All Big Six Clifford Plenum Divider

Relates to all big sixes

old28racer

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I just got done making a modification to my 240/300 inline 6 Clifford intake manifold. I have listed the mods & included some pictures. I have removed the center section of the Clifford 4V adapter to form an open plenum. Then I turned the 4x2 carb adapter 90 degrees. Formed a wood divider template to fit the side walls & floor.

1- Added a plenum divider so 1 throttle bore feeds #1/2/3 cylinders & the other #4/5/6. cylinders. The setup will work with both my Holley 2300 & Autolite 2100 1.08 venturi carb.

2- I JB welded & filed flat the 2bbl adapter top to make sure I had a good gasket seal & no vacuum leaks.

Comments on anything I can do to the setup to improve on operation.
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old28racer

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greg, thanks for the reminder. I hope that the metal divider placed in the notch that is cut in the 4x2 adapter will help keep it in place along with the carb base holding it down. We will see. With continuing medical issues it will be a few months until I get the intake in place.

First thing is to get a fresh AFR baseline with the current 300 log intake and 1x2 carb adapter. #2 is to change the .062 factory jets in my Holley 2300 to a pair of .059 & get a new cruise AFR. #3 is to install the plenum divided Clifford & my 2300 with the .059 jets. #4 will be to try my Autolite 2100 1.08 venturi carb in place of the 2300 and get new cruise AFR numbers.

Will make the 2300 .062 to .059 jet change the first of Aug.
 

pmuller9

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Does the divider plate just sit on the plenum floor or does the floor also have a notch for it to sit into?
 

old28racer

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Ya Paul, I was able to cut a very shallow groove into the floor so I don't think it coming loss will be a problem. I'm just going to try it and see if I need to do something more.
 

sixtseventwo4d

Well-known member
This will sound like I'm defeating your purpose but perhaps others will add to my thinking here. Drill a hole (3/8?) though the plate or maybe just notch it at the bottom vertically a small margin. My thinking here is to reduce the pulse waves between the front 3 and back 3 cylinders, maybe it won't be as prone to loosen. It may also help to balance venturi contribution at idle.
 
This will sound like I'm defeating your purpose but perhaps others will add to my thinking here. Drill a hole (3/8?) though the plate or maybe just notch it at the bottom vertically a small margin. My thinking here is to reduce the pulse waves between the front 3 and back 3 cylinders, maybe it won't be as prone to loosen. It may also help to balance venturi contribution at idle.
The divider cast into the EFI manifolds has much bigger pass through holes where the vacuum and PCV bosses are. The pressure differentials we're dealing with here are very small. They're basically just sounds. Anything that mostly blocks off the path will have a large effect. I don't think a 3/8 will do much of anything either way.
 

old28racer

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Ya guys like I said before I am going to try the system like I have it now and see if I need to make any changes. With the bottom of the divider in the grove on intake floor and the top in the notch cut in the 2 bbl adapter, JB Welded & held down by the carb base plate I think it will stay in place, we will see. Thanks for the comments.
 

Firepower354

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It looks like there are gaps around the edges of the divider, in places? That would eliminate the need for holes/notches added.

I think it would benefit, ditching the 1-1/2 of spacer/adapter plates, for a single flat aluminum plate, bolted to the intake, studs for carb. Less plenum volume, lumpy side walls, should improve response a lot.

Even a plenum "pig" between 3/4 to make it like the upper level of a DP, but at full runner size, seems good for torque.
 

bubba22349

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Hi old28racer, I really like this idea of the plentium diveder! I don't know if you have already fixed this but I noticed in the above pictures where you machined out the carb plate that the bottom edge is straight down into the plentium. If you were to put a small radius on the bottom edge next to the plentium it might help the flow or turbulence some. On your center diveder it looks like a little tapering of the top edge near the center part of each the throttle bore openings would also help the flow but mabe you already have plenty of clearance for the different Carb's throttle blades to open.

I can't wait to see how well your intake mod performs I always though they should of made an intake like that, kind of like a 180 degree type intakes that worked so well for street torque on the V8's back in the late 1960's and into the 1980's engines (but I guess this would really be a 120 degree intake design on the six?). Best of luck.
 

old28racer

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Firepower354 -- Yes there is a slight gap between the divider walls & the intake wall. I am hoping they work like a balance tube, no more than a few thousands at any point.

