I was using a Offenhauser dual port intake on my 66 F100 build all last summer and fall, but now I have ditched it for a customized Offy C open plenum manifold.

This is why:

The build specifics are:

EFI heads.
EFI exhaust manifolds
9.6 'or so' compression. Deck milled .030, Head milled .030, Piston tops cut back .030. Custom push rods
Comp cams 66-237-4 cam Single pattern, .447 lift, 260 duration, 110 lobe separation.
Steel timing gears.
Stock valve train.

Offy DP manifold with a Summit 500 cfm Autolite clone carb. .068 main, .071 secondaries, .042 acc pump nozzle, .045 idle feed restrictors. The carburetor is mounted facing the engine.
Also, there is a heat plate that attaches to the intake manifold. I run heater hoses to it so that the manifold gets 'some' heat.

Not a drag race engine, but an excellent highway cruiser engine.



During the summer months, the engine ran fine. Actually, it ran great! But once the outside temp dropped to 50 or below, the engine started to miss. Once it was 40 or below, the engine would barely run at all.

I finally pulled the spark plugs and found that the outside cylinders #1, #6 were running lean, #2, #5 were okay, #3,#4 were completely gas fouled out. On a much warmer day the #3 and #4 spark plugs didn't foul out, but they were still running richer than the others.

With the carburetor mounted correctly, facing the engine, the runners for the #3 and #4 cylinders are only a few inches long.
My working theory is that they are not long enough to absorb any heat from my puny antifreeze powered heat plate. Especially when it's 40 deg, outside.

So I switched to the Offenhauser C Series intake manifold (with modifications).

Now the truck will run just fine all the way down to 30 deg. It's still cold-natured and it probably always will be since I am missing the exhaust powered heat riser.

I have included some pictures of the Offy C manifold mods to turn it into a proper dual plane manifold with excellent idle and low speed manners.


The stock manifold is designed for the carburetor to mount in a north-south orientation.



First order of business was to cut out the main web.


Once the web was cut out, I started working on a groove that my divider plate could rest in.


Fitting the divider plate.



I am using an Offenhauser 5832 adapter to mount the carburetor east-west. The divider plate comes up though the adapter and helps support it. CAUTION. If you decide to do this, don't use something flimsy for your divider plate. In fact, it's better if you have it welded in. At idle, with 15 hg of vacuum, there is a lot of force on the divider so it needs to be in there securely.


1/2 inch thick 4 hole phenolic spacer to help with summer months heat soaking.


The carburetor is facing away from the engine. This way the primaries are farthest away from the #3 and #4 valves (soaking up as much heat as possible). The runners are not equal length, but they are much closer than they were. Equal length is not a great thing for a street car anyways.


Mounting the carb in this orientation let's the throttle and choke linkage be MUCH shorter. Makes for a easier feel on the gas pedal.


On the engine.


I have been driving it for about a month now with the C series manifold and so far I love it. No way would i switch back to the DP manifold.

Once spring is here, I will see what kind of mileage I am getting.
The spark plug fouling issues have disappeared except when it's 30 or below outside.

 

Great photos! Thanks for sharing.

 

That looks really nice. Did you secure the divider to the intake in any way besides trapping it under the carb?

 

Really like the attention to detail! Would be interesting to see if I could get similar results with my moody intake? Really interested to hear more feedback once you drive it more and can post up some mpg numbers. Thanks for posting this!

 

Good write up. I've been thinking of building an intake manifold using the lower half of an EFI intake manifold with tubing attached to the top and an offset part for the carburetor. (I'm being cheap).
I was thinking about how to make sure there was even fuel distribution with what I have in mind.

 

The first iteration of the divider plate was just trapped in the groove. After a few days of driving it started to vibrate. A lot.

I took everything apart and found that the divider was vibrating so hard that it was cracking. In a few more days it would have split completely and sent chunks into my engine. The first divider was 16 ga steel sheet.


The second divider I made out of 16 ga aluminum. I should have made it from 1/8" or thicker but so far it's working.

I cleaned everything and used a generous amount of JB Weld putty to hold the divider in the intake. The divider is also supported at the top where it fits through the adapter.

If I had known beforehand how much stress the divider sees, I would have made it thicker and welded it in.

 

That is why I asked.
Do not underestimate the destructive powers that the intake pulses in the intake plenum can have on your divider plate. In every instance that I can think of where plates were placed inside an intake (e.g. diverter fins, port throttles, etc.) there have been instances where those pulses have torn things apart. Yes, I would increase the thickness of the plate to at least 1/8" and weld it in fully then check the intake surfaces for flatness and correct if necessary.

