Dual Plane Manifolds.

[Formula Jr]

Whilst rebuilding the old 4160 Holley with vacuum secondaries, I came across a conundrum that I hope some of you more knowledgeable gearheads might answer.
My engine is a ford 351 with a Ford dual plane aluminum intake manifold. The idle was
never quite right with this combo when the engine was warmed up and the secondaries would only open fully with two people on board. Now after rebuilding the carb - a very easy task by the way with Holleys, once you've fabricated a driver for the special secondary metering plate screws- I noticed that the carb; because of the Throttle cable holder plate and the base gasket, was never completely seated on the manifold plenum (divider). It had a 1/4 inch gap between it and the plenum divider. So, I figure, what's the deal with a dual plane intake if you're not really separating the banks. So, being the type of person that this would bother, I set out to ask a few people what the proper setup was. All of them, once I pointed out the gasket and seating problem, simply said - "well, I never thought about that....you usually cut down the divider some anyway." Okay, so then, why are the intakes sold with these nice flat machined plenum dividers? And how come you can not find a stock base gasket to actually divide the carb in half so one side truly feeds one bank independently of the other.
I removed the throttle holder plate and custom made a nice true divided base gasket and tested it out with the muffs. Idle was purrrfick and the little bog on acceleration was gone - still have to fabricate a throttle cable holder of some sort before sea trial but the engine sounds much better. But the original question is still there. What's the true purpose of a dual plane manifold if everyone just sort of ignores it or cuts down the plenum (pellum) divider?



[This message has been edited by Formula Jr (edited 04-07-2001).]

[BigGrizzly]

First they do make that gasket. Next the space you are talking about isor balence between the left and right sides of the carb, more so for the secondaries. If you had a car you eith a drivability problem you would put pn aplate with 4 ventories, as in the early Fords. The 4160 has some balance in the baseplate, in my opinion not enough. I think you solved your problem with the rebuild. If the secondaeies still kick in too soon then chang the vaccum actuatoe spring to a stronger one (don't loose the little ball). they are readially available.

The reasion they machine it dead flat is because its cheeper. Plus and gives you the option of using 4 hole spacer or one large pellum chamber. To this day I have never seen a boat run better with the 4 hole plate yet. Som say they do but I have proved all wrong so far.
I do hope you solved your problem. I had one like it once ans found a bad gasket on the metering plate,( I think).

Randy

[RickR]

Formula Jr
Did it make any difference on the top end?
BBCs are supposed to benefit a little on the top end if you cut the divider 1/2".

------------------
RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>

[Formula Jr]

I think I'm using the right words here, but I'm probably wrong.
The Pellum is the open air space directly under the carb before the fuel/air is directed to the individual cylinder intakes?
The word plenum I'm using, doesn't jibe with the physics involved since that applies to pressurized vessels, not lower pressure areas.
What I've read and heard so far, is that Small Block Fords need all the vacuum they can create. Thus the smaller CFM carbs. My thinking is that by isolating the right and left cylinder banks, I maximize the vacuum created since there is less volumn of air to dilute the vacuum created on any particular intake stroke. Ideally eight individual carbs would maximize this. I'm looking for High End, but I can't test the theory till I make a new Throttle cable holder and run it under load. This gearhead stuff is cool.

[RickR]

Formula Jr
Your discription is correct.
Divided plenum will help create a better vacuum PULSE at low and mid rpm's enabling a better idle and throttle response but

Open plenum intakes work better at high rpms by using all four carb venturies for each intake pulse to increase the efficiency (lower the vacuum) of the carb.

AIR IN AIR OUT!



------------------
RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>

[boldts]

Formula Jr.
I don't know if this will help you, but here is what I am running on my 302 Ford engine in my Donzi. My boat has the Holman Moody parts that were added to the engine even though it no longer has the original engine block. My intake came with a 1" carb spacer that has 4 equal sized holes in it. By the look of the gasket on it, it looks like it mated solid with the intake manifold. I am running a 650 CFM mechanical secondary Holley. Last year I installed a K & N air filter that is about a 1 1/2" to 1 3/4" high by 11" dia. Because of the new filter and the additional height, I had to take the carb spacer out from underneath the carb. (I also had to remove a little material from the engine cover.)

The big thing that I noticed from what was done was that the boats out-of-hole punch really improved. I didn't gain any speed that I could tell, but I didn't loose any either. Another note, this is not a marine carb, but I have added 2 hoses to the carb overflow vents to put the fuel back into the intake. I really like the mechanical vs the vacuum secondary. No lag time when you hit the throttle. Terrific if you like to ski behind your boat. The 650 CFM uses a little more fuel than the old carb that was on it. I believe a Ford product. All I know is that these modifications really woke my engine up and made it much more reliable. She starts with 1 or 2 pumps of the throttle and idles like a kitten.

Scott Boldt

[BigGrizzly]

Hold it folks, Bolts your comparing apples and oranges. your using a holley doubble pumper, and it uses alot morw fuel.Rhe 4 holed spacer was never a Holman & Moody part installed on a Donzi. With that Double pumper it won't make no difference which spacer you use. I have them all- aluminum and phenalic. just so you know that engine will run the same top end with a 500cfm, maybe better. My Corsican run better and faster with a 600 than a 750. I would put a 500 on but it is on my 1971 351 MACH I, and it runs great to spite the fact it came with a bigger carb. Rmember a boat is not a car, think how many rpms at 60 in a car vs 35 in a boat. So much for bottom end

Randy

[boldts]

Big Griz,
Thanks for your information. I don't know exactly what came with the HM option. The spacer was on the intake when I bought the boat so I assumed it came from the HM folks. Next time the boat needs a carb, I'll try the 500 CFM. Even with the 650, I don't think my gas mileage is any worst than when the engine had smaller carbs on it. The only difference is the wide open throttle runs which like most other Donzi owners I love to do as often as possible. I still wouldn't trade the mechanical secondary for the vacuum. I love that throttle response!

I look forward to learning more from you Randy at the Awakening in KY. Knowledge is a wonderful thing. I just hope you that have it don't mind us guys who don't to learn from you.

Scott Boldt

[BigGrizzly]

Scott don't mind at all. You shouldn't have to pay the price and time I did. Thats what this board is about, free help.

Randy

[Formula Jr]

Got the throttle cable holder installed - noted that I got to get a new cable - and using a digital reference tach, my little experiment payed off big time. The high speed miss is gone, the bog is gone, and I picked up 130 more RPMs at WOT. Mid-range acceleration now lets me hop out of the water on a porposing trim like a jersy skiff. I run out of water on Dexter Lake before I can nail down the right tab and drive combo; Jrs chine walk badly over 50 if you don't get the trim right cause the lower strakes terminate too early to accomadate the old style water intake systems, but man, its never run this well and the squirlly steering got fixed by the new steering link bushing. So, I'm pretty certain at this point that the space between the throttle plate and the intake manifold was the cause of the high speed miss that had plauged this engine for over six years and thru three new, but identical carbs. The water coolent issue was the pointer that lead to looking at why the banks were trading off on the miss and that lead to looking at carb balance. This was also the first time people came up to the boat and started asking "What the heck is that." Cool! The lake is getting smaller every day.
http://www.zing.com/picture/p12019ef59c30ff3b737d29f41c1c8903/fea36d3f.jpg.orig.jpg
http://www.zing.com/picture/pd45deb880bdadfce0ef7152505d73c2d/fea36d5e.jpg.orig.jpg

[This message has been edited by Formula Jr (edited 04-16-2001).]