I get what your saying about a flat aluminum adapter plate to reduce plenum volume, might try that later.

As I am new to this plenum volume game I do not know what a plenum "pig" between 3/4 is or looks like. Please let me know.

bubba22349 -- I am going to try the Clifford intake with the 4x2 adapter like it is set up the first time.

I am going to run my truck with my current very restrictive 1x2 adapter & stock 300 1bbl log intake. Will run my Holley 2300 2bbl carb with .059 jets & 6.5 power valve to get a new baseline on my AEM AFR gauge.

Then install the divider plenum Clifford intake with my Holley 2300 set as above and get new AFR numbers.

Then remove the Holley 2300 and try an Autolite 2100 1.08 venturi 2bbl with a 7.5 power valve and .057 jets and get new AFR numbers.

As for the divider/throttle plate clearance it is fine with both carbs, the Holley 2300 has a bit bigger plates than the autolite.

This may take me all summer as I am still having medical issues.
Thanks for the comments.
 

bubba22349

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Old28racer, I do think this is going to be a very good experiment, I believe it will run better and get better MPG with either of the two carb compared to the stock log intake with its adapter. Sorry to hear your still having the medical issues and do hope that getts better for you real soon. Best wishes on a full recovery!
 

old28racer

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bubba22349 -- Thanks for the good wishes. Ya I am looking forward to getting the divided Clifford intake mounted & my 2300 2bbl installed. With the divider installed the cfm velocity should be much better than with the 4V open plenum. As it will move more air I may have to richen my jets back up from the .059 to .061 or maybe even back to the factory .062 jets. My AFR gauge will tell me what to try. Better MPG would be a plus as my around town is a low 9-11 mpg & my highway a so/so 16-17 mpg. If I could get 14/20 I would be reel happy. I will also be installing a freshly recurved HEI distributor done by Billy the "distributorman" on this site that should help performance & a better burn.
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MechRick

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Reducing plenum volume and number of carb bores facing each cylinder should increase signal. You may have to jet it leaner. I just went the other way on a dual plane equipped 5.0 by adding a 2" open spacer between the carb and intake. I found primary jetting to be unchanged, but the secondaries went from 68 jets to 73's.

You might find a nice torque boost down low.

My only concern is removing the resonance between 1-3 and 4-6 might mess with cylinder to cylinder fuel distribution. It's something you can rule out by looking at the plugs after a good test drive.
 

old28racer

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MechRick -- Thanks for the comments. I will be real surprised if I have to go richer on the jets as my current 1bbl log intake with a 1x2 adapter is so flow restrictive and the Clifford flows so much better I think my AFR gauge will call for bigger jets. I am not looking for a performance gain, I just wanted to try running a plenum divider to feed the 1/2/3 & 4/5/6 cylinder better. I will keep a good eye on the plugs also. I am hoping to run a cruise 45-50 mph AFR in the 15.5-16.0 range. This will improve my mpg in general.
 

JPierce

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Interested to see the results. I like the industrial look of the engine. I'd like to finish my engine with a bronze look similar to your cylinder head. Is your head painted or is it a clear coat?
 

old28racer

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JP -- the head has no coating on it yet. It was hot tanked & bead blaster before complete rebuild with bigger valves. I had planned on painting it the same industrial gray as the block, valve cover will stay black.
 

Blairsville Ed

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When you get the motor running, watch the main Venturis with a flash light as you slowly increase the throttle opening. You need both Venturis to start flowing at the exact same time. This is more of a problem with the Weber carbs but a very slightly bent or miss aligned throttle plate could cause one side to flow before the other. Ask me how I know……It causes a poor throttle up that is very hard to diagnose.
 

Frank

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Firepower 354- good afternoon. - What am seeing on the pic of your DP manifold? Is there a mod in there, or am I confused by shadows? I am not being derogatory. I am always interested in mods to this diamond in the rough manifold, just not sure what I am seeing. . Thanks
 
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