Inliners International Forums General Discussion Hi Performance Edmunds 2x1 248 intake on a 302 and other stuff

I have been working on a engine for my granpa's 55.1 GMC forever. I am finally to a place I can afford a 302 and military ones are surfacing locally. About 6 years ago I lucked upon a Edmunds 2x1 labeled "248" on the bottom. I realize this is a small port intake. Would it be a huge restriction to run this with adapter rings to the larger 302 port? I could have it enlarged internally but it hurts my heart a little to extensively modify something that has lived this long. I plan on using 2 Zenith 228 carbs. I have a 11475 and a 14475. I am not sure if these are the same or not. They show for the same applications in many places. Not many threads about running dual Zeniths to be found. I plan on increasing compression to 9.25, performance cam and mildly porting the head. Is there much to be gained with the cast iron or tube header? The stock 302 exhaust is 2.5" and is about the right size for the horse power that this engine will likely make. Any thought are more than welcome. Thanks

My 270 is opposite of your situation. The haed is small port and the intake is big port. The rings transition from large to small and I feel that for the street it is actually a benefit because it probably increases velocity entering the port. I don't know if that would work going the other way. I would not feel bad at all hogging out the intake to do what you want it to do. That is what it was built for, the reason it exists.
I have several Zenith 28/228 carbs. I will use 2 on my Chevy four cylinder. They were used a lot in the "Day". They have separate and changeable venturies and some have external adjustable main jets. I have used then in pairs in the past but have been told by a carb expert that it is hard to tune more than one at a time. I like them.
Yes, there is power to be gained with headers. To work best they should be tuned to the rest of your build and the use the engine will actually see.

I assumed it would act like putting straw in a marathon runners mouth and asking them to run. I will see if it has the meat to open it up. I will look at headers. it may be a down the road thing just because the cost to rebuild will be plenty.

well I went to the garage to measure and I can't open it that far. Guess I buy rings or sell this one and buy a big port intake. There is a 3x1 nicson on eBay but it is a little to high for me at the moment.

The problem I have with the two sizes is sealing it at the head. I'm not savvy to all the flow science but it would seems that flowing from the small intake into the large head might cause turbulence entering the head and slow the velocity of the charge because it would expand the fill the large port. The valves are the same size so that may not be the case. It's just metal and someone can modify the intake. Eddie Edmunds would not mind.

I would have to add substantial material to the outside to open it that large. I was looking in my old Fluids book about pressure recovery in diffusers but it looks like the length from small to large would have to be long and at a shallow angle to prevent turbulent flow.

There's always turbulent flow in an engine, but I agree that a tapered cone transition piece would have to be fairly long to minimize disruption.
I might point out that:
a 2 X 1 manifold requires both carburetors operating simultaneously on a common plenum (2 X 1 separately is not possible due to the siamese center cylinder pair)
a 3 X 1 with a common plenum may run on the center alone (with possible distribution problems)
a 3 X 1 with independent runners requires all 3 operating
So there are operating differences as well as appearance.
How are your fabrication skills?

My Fab stills aren't bad and I know a few good machinests. I wonder how the McGurk/Howard Style intake where the runners drop out the bottom of the plenum work vs the oem/fenton/nicson/edmunds where the runners run out the side of the plenum. If you built it like some of the tunnel rams where the top unbolts you could more easily make it a 3x1, 2x1, 3x2 5x1, 5x2 etc to see if one improves. I will have to look into it a bit more.

An (*almost) individual 3 X 1 is the easiest to make, stealing the port runners from an existing aluminum manifold and adding a flange to each cut end. Many possible choices but all are simultaneous operation so size & jetting are critical, each cylinder pair needs more CFM than you might suspect.
* frequently, a small balance tube is added

A top with multiple patterns means a common plenum, so the carb CFM is cumulative.

I looked on Mcmaster carr and 1.5" sch 10 90 elbow are 1.68" internal and flanges would be easy to get cut and welded on. How would you heat the individual runners? Maybe box around the carbs flanges to heat with coolant?

A common air filter box with a thermostatic door (JY, low $$) ducts air pre-heated from a sleeve over the exhaust, just like everything had 1967-85.

Greetings . . .

Based on the original the post it sounds as if you have yet to acquire an engine – and that some 302 engines/parts may soon be available in your area. Seems you are still weighing options before purchase . . .

The old Patrick’s site/catalog had a very succinct write up on stock GMC heads:

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2136477

Open (Hemi) chamber, small ports (1 7/16"). Served as the only head, apparently, for '228, '236, '248, '256 & '270 from 1939 thru approximately 1954, with the sole exception of some '270's which got the new bigport "H" head (below). Compression was established by size of lump on the piston.

2193417

The legendary "H" head - open chamber, with large 1 3/4" intake ports, introduced in 1950 and used only through '52 after which (819) was used. Great for performance! Forged, custom piston shown.

2193983

'302 head - big ports, "D" chamber, always with a 2-barrel carb, it seems. Installed in larger GVW trucks (never pick-ups) from '52 thru '59. An accompanying governor limited rpm's to 3,200. Also, a great performer, with "D" chamber. Custom piston pictured.

2194819

Small ports and small chamber. This head was apparently introduced about 1954 to allow lighter, flt-top (or even dished!) pistons on '248 and '270 engines. '236 and '256 engines were already gone and now '228 and '248 were dropped, leaving the '270 to serve in everything down to 1/2 ton. These (6) engines are the "small-block" Jimmy family; sharing rod and main bearing sizes, connecting rods and head-bolt pattern.

Theoretically, any of the above heads could be installed on any of the blocks. However, the '477 head would be limited in performance by the small ports, and pistons would require some sort of lump (or, "pop-up) to make adequate compression. The '417 and '983 heads make excellent performance application with their large ports and good chamber design, though they are best installed on their respective blocks: '417 on a '270 and '983 on a '302 due to piston confiuration. By noting the photo, it is apparent that the '302 head would interfere with the '270 piston, and the "H" head would make less compression with the '302 piston. The 2194819 head is a poor performance candidate by virtue of its small ports and restricted chamber. (note that parts number reveals introductorysequence ) None of the stock GMC pistons is recommended for a performance engine, due to their excessive weight.

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Eddie Edmunds cast that manifold for the then common 477 head.

Fast forward to today and you’ve got an uncommon Edmunds manifold in hand. And ebay has a NOS small port head available:
NOS GMC 2136477 Head


regards,
stock49

I actually have 2 248s, one with a 477 and one with a cracked 819(stock to the truck). I also have a civil 302 long block with 2 bent rods and in need of 2+ sleeves but with a good head. I am making arangement to buy a 52 military 302 that runs but is 5 hours from me. A 58 270 is for sale by me but the engine is condition is unknown and I get to pull it for $600. I am thinking I will build the long block and keep the stock manifolds until I can build or find an intake and header.