
 http://www.network54.com/Forum/119419/message/1250831614/Offy+Dual+Port

The primaries feed the longer path lower runners and the secondaries feed
the shorter path upper runners. When cruising, the manifold operates on the
smaller area, longer path, runners. This provides high velocity flow for
good fuel economy and throttle response. Under high demand, the secondaries
open and provide additional flow through the larger top runner portion.
It's an interesting approach but the packaging required to fit within a carb
intake envelope does compromise the design somewhat.  By reputation, it's a
good fuel economy, low end response intake.  Makes a good truck intake but
it's not a maximum horsepower design.  Many years ago, David Vizard did a
big intake comparison (on a 350 Chevy but the trends probably apply).  The
Dual Port was near the bottom of the list when comparing peak horsepower but
the area under the curve was very wide and the fuel consumption was the best.
I've seen similar test results on other engines.  On a mildly cammed, big
port, low compression 351C, the dual port design should work well but on a
higher performance engine, they will limit RPM and kill top end power.
The ratio in areas of the the upper and lower ports varies by engine.  Some
are split evenly.  On the the 351C, the split in area is perhaps one third
to two thirds. 1/3 to 2/3.  That would lend itself well to spread bore
carb designs with similar venturi area ratios.  The dual port is designed
for an idle to 5500 RPM range but an Offy tech rep claimed 300 to 400 RPM
can be picked up by milling back the port divider 3/4" (keeping it square)
at the head flange.  He also mentioned that if you open up the the primary
and secondary holes up to a big oval instead of two circles, performance
will increase with no loss of low end.  It would be interesting to send a
dual port out to Extrude Hone to see what they could do with it.  For the
dyno program, I've got spread bore versions for both 2V and 4V heads.  The
2V is unported but the 4V has had some port work.  Even with the port work,
I expect that intake will be out of it's element on the dyno 351C due to
the cam and compression of that engine.  I've got Quadrajet, ThermoQuad
and Holley 4011 spread bore carbs to fit that intake and I've thought about
installing it on my white Pantera's mostly stock, low compression open
chamber 4V engine but I've got a high port headed stroker going together
for that car and don't know if I'll have time to play with the dual port.

I know of several people who have run a Dual Port on 351C's and 400's (one
in a 4000 lb Torino, two in Panteras, and one in a Detomaso Longchamp, one
in a Bronco).  All seemed to like them in their specific applications, noting
very good throttle response and fuel economy.  However, the Dual Port will
limit the upper end, so how well it will work in your application is highly
dependent upon the rest of the engine build.

Offenhauser holds the patents for the dual port design and is the only company
I know of that made dual port intakes.  For the 351C, they made them for both
2V and 4V heads to suit square bore and spread bore carbs.  Most of the spread
bore versions were for Quardajet and ThermQuad carbs but they also made a
version for Motorcraft 4300D carbs.  Here's a quick list of the
various versions
of the dual port that Offy made for the Ford 351C's and 351M/400's:

Offenhauser Dual Port 351C-2V (6110-DP)
- Holley square bore carb bolt pattern (same as Autolite 4100,
Motorcraft 4300A,
  Carter AFB/AVS, etc.)

Offenhauser Dual Port 351C-2V (6111-DP)
- spread bore bolt pattern (QuadraJet, ThermoQuad)

Offenhauser Dual Port 351C-4V (6013-DP)
- Holley square bore carb bolt pattern (same as Autolite 4100,
Motorcraft 4300A,
  Carter AFB/AVS, etc.)

Offenhauser Dual Port 351C-4V (6014-DP)
- spread bore bolt pattern (QuadraJet, ThermoQuad)

Offenhauser Dual Port 351C (p/n?)
- Motorcraft 4300D spread bore bolt pattern version

Offenhauser Dual Port 351M (1975 and later, 6141-DP)
- Holley square bore carb bolt pattern (same as Autolite 4100,
Motorcraft 4300A,
  Carter AFB/AVS, etc.)

Offenhauser Dual Port 351M (1975 and later, 6142-DP)
- spread bore bolt pattern (QuadraJet, ThermoQuad)

Offenhauser Dual Port 400 (1971 and later, 6033-DP)
- Holley square bore carb bolt pattern (same as Autolite 4100,
Motorcraft 4300A,
  Carter AFB/AVS, etc.)

Offenhauser Dual Port 400 (1971 and later, 6034-DP)
- spread bore bolt pattern (QuadraJet, ThermoQuad)

BTW, the Dual Port is a different intake from the Equa-Flow 360. Externally
they look the same but the 360 is a split plenum (left/right) single plane
intake with a notch in the center divider. The Dual Port is the one with the
ports divided top and bottom and the plenum divided fore and aft.  The Offy
Equa-Flow 360 performed poorly in my dyno testing.

Dan Jones