Phil.  Some comments from the peanut gallery . . .

I'm intrigued that Ford seemed to be married to direct linkage on y-block and FE dual quad set-ups while GM scrubrolet seemed to be tied to the progressive linkage methodology.  I don't know if that was true across other GM divisions (like Cadillac) or just scrubrolet.

My understanding is that Ford's FE factory triple set-ups used idle circuits in all three carburetors.  That was so different from the thinking of most hot rod triple set-up architects who advocate eliminating the idle circuits in the end carbs and using the carbs as dumpers.

Food-for-thought from a fellow y-block hobbyist . . .
- The Edelbrock y-block FM255 intake separates the two four barrel carbs more than other Edelbrock and other aftermarket manufacturers did. 
- There's ~ 8 1/2 inches between the carburetor centers.
- Each carburetor's primary throttle plates appear to be equally spaced from the four cylinders that each carb feeds. 
- In using progressive linkage with the rear carb as the primary, the airflow from the rear carb primary plates to the front intake ports' branching is ~ 7 5/8 inches. 
- The distance from the rear carb primary plates to the rear intake ports' branching is ~ 1 3/8 inches. 
- This suggests that the rear primary carburetor's air flow will favor the rear four cylinders. 
- Additionally, with progressive linkage, the front carburetor's actual opening of primary and secondary throttle plates will likely be quite limited in the vehicle's normal street and highway use.

Is progressive linkage of the carbs on this particular 2x4 intake manifold layout ideal for equal air flow to all cylinders?

Best Wishes always with what you're doing!      


NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

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The following is in no way disrespect for TOM EATON and his experience/knowledge.

I think the FYB 8V Systems were released for race mainly. The use of direct linkage would be advantageous in that scenario. GM used progressive on their early systems (8V and 6V) to make them more street-able. As bad as I hate to say it, GM did offer better street engines. FORD did not. If you wanted a fast FORD, you usually had to build it. That was until 1968 anyway (428CJ).

You run on the primary carb primary circuit until you want more acceleration. You use two idle circuits but only one accelerator pump for normal driving until higher RPM cut-in. That primary carb will supply sufficient fuel at low street RPM as the fuel mixture is a vapor and will be drawn to each cylinder due to a common plenum. Each cylinder will not be perfectly balanced (according to intake plenum design) but once both carbs are cut in, most everything will be good (if setup properly). I had a 67 GT-500 that when all eight opened it sounded like a shotgun.

The only way to win the argument is to install both linkage systems, tune and put on a chassis dyno.

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Since I run progressive I'd like to throw this in. When I still had my manifold off  I taped of all the ports with blue masking tape and started poking holes and using my air comp and a nozzle to see if the front of the rear carb had passages to the front. I checked each one, one at a time removing the port flange tape and poking a hole and adding air at the carb plate. I didn't care how much flowed, just that it did. I was making an assumption that if there was a passage, enough would flow at idle and cruise no matter how many inches it was different from the rear.  All of them showed air flow so I also assumed then I could go progressive like the 245/270 hp "C" engines. In 5 years there is no difference in plug readings. I run 45's.
Remember these engines are just "air pumps" and don't know much or care much about effiency at the lower RPM's where we normally drive them. I'm not at a drag strip. Unless your racing and using a tunnel ram type manifold you don't don't do this for a performance advantage. If I was looking for that I'd put on my Blue Thunder or a Mummert 4 barrel. I did it because it's neat looking.. After 5 years of have guys look under my big red Purolator air cleaner I've gone to individual air cleaners buy using Charlie Prices bases, Empi VW filters, and 2 7" polished aluminum pan lids for a kitchen supply store..
Dual quads on anything is always worth a second look and a smile....The 50-60's at there finest.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

Thanks for the opinions.  I noticed the greater plenum separation in the FM 255 manifold myself. The air compressor test is something I would not have thought of.  Since my carbs will be mounted forward, it won't be difficult to try direct and progressive linkage and go with what works best.  My TBird is street driven around town and to car shows.  It's not a racer.  I do step on it hard.  I don't race because I'm sure I would grenade the T86 tranny and that's something I don't want to happen.  I test mounted the carbs this afternoon and took some measurements.  One of the carbs in these photos is on my car now.    The second one needs cleaning up.  I will be changing out the vacuum secondary tops so I can connect them.   Looks awesome.

Anyone know where I can buy this or a similar part so I can connect my stock Thunderbird linkage to my dual quads?

Phil.  Those pieces I made for the dyno so the linkage could be easily adapted and adjusted.

Lorena, Texas (South of Waco)

The Ford Y dual quad setups had direct linkage to both carbs as supplied by the factory on both the forward facing and rear facing applications.  It would be interesting to hear why that was the ultimate choice by the engineers as to whether both linkage setups were tested and the direct linkage deemed the better of the two or if the direct linkage was simply more expedient and less expensive to provide.  You’ll have to keep in mind that there were several thousand dual quad equipped 1957 Fords made with the dual quad option and those all had direct linkage on the carbs.
 
When the FE’s came out with three deuces and dual quads, progressive linkage was used on all of those.  On the FE three deuce setup, progressive linkage was a natural as the primary carb was centered on the manifold thus making fuel distribution to the front and rear of the engine much more equal.  On the dual quad setups, the two carbs are facing to the rear to facilitate distributor clearance and on those setups, the front carb is the primary carb with the rear carb being in the idle position for the first third of the throttle opening.  That put the primary bores on the front carburetor in the most favorable position to feed all the cylinders in normal driving.  Having the carbs face rearward did mandate the use of a bell crank on the throttle linkage to move the linkage to the passenger side of the engine.  This bell crank scenario is also seen on the Thunderbird Y dual quad applications where the carbs are also facing to the rear.  Where the FE with dual quads had progressive linkage, the 'E' code Thunderbirds had direct linkage.

I have a dual quad FE in a ’66 Fairlane and drove it for awhile with the original progressive linkage but eventually got tired of the off idle hesitation with the progressive linkage when just driving the car around normally.  I went with direct linkage on both carbs and drivability improved immediately.
On your Y setup, try it both ways and find which one you are happier with.
 
Here are a couple of pictures of the direct linkage on Y dual quad setups.
 

  o

Lorena, Texas (South of Waco)

I'm having a difficult time finding the part in the photo below.  Lots of suppliers make a rod with ends to connect both carbs,  The Thunderbird linkage requires a hole mid way.  There is 8 3/4" between the carb linkage holes. Does anyone reproduce this linkage or must I fabricate it?

Phil, if you are going to use those carbs facing forward on that manifold in a Bird, take the chrome trim from the hood scoop and slowly lower the hood closed while watching for interference with the fuel bowl on the front carb.  The factory reversed the carbs to fix that problem, and your manifold looks to have the front carb further forward than the factory manifold.  Air filter fit may also be a problem.  Be careful, don't damage any parts of your Bird.

John - "The Hoosier Hurricane"