How do you make a homemade updraft intake for a 302 GMC?

I know this entire idea is pretty dumb, but it is what I want to do. I have a 1952 302 GMC that I am putting into my 1920's American Lafrance firetruck speedster. The problem is hood clearance, and I want to run dual updraft carburetors. I can fabricate just about anything, but I don't know the dos and don'ts of inline intakes. I already have an intake off of a COE truck that has an updraft carb, but I want to use two carbs and open exhaust. How do I build my own intake? I want a cruiser not a drag car. Why do I see so many aluminum intakes with water jackets and do I need to have a water jacket if I make it out of steel?

 

If you run it on the street,you'll want water. It is for warm up. You can weld up just about anything for a manifold, just add a balance tube between the 2carbs for air.

 

not a dumb idea at all...many early racing engines used dual updrafts. for inspiration, look at the Winfield model a/b ford 4 cylinder intakes. since that was a 4 cylinder with 2 ports, those intakes had one "up and over" hoop going from one port to the next and the downspout for the carb was at the high-point of the hoop. now just make two identical hoops (with your port spacing) out of aluminum tube and siamese the center port together for sort of an inverted W shape.

 

Also, I have two nice old Bendix / Zenith iron caburetors that would have been used on a 270 GMC COE updraft system. If I use these two carbs, do I need to modify the carbs? Also, what do I do about the vacuum advance seeing as how it was designed to run off of one carb? My goal is to make this engine look a bit older than it is. I don't plan on racing it, just driving it. I want the fire truck ch***is to look like a 1910's racer.

 

If you had an aluminum twin carb manifold and you made spacers to move it out a little, for room for the exhaust manifold. I bet you could turn it over and run it as an updraft. Then you would need the carburetors. I don't remember any twin carb intakes for the GMC. But maybe.

 

Actually some of the GMC sixes came with an updraft (single) carburetor. I saw one in a junkyard in Kansas.
V/8

 

The GMC six that I saw had been swapped into a Chevy truck. After doing a google search it seems that the updraft carbs originally came on the COE trucks.
V/8

 

Not absolutely sure about the 302, but 216 and 235 manifolds can be put on upside down. Welder conversions did that for many years. The exhaust points up and to the rear, but that's easy to bend back down.
The vacuum advance will work fine with 2 carbs. Just plumb it in to manifold vacuum.
On a street rod, if you custom-built a header there'd be no problem with an upside-down intake. I'm sure somebody made a dual carb manifold for the Jimmy. You might have to build a bolt-pattern adapter setup to get the updraught flange to fit.
Finally, PICTURES! We live and breathe pictures around here, especially fun freaky builds like yours!

 

ellis made a dual carb intake for the gmc. i have one hanging on the wall.

 

I don't think you can do that - the old Chevy 6 distributors worked off a ported vacuum source on the carb, the vacuum signal came on as the ****erflies open, with manifold vacuum the signal goes off as the ****erflies open

 

The water jackets in manifold heat it. If its not the fuel will not atomize and will condense.

 

I have a NOS Zenith 2 barrel side draft carburetor, that would have been used on the larger GMC "Cannonball" gas engine COE's, with the larger 318, 426, or 503 inline 6. Is the updraft 302 manifold you have in good shape. I've been looking for a spare. i have a stock '52 GMC 470 series 3 1/2 ton COE.

 

Just build one like mine, but upside down.

 

I had wondered if that could be done.I have seen several dual carb intakes for the downdrafts.

 

The intake manifold that I have is in good shape, but one of the exhaust donut ring ears is cracked-but repairable. I am planning on building my own exhaust system anyway.

 

That is what has had me concerned. Both of these carburetors that I have have the vacuum port for the vaccuum advance. I could plug the vacuum advance into one of the carbs and plug off the other port. My fear is that there want be enough vacuum to run the advance now.

 

Here are some pictures of what I have to work with. The fire truck is a long ways from completion. This is a picture of the 302 the day I brought it home. As you can see the COE updraft manifold that I have is only good for one carb. I have several of these nos Bendix-Zenith carbs.

 

Here is a quick sketch of what I have come up with. I want this engine to look like a much older engine than it is when I get it done. I would like to use heavy copper tubing to make the manifold. If I make it W shaped, then I can run my exhaust pipes straight out of the side of the hood. I can make small water jackets around each inlet pipe to help it warm up. Good idea or bad idea?

 

I really don't know anything, but since you asked. It looks good to me.

 

If you run the exhaust close enough to the intake, you won't need the water heat. 250,292's use exhaust for intake heat, so you might be able to make a heat chamber around the bases of each carb. Something like a tube in a tube with sealed ends, plus exhaust heat works much faster and gets much hotter then water heating.

Joe

 

One carb. vacuum should work for the advance. If you are welder friendly, the intake should be no problem. You could go with weld black pipe fittings for your el's which look like tight radious mandrel bends. Would give you the appearance of a casting.(shown in Ooltewahspeedshop photo). Available at a good plumbing supplier. You could also go with mandrel bent tubing. You might want to ask Ooltewahspeedshop if they would sell you some flanges to give you a start.

 

Yes, one carb should provide enough vacuum for advance. However, it might be a little ragged, what with only 3 cylinders ****ing on it. If so, use a tee and hook both carbs to the distributor vacuum.

Bill

 

I'm of no help on the carb question, but I do like the way your project looks already. Keep us posted!

 

Until the engine is up to full operating temp and the engine heat transfers to the intake.

If you have the fab skills I'd think using the stock updraft as kind of a pattern and welding up a dual carb log style manifold would be the solution. Running a heat tube through it and using the engines heater outlets and inlets would simplify that. If you have room on the side you could even make the runners from the body of the intake to the head a bit longer for header clearance.

 

Big series 270 - 302 GMC from the early '50s used a full mechanical advance distributor that would solve your advance problem. I doubt you need any intake manifold heat in Texas. An Ellis dual intake has no heat riser flange & would work well - still pretty common & costs less than a McGurk or Nicson

 

Thank you to all of you for your help. I'm listing some pictures that I pulled off of the web to give you an idea of what I'm going for. I'm pretty sure I want to make the intake out of copper at this point. I can fabricate just about anything. I want my exhaust to end up looking like this 1908 Mercedes when I'm done. Over all, I want this American LaFrance to resemble the 1909 ALCO racer. Low and long is my goal with the car. I plan on doing a bunch of cosmetics to the engine to make it look older. I have seen some of these dual carb set ups with a small pipe that connect to both the inlet necks just above the carbs. What does that do?