 

Thought I would give an update on how this intake and carb combo is working out.

It's my daily driver in a 1966 F100 with a M5OD trans 3.5 final gears.

I have a 15 minute run to work on the interstate at 70 mph and it handles it fine.

The mileage is not what I was hoping for at 13.7 mpg. If I slowed down to 60 I am sure the mileage would improve, but that's a good way to get run over on these roads.

 

What RPM are you turning, around 2700? If so, I turn about the same in my EFI, and get about the same mileage as you at that speed. That's about the best a 4.9 will do at that RPM.

 

70 is about 2300. 60 is right at 2k

 

I was getting mid 14's with my 81 F100 flare side at 65 MPH with a 300 w/ stock carb, EFI exh. manifolds to stock system on back to tail pipe / T18 / 2.47 rear gear & 235 / 75 / R15 tires, forgot what the RPM was at that speed but 70 MPH the RPM was higher than I liked and the MPG down.

Now I said "was" as I swapped in a NP435 with Advance Adaptor's range splitter over drive unit. I just got the truck back on the road with AC, to hot to drive with out it before, so I will be getting some MPG after a few fill ups. I can say at 65 MPH the RPM is about 1800 in OD, think 70 MPH is about 2000 RPM.
Dave ----

 

im looking for an intake would your's be for sale possibly I've got a 85 f 350 dually im wanting to up hp and torque what do you think any thoughts would be great if you're interested in selling please email me thanks tim

 

I've been running a Clifford intake and an east / west mount 600 Holley since 1998.
I just did today come across Farnsworth's thread by snooping through the 300-six forum section.
Hmm, I've never really thought about checking my plugs for burn color like Farnsworth, buttt, I have learned to live with a hesitation off of initial start from a stop if I floor the throttle. I at first played around with holley accel pump squirters and cam ramps to no avail. Then kind of learned to drive accordingly upon trying to make a "quick" start on the green light in a 6,200 lb pickup with 300 cubes grunting.
Although I have enjoyed the little 300, it no where compares to a 385 series 429 in my E-250.

Amazing how much resonance or pulse vibration in the divider plate.
Oh, too, my 300 lives in my '77 F-350 longbed Crew Cab with a heavy chassis, and I gutted the TTB crossmember and converted to a '79 D-60 disc brake w/ kingpin axle assembly.

Back to my 300 experience:
When having my machining done, my block is now .040, with hypertectic pistons.
I had the rotating assembly balanced, and some mild porting on the head.
I installed a Comp cam (which a lobe on #1 cyl went flat in 4-years) and replaced w/ a Clevite.
Headers are by Clifford too, and with no issues in 23 yrs service.
One mistake i made looking back: I didn't modify the oil squirter which sprays oil on the timing gears.
The oil sprays straight out the end. I should have plugged the end and drilled a hole in the side, directed at the metal timing gears. Hahaha, and I do enjoy the distinct whine of those gears !
Had i made this mod, the lobes on the comp cam may have lasted longer, who knows.

What I like about having a 4-bbl carb on my 300 is the Secondaries for towing and running along happily at 3600 rpms, as the 300 cubes try pulling the trailer up the grade.
The 4160 Holley no. 3310 four barrel w/ manual choke is very Cold Blooded until warmed up.

I have a later model split factory 300 exhaust in two pieces I purchased used as back-ups in case I had problems with the Clifford headers, but have had no problems yet with the Clifford Headers.

Oh, I am not running a fan shroud, and have never had any cooling issues. 300's run Coool.

Almost forgot: I took a 300-mile cruise in 2020 up into the Sierra Nevada mountains to retrieve (purchase) an early *****'s F-134 engine to get some very rare transmission parts I wanted.
My fuel mileage average was just over 13-mpg best I could figure. no overdrive w/ NP435 trans and 4:10 axle gearing w/ 255-16 tires. If I were driving 65 on the freeway, my mileage would probably drop from extended upper rpm's. I don't have a tach, so all is a guess unless I run the numbers on a rpm calculator. I ran the numbers and at 65 with 4.10 gears and 33" tires, i'm doing 2,700 rpms.
Heck, I figured I'd be into the low 3,000 rpm range. Amazing.
I guess the direct mounted fan is making lots of noise I could loose if I went to a fan clutch.