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PostPosted: Sun Aug 07, 2022 12:32 am 
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Supercharged
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Joined: Sun Nov 03, 2002 9:20 pm
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Location: Fircrest, WA
Car Model: 76 D100
I have seen this topic discussed before on this board (about 12-14 years ago) but the discusion then focused on things like milling and tig and mig and other things I just don't have the tools for.

I spent a few hours and a few dollars converting an Offenhauser intake I got for cheap (thanks again, Wes!) from a single plane intake to a fairly ugly but functional dual plane. This is the cheap and dirty version of the conversion, and I will give you tips on how to do it even easier and cheaper.

*Note: a previous owner of this intake had it powder coated (oooh, shiny!) and cut the throttle bore + out of the middle of the carb opening. This actually helped me because it gave me easy access to where I needed to place the dividing wall. If your intake has the throttle bore divider still in place you will need to cut the middle part out or cut it all out or make a notch for the divider.

(1) Notch the opening for the intake. Try and get it as wide as possible without cutting into the wall around the central inboard intake-to-exhaust bolt.

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(2) Measure and cut/grind (depending on your tools. All I had access to was my bench grinder when I did this) a piece of metal (I used flat steel stock from Home Depot. It was cheap and I already had it) into a dividing wall. Make sure it fills as much of the plenum as possible but is still flush with the carb mounting surface.

Secure the dividng wall in the intake with some high temp RTV (you want that wall to be removeable in the future "just in case," right?).

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(3) There will likely be a gap at the inboard and outboard sections of the divider wall. Fill it with more high temperature RTV.

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I left the gap at the back open (a) to serve as sort of a balance tube and (b) because there is a vacuum fitting right behind it that I want to leave functional.

(4) Start building your adapter stack to mount the carb sideways. I did not get any carb mounting plate with this intake so I had to come up with my own as I completed this project. NOTE: If you are using an Offenhauser intake, YOU MUST USE A FOUR HOLE SPACER OR ADAPTER IMMEDIATELY ON TOP OF THE MANIFOLD OR IT WON'T SEAL. The indent for the inboard manifold attachment bolt dips far enough into the plenum area that an open spacer or mounting pad will leave a big vacuum leak.

I started with an old large to small spaced four hole adapter I had and I drilled the holes to match the mounting holes in the intake. This gave me a way to seal the plenum under the carb. I then actually paid to buy a new pair of trick carb mounting adapters that spin the carb 90 degrees while having the carb mounting provisions in the standard location. They are an Edelbrock piece intended for tunnel rams, IIRC. Fortunately, they are sold as a pair in case you mess up the first one. The downside to the 90 degree adapter is that it is an open design. If the 90 degree adapter had been a four hole design, it would have sealed the plenum and I would not have needed the first four-hole adapter.

If you are building your own carb mount from a single piece of aluminum and doing all your own drilling and such, you could get a few four barrel gaskets and carefully mark and drill the carb mounting pad 90 degrees off and skip having to get a second adapter plate. I worked with the materials and tools I had on hand and didn't have that option. I have the hood clearance and want the benefit a carb spacer gives so I don't mind the extra height.

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My setup from bottom to top goes: manifold - gasket - four hole spacer in normal orientation - gasket - 90 degree carb adapter screwed to manifold through the four hole adapter and bolted to the four-hole spacer using the normal orientation carb mount holes.

Most expensive part of this was the 90 degree adapters, but I have under $100 into this and now I have a functional "dual-plane" four barrel intake with the carb mounted properly to allow true dual plane operation.

Once I get my rear manifold cut and welded and install my OG Dutra front exhaust manifold, I can run dual O2 sensors and have effectively two three cylinder engines. Should make tuning a bit easier.


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PostPosted: Sun Aug 07, 2022 3:29 am 
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Far out. Interested to hear how it runs. I guess you will use a 390 4bbl on this or maybe an Edel 500?

Lou

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PostPosted: Sun Aug 07, 2022 10:33 am 
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Supercharged
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Location: Fircrest, WA
Car Model: 76 D100
I have an old Carter AFB that is 500 CFM that I am running right now on a Super Six intake with a 2 to 4 barrel adapter (I know, not optimal). I also have a Holley 2300 Economaster 350 CFM carb and a Holley 4360 Economaster four barrel that flows 450 CFM. The Holleys both need some work before they can run, but i may try them out. Short term I am running the Edelbrock.


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PostPosted: Mon Aug 08, 2022 4:57 am 
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Reed, wouldn't that open center of the 90* adapter kinda negate the concept of the dual-plane plenum? Seems that you'd get unwanted mixture between the dual planes before the charge even got to the plenum.
But, I'm not an automotive engineer.

Roger


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PostPosted: Mon Aug 08, 2022 2:20 pm 
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Supercharged
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Location: Fircrest, WA
Car Model: 76 D100
Roger- no. The divider goes from the floor of the plenum to the carb mounting surface, so the two planes, or, in this case, more accurately the two halves, remain separate as long as the division is maintained up to the bottom of the carb. I did intetionally leave about a 1/2 inch gap at the bottom of the carb for pressure balancing. Will it help/hurt? I have no idea. I build intakes using RTV sealant. I don't have sophisticated measuring devices.
:mrgreen:

I can always build a thin air dam out of JB weld putty if necessary.


