Downside of updraft?

Well, this may be an easy question for some, but I have been scratching my head on this one.

Many vehicles of the 20's/30's came from the factory with an updraft carb. Somewhere in the mid to late 30's most manufactures used downdraft. Some european cars have used sidedraft.

What was the shortcoming of updraft? Gravity, flow, atomization, something else?

I did search and attempted to google research, but nothing really stands out.

THanks!

 

I think a combo of those.... I've got quite a few old John Deeres w/ updrafts that work great but they rarely see the same conditions a car would... (if you get to speeds over 20 on an old Deere I suggest you jump)

 

Thanks for the response. I may not get a definitive answer, but curiosity is driving me to find one.

 

they dont work well sitting ontop of v8's

 

Lets use the dodge/plymouth flathead 6 as an example. Certainly could have used updraft or downdraft.

V8 may not always be the platform in question. But seeing that you brought it up, why did some use sidedraft on v8?

 

I wonder if it has to do with the intro of the V8. An updraft wouldn't work on a V8 but a down draft would work on a 4,6,etc.. just fine. Why use different carbs when one 2barrel design would work for both? I don't really know but this sounds logical, right? I think side drafts were mostly used for hood clearance, I could be wrong though.

 

Actually, all of the early V-8's used updraft carburetors.

The biggest difference is gravity, or the lack thereof. Using an updraft carburetor means that the vacuum of the engine must overcome gravity to pull the air/fuel mixture into the engine.

One big disadvantage to the updraft is that once the engine is turned off, all fuel in suspension in the intake manifold drops down into the throat of the carburetor. Often there is enough to have fuel run out of the air intake.

The updraft carburetor was used in the beginning because of the lack of a pressure fuel pump. Gravity feed was the way these were fed (the carburetor lower than the fuel tank). Although there were some innovative (if not too successful) methods of pressurizing the fuel tank. One was a hand pump that pumped air into the tank; another split off part of the exhaust!!!!!

Once a good pressure pump came along, the updraft was used only for special needs (i.e. cab over engine or marine with a closed bilge).

Jon.

 

Jon, thank you. While I had thought that it was possibly gravity, you filled in the gaps with sound reasoning/fact.

Thanks again!

 

not a disadvantage but you cant drop nuthing down an updraft. i think paper was expensive and air filtraion wasnt its finest

 

Gravity, that pretty much explains it. Did you ever try to run a "T" up a steep hill. It don't work because the carb is then above the gas tank. SO, you turned around an backed up the hill.
Jr.

 

KOOKEE...I think back in those days the hand some real issues with carbs. in general. for 1 thing the knew an updraft carb. couldn't flud and cause a hydrolic situation. but like you i realy don't understand the sidedraft at all. there was a post here that said his carb. leaked so bad that gas was realy bad in the intake manifold. he said it bent piston rods, and he posted pics. that was the first hydrolic i've seen with gas...POP.

 

The only downside to updraft is when flooded, the excess fuel is outside the motor. It can be a fire hazard. As far as function, there isn't a downside. On a downdraft spilled fuel goes into the intake manifold, is contained.

 

29Nash, I read that on Wikopedia. Fire hazzard, but at least the engine wasnt flooded! Not sure that is a good trade-off.

Sidedraft. Now I know these were not develped for better hood clearance, but what was the development edge or problem solved through their use?

Thanks all for taking the time to educate or speculate.

 

From the Datsun 1200 website:

 

I wouldn't depend on what Wilkopedia says as much as my personal experience with lot's of cars with updraft carburetors. Since about 1946 when I first drove, I've had more carburetor fires with updraft than I have with downdraft. So, discounting fire hazard(your conclusion)that narrows it down to ZERO downsides of updraft carburetors.

 

T-Time: Thanks! That explains alot and appears quite logical.

 

I had several times, when living in the Islands (St.Thomas)that I ran out of gas going up a hill in my 29 PkUp and had a 1/2 Tank of gas. And I was running stock configuration. An electric fuel pump fixed that problem. Also if the Tank valve was not shut off, and the carb float did not shut of the gas when you shut her down, you found all the gas on the floor, (fire hazard) the next morning.

 

I firmly believe, that updraft carburetors are just un-natural, completely against all laws of nature. This conviction also applies to inline five cylinder engines.

 

You should check out the new "Hot Rod Deluxe" mag. It has some vintage photos of engines (most from the 50s) and a few of them (sbc by the way) have some strange fuel system setups.

 

Having to stand on you head to tune it.

 

This is the best response yet.

 

All I know is the updraft on my dad's Graham-Paige with Continental six never stays in tune! haha... all the updrafts on our Ford 8N's / John Deere 60 / John Deere A work flawlessly and never get any attention.

I've got a pair of SU side drafts on my '67 MGB GT. No carb flows as well as a side draft into a small displacement engine. This is why you see side drafts on the old race cars. They're incredibly simple to work on too... almost too simple. Think, "easier than Carter 4 barrel" simple... The only issue is people don't seem to know not know how to trouble shoot properly and starts screwing around with the carburetors because their "funny lookin'", so that must be the issue... haha.... poor bastards...

-Chris

 

I read somewhere the updrafts were popular because the early carbs were a little careless about atomization and the larger drops would not be carried up and into the manifold, making for a smoother running engine.

Sounds good to me. A smooth running engine is important if you don't have motor mounts.

Bill

 

What's wrong with inline 5's? I almost sold my soul to a car dealer to buy a Colorado w/ a blown 5 cylinder.. ran like a raped ape...

Now back to updrafts!! (nothing wrong in my mind with them either!!)

 

I am restricted to running 4 cylinder engines in one class of racecar. I used downdraft carburetors and they worked well except that after shutoff gas sometimes leaked into the lowest cylinder. The puff of smoke on starting seemed nice but the fuel hydrolocked the engine and split an expensive aluminum block.

To avoid this, I changed to updraft carburetors...no more broken blocks.