4 hole adaptors are good for dual-plane manifolds where half the carb works for half the cylinders........
Just slap it together
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| quest. re carb intake openings in offy manifold types https://slantsix.org/forum/viewtopic.php?t=23833 |
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| Author: | rock [ Sat Jun 30, 2007 11:21 am ] |
| Post subject: | quest. re carb intake openings in offy manifold types |
Hello folks, Because I am fixing to change from a 500 cfm Edelbrock to a 390 cfm Holley 8007 I am thinking about my Offenhauser manifolds. Currently the Eddy is installed in one of the Offys that has the large square hole under the carb and I used a 2 inch riser spacer with a square hole. I wavered at installation time about using a 4 hole adapter for primary and secondaries, but had the square hole adapter and manifold on hand. As time passed I now I have a Offy manifold with the 4 holes for the squarebore Holley and am thinking about buying a 2 inch 4 hole riser. That prospective purchase has made me revisit the issue of how important having each butterfly valve dump into its own intake manifold hole really is. While I have done some reading there basically seems to be a lot of opinion, not a lot of solid reasons why one or the other is recommended (separate holes vs. one large hole). Since mine works fine now with the large square hole intake, what is the thinking of folks here who may have experience with both kinds of intakes or risers...is one as good as another? (For the Holley I am going to install it turned 90 degrees so the fuel dumps closest to the head). Thanks, rock '64d100 |
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| Author: | emsvitil [ Sat Jun 30, 2007 1:08 pm ] |
| Post subject: | |
4 hole adaptors are good for dual-plane manifolds where half the carb works for half the cylinders........ Just slap it together
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| Author: | rock [ Sun Jul 01, 2007 4:43 am ] |
| Post subject: | Ed, thanks very much, you confirmed my thinking |
Ed, Thanks very much, you confirmed my thinking. Since you have done more tuning than anyone I know of your opinion counts with me. I had "logicked" that the 4 hole would almost be required for dual plane but not really needed for a square hole single plane. I am one day gonna try the deal of TIGging in a divider in the space under the carb in another square hole Offy. I know Offy has done extensive development engineering 9I hope) and there is probably a reason they don't sell a slant intake with a divider plate, but I don't have access to that engineering. So I get to 'speriment but that is fun for me! rock '64d100 |
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| Author: | Doctor Dodge [ Sun Jul 01, 2007 7:07 am ] |
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The individual holes under each throttle blade can be used as a mixture distribution tuning tool. I have found that the longer tubes under the venturis helps to get the A/F mixture more "homoginious" or in a more evenly distributed condition, before the "mist" enters the plenum area. This can be helpful if you need to sort-out unevent mixture distribution in a "wet" intake manifold or you want to see how "staggared" carb jetting is working. I would agree that you should try what you have first and get a baseline, "read" the spark plugs to see if there are any mixture distribution concerns. If you do have any rich cylinders (some dark plugs) then try the 4 hole manifold / carb adaptor. DD |
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| Author: | Rug_Trucker [ Sun Jul 01, 2007 10:46 am ] |
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Why are you dumping the Eddie carb? Spacers on an A body bring to mind hood clearance issues. Am I correct? I have a stack of 4bbls. The smallest is a '66 383 AFB. I don't know the CFM of it. I do know it is smaller than the #1405 600CFM Eddie I have. I took a micrometer and measured all the throttle bores in my 4bbl collection. I calculated all the actual throttle bores. I have the '66 383 AFB, a '69 440 AVS, 70 383 AVS, the 600 Eddie, and stacks of 800, and 850 CFM Thermoquads. |
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| Author: | theo [ Sun Jul 01, 2007 11:32 am ] |
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rock : just an idea I once did on my son's ford 390. I had an open plenum manifold, put a four hole spacer under a 600 cfm holley. I NOW DO PORTING AND POLISHING,after years of instuction from my friend Herb Del Grande of head development grinding and a year at De Anza auto tech engine tract with a very knowledgable Gary Lewis. Air or air fuel mix does not like edges or corners, so try radiusing the bottoms of the holes a 1/4 -- 1/2 inch to remove the edges to influence the air / air fuel mix to enter the plenum and move through the plenum to the ports with less turbulance. don't expect gobs of improvment, but every little bit helps. 
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| Author: | rock [ Mon Jul 02, 2007 5:07 am ] |
| Post subject: | DD, Theo and RugTrucker, thanks and yep to porting it |
DD, Theo and RugTrucker..THANKS! Rug Trucker, not getting rid of the Eddy,just swapping it because I am an endless tinkerer. I also like the possibilities inherent in the tuning combinations of the Holley better than those I am limited to in the Eddy. Be assured the Eddy works great. And no, not a hood issue. Trucks havenear about "sky's the limit" on spacers! After lworking with Holleys I may NOT like all the possibilites, but as one of the gang of tuning fanatics on this forum I probably spend more time tweaking, tuning and measuring than straight out driving! I am looking for that perfect curve, like Emsvitl is looking for. Theo, I am a port and polish believer too. The Holley baseplate with the throttles has a (to me) unacceptably prominent sharp casting or sawing remnant inside each hole as fuel/air enters the manifold. I ported these ridges out, smoothed the break a bit then polished the interiors. I also ported and polished the Offy manifold. Much easier to do that aluminum than doing the iron head! And DD, as always, Thanks! You have given me another idea... rock '64d100 |
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