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PostPosted: Mon Aug 08, 2022 7:05 pm 
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Cool project, nice-looking result, but using RTV inside the intake like this makes my beard stand on end. Yes, RTV is gasoline-resistant to a degree, but that's for incidental exposure, not a constant steady high-velocity gasoline shower. I really think you should clean that RTV all the way out of there and either leave it (and any substitute) out, or use a more suitable material—preferably one that involves welding equipment, not a squeeze-tube. The consequences of sucking glue of whatever kind into the engine can be worse than they might seem; RTV is made out of silicone. When silicone is burned, you get silica, a high abrasive that can do a real job on things like cylinders.

I might also quibble a little with calling this a "dual plane" intake. It's a divided single-plane intake. Here's a dual-plane, dual-carb Slant-6 intake:

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PostPosted: Mon Aug 08, 2022 11:57 pm 
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Supercharged
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Location: Fircrest, WA
Car Model: 76 D100
OK, I will tear it down and remove the silicone. Lacking a mig or tig welder, I will have to break down and try Durafix aluminum brazing rods to secure the divider.

As far as "dual plane" goes, that South African manifold looks pretty nifty, but it seems like an over complicated way of achieving the same effect as I did. I think balancing the micture in that dual carb intake with the stacked plenums would be a nightmare. It seems to e that splitting a stock arrangement manifold down the middle and running a transverse mounted carb achieves the same goal of a separate intake plenum for each half of the engine, but since it is an in-line engine as opposed to a V shaped engine, the plenums do not need to stack. Then again, I am by no means an expert on this stuff and could be missing some advantage to the stacked runners on that South African intake.

I will keep making slow progress on this project as the summer goes on and will report back what I find.


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PostPosted: Tue Aug 09, 2022 1:57 am 
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Reed, you are doing great work here, a nice, innovated approach.
The video at the link has graphics that explain how a dual plane intake improves the vacuum signal to the carb over a single plane intake.
The important concept is that the two plenums must be completely separated. Notice that the carburator base on the dual plane intakes shown the
plenum wall goes to the carb-carb gasket base. Any leakage or balance tube that connects the plenums will dilute the effect of the dual plenum.

https://www.youtube.com/watch?v=2UfZXXrrPTk

I used aluminum rod with a butane torch to fill gaps for an aluminum project I was working this past spring, I used the Benz-o-Matic rods from Lowes with good success.
I have also seen the aluminum gas torch welding rods at Harbor Freight.
The video at the link compares various brands of aluminum gas torch welding rods.

https://www.youtube.com/watch?v=fKIKsDfRAcs

Loctite has several products that are advertised as gasoline resistant.
the link shows one of them. Personally I would call the Tech Line at Loctite, explain what you are trying to accomplish and get their input for the sealing product solution.

https://www.loctiteproducts.com/en/prod ... pound.html

Good Luck with your project. I am certainly interested in how it works out. I may do something similar on the slant in the D150 over the winter.

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PostPosted: Tue Aug 09, 2022 4:28 am 
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EFI Slant 6
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Car Model: 1966 2D Dart
I have had a divide plate inside of Clifford manifold. The manifold was built to sequential efi and it had a 318 throttle body on it. Divider improved the idle and off idle a lot with a wild camshaft. There was no difference in performance (1/4 mile et and speed) with or without the divider plate.

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PostPosted: Tue Aug 09, 2022 9:20 am 
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Supercharged
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Location: Fircrest, WA
Car Model: 76 D100
Thanks for the kind words. I have always been a bit leery of sealing it with RTV sealant, so I will just go back and use the Durafix rods and some Mapp gas. I may re-fabricate the dividing wall out of aluminium instead of steel since Durafix does not stick to steel.


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PostPosted: Tue Aug 09, 2022 12:46 pm 
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I would feel fine about using a few little spots of JB Weld to fix it in place. Really, it cannot move once the plate is on top of it.

Thanks for the cool posts, everyone!

Lou

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PostPosted: Tue Aug 09, 2022 1:35 pm 
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The divider could be sealed around the edge with a piece of fuel hose split down the side

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PostPosted: Tue Aug 09, 2022 11:52 pm 
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EFI Slant 6
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Car Model: 1966 2D Dart
In my version the divider was not sealed at all. There was 1 - 2 mm gap around and the system did its job well.

In the past the multi carburetor systems have balance tubes in between ports doing something.

A small leak at the divider area does the same "something".

I had port injecting EFI not carburetor.

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PostPosted: Wed Aug 10, 2022 7:31 am 
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Supercharged
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Location: Fircrest, WA
Car Model: 76 D100
lgu32 wrote:
In my version the divider was not sealed at all. There was 1 - 2 mm gap around and the system did its job well.

In the past the multi carburetor systems have balance tubes in between ports doing something.

A small leak at the divider area does the same "something".

I had port injecting EFI not carburetor.


Thanks for sharing this. I have been wondering if the usual carbureted intake principles applied to an EFI system.


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PostPosted: Sat Aug 13, 2022 6:55 am 
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EFI Slant 6
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Location: Helsinki Finland
Car Model: 1966 2D Dart
The vacuum outlet port used for example didnt hit to the divider but was in side of 1-2-3. As well I had my oxygen sensor in my D-duals front. So my 225 was doing a master and slave run. The first three cylinders was controlling everything and the 4-5-6 has to adapt to what they will get. It was working well. The marginal leakage in between 1-2-3 and 4-5-6 didnt cause any problems.

viewtopic.php?t=53